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View Full Version : BG Speed Demon 850 problems - carb experts please HELP



dhardison
07-19-2006, 01:33 PM
I finally got the GM 572/620 installed and took it on it's maiden voyage this past Monday. The engine comes with a BG Speed Demon 850 (#1563010 - mech secondaries). The carb idles fine (900-1000rpm) and when you're "on the gas" it runs strong and smooth all the way to 6000rpm. The problem is when I'm trying to "cruise" or hold the throttle still for a constant mph. When holding the throttle still, the carb surges and stumbles, NO popping/backfiring through the carb, more of a surge. It feels like it's going from lean to rich, lean to rich which makes the car surge/stumble. It doesn't matter if I'm doing 10mph in 1st or 75mph in 5th, when holding the throttle still it's the same result. As I mentioned it idles fine, but when taking off I feel the same stumble between ~1000-2000rpm.

I followed the manual that came with the carb and set the butterflies (.020 slot showing) and idle mixture screws (3/4-1 turn out) as recommended. Out of the box the carb was jetted 85/93 with a 6.5 power valve. I have the inital timing set to 20 degrees and it advances to 36 around 3500rpm. Here are the things I've done so far at the recommendation of BG tech support:

- I went back through the manual and double-checked the baseline settings
- I changed the primary jets from 85s to 87s - thinking we might have a lean condition - no improvement/change
- I changed the jets from 87s to 83s - since going up didn't help I figured I'd go the other way - no improvement/change

Any thoughts on what might be the issue? I'm thinking of plugging the power valve next to take that out of the equation.

Here are the specs of the carb:
MODEL #850
PART #1563010
VENTURI SIZE:1.562
IDLE AIR BLEEDS PRI:.070
IDLE AIR BLEEDS SEC:.063
H.S. AIR BLEEDS:.028
BOOSTER TYPE:DOWN LEG
BOOSTER LEG:.184
SQUIRTER SIZE:31 STD.
BUTTERFLY SIZE:1-3/4
AIR HOLE SIZE:NONE
PUMP CAM ## 330
PUMP CAM COLOR:PINK
METERING BLOCK # P.A-12400
METERING BLOCK # S.A-12401
SIPHON BREAK:.028
E-HOLE POS. #1.031
E-HOLE POS. #2 N/A
E-HOLE POS. #3.031
E-HOLE POS. #4 N/A
E-HOLE POS. #5 .033
P.V.C.R. SIZE:.059
TRANSFER SLOT RES: OPEN
IDLE FEED RESTRICTOR .035
MAIN JETS PRI: #85
MAIN JETS SEC:# 93
POWER VALVE PRI: 6.5 STD.
POWER VALVE SEC: N/A
LINKAGE TYPE: PROG.
N&S SIZE/TYPE: 110-VITON
ACC. PUMP DIA. SIZE 30CC

Thanks!

Tig Man
07-19-2006, 04:55 PM
Sounds like it's either too lean or need to change the power valve to a lower vacuum!!

Mark

07-19-2006, 05:06 PM
What Mark said. I bet the power valve needs a-changin'. What's the manifold vacuum at idle?

Can you list some of the engine specs?

dhardison
07-19-2006, 06:40 PM
Unfortunately I don't have a vacuum gauge to measure manifold vacuum. One of the BG techs also mentioned changing the power valve from the stock 6.5 to a 4.5. Is this change in the right direction (i.e. going numerically lower)?

The engine specs are listed in the URL below (the 620hp model, NOT the 720).

http://www.sallee-chevrolet.com/frame.html?/ChevyBigBlockV8s/572.html

Thanks!

07-19-2006, 09:01 PM
Is this change in the right direction (i.e. going numerically lower)? Probably, but you won't know for sure until you buy a vacuum gauge.

dhardison
07-20-2006, 06:35 PM
I took the possible power valve problem to an extreme and plugged it off completely. Still no change in behavior. Here's what else I've tried with still no positive result.

https://www.pro-touring.com/forum/showthread.php?t=20927

If you have any suggesstions I'm all ears!

07-20-2006, 06:56 PM
What do the plugs look like?

dhardison
07-20-2006, 07:08 PM
The plugs are all light to medium brown in color. Not too light, not too dark, seem to be just right. No deposits, oil, etc on the plugs either.

07-20-2006, 07:18 PM
Plugging the power valve will lean out a carburetor not jetted for it something awfull. If I remember correctly, the correction factor is something like 8-10 jet sizes.

You really need a vacuum gauge dude.

formula
07-20-2006, 07:30 PM
a vacuum gauge is god when it comes to carbs. It'll tell you everything you need to know if you know how to read it. Best 25 bucks I've ever spent.

dhardison
07-20-2006, 07:39 PM
Plugging the power valve will lean out a carburetor not jetted for it something awfull. If I remember correctly, the correction factor is something like 8-10 jet sizes.

You really need a vacuum gauge dude.

Yes, the power valve is good for 8 jet sizes with the Demon carb. But the power valve shouldn't come in to play when holding a constant speed. It only starts opening when you're accelerating and vacuum is moving toward zero.

So in theory just plugging the power valve without jetting up 8 sizes will cause a lean condition when accelerating, but when holding a constant speed it shouldn't cause a lean condition because the PV should stay closed. Right?

dhardison
07-20-2006, 07:42 PM
a vacuum gauge is god when it comes to carbs. It'll tell you everything you need to know if you know how to read it. Best 25 bucks I've ever spent.

Agreed I need a vacuum guage. When I get one, what type readings will I need to obtain? Idle vacuum? Vacuum when trying to hold a constant speed?

How do I use this new vacuum info to help diagnose my problem?

Thanks for the help guys!

07-20-2006, 08:51 PM
When you get the vacuum gauge, hook it up to a long enough line for it to reach inside the cab. Check idle vacuum, off idle vacuum, and cruise vacuum.

dhardison
07-21-2006, 11:03 AM
I hooked up a vacuum gauge and here are the readings:

idle (950rpm) 6.5
off-idle (1600rpm) 11
cruise (2000rpm) 11

I noticed that when I accelerate and the vacuum drops at or below 6-6.5 the stumble is gone. Same thing when decelerating when the vacuum goes above 6-6.5 the stumble returns. I have a 6.5 power valve it her now so the way I figure is when the PV is opening at 6.5 the extra fuel is aleviating the stumble. My next plan of attack is to leave the PV at 6.5 and go from 85 to 90 jets in the primary. This will provide additional fuel at cruise without the PV needing to kick in. If this works I'll adjust the secondary jets accordingly. Does my logic make sense?

07-21-2006, 11:10 AM
I have to ask, have you ever owned an engine like this? If not, is it possible that you're feeling the cam? 2000 RPM is lugging an engine like yours.

(Disclaimer, I've never owned, or worked on, a BBC)

The power valve is incorrect anyway, whether it gets rid of the problem completely or not is anyone's guess. Throw a 4.5 in it and see what happens.

dhardison
07-21-2006, 11:27 AM
I just tried the 90 jets and if anything it was worse.

I've owned many BBCs, none with this many cubes or a hydraulic roller cam (the cam isn't very radical). It doesn't just do it at 2000 rpm, if I'm 'cruising' ANY rpm in ANY gear it stumbles. The stumble is not tied to a certain rpm, mph or gear selection. I'll try a 4.5 and see, but right now I'm taking it out to test-n-tune night at the drag strip, since there I won't be 'cruising'. :naughty:

07-21-2006, 12:01 PM
I wouldn't do that if I were you. Motors running way lean at full throttle still feel strong if you don't have a base line ass reading with a good tune. They run strong right up until parts of the piston leave the car via the exhaust.

Try the power valve and we'll go from there.

JoshStratton
07-21-2006, 12:33 PM
but right now I'm taking it out to test-n-tune night at the drag strip, since there I won't be 'cruising'. :naughty:

Hey Dan...Are you going to NationalTrails?

If I wasnt working on that dang barn I would come watch. You going next weekend too??

dhardison
07-21-2006, 12:38 PM
National Trails is having some big 3-day race. I'm going to Mt. Vernon for test-n-tune instead. Hard to say about next weekend......

Steve68
07-21-2006, 03:26 PM
Put the Holly on and then see what happens!!, BG's I have heard are tempermential, (disclaimer) never worked on one but watched the kid down the street put one on a Mustang and it was nothing but problems!

I have messed with a bunch of Holly and I can usually get them to work,

07-21-2006, 03:47 PM
Steve, in my opinion, stock Holley carburetors trick people into thinking they know how to tune any carburetor. The reality is that stock Holleys don't run as well as others, but they don't run well equally on most motors, which equates to very little tuning time until it's as good as it's going to get. Yes, Demons are extremely touchy, but if you know what you're doing, you'll be glad they are. IE 1/8 turn on the idle mixture screws actually does something! They are a lot of work to get right though.

If you ever want to see how a carburetor should run, have one built professionally. Then you'll understand how bad stock Holleys really are.

dhardison
07-21-2006, 09:20 PM
Matt, I tried the 4.5 power valve and still have the surging. Only now instead of the surge stopping when vacuum reaches 6.5 (as with the 6.5 power valve), it now stops surging when vacuum reads 4.5. So the surging stops when the power valve begins opening. I can only assume if I installed a 10.5 power valve the surge would clear up at the 10.5 vacuum mark (for example). I've considered once again plugging the power valve completely and jumping from 85 to 95 jets in the primary to compensate for the now plugged power valve.

dhardison
07-21-2006, 09:23 PM
Put the Holly on and then see what happens!!, BG's I have heard are tempermential, (disclaimer) never worked on one but watched the kid down the street put one on a Mustang and it was nothing but problems!

I have messed with a bunch of Holly and I can usually get them to work,

I put the Holley on earlier this week and the surging problem was still there although it didn't surge nearly as bad. The out-of-the-box (i.e. untuned) 930 Holley DRASTICALLY reduced the power as felt with my 'seat of the pants' dyno.

07-21-2006, 09:31 PM
Leave everything the same, but go up 10 jet sizes front and rear. This sucks long distance. :)

dhardison
07-21-2006, 09:56 PM
This sucks long distance. :)
Hehehehe, agreed.

So leave the power valve at a 4.5 and go 85->95 primary and 93->103 secondary? I don't think I have anything bigger than a 99 jet.

It may be a while before I can test this change considering that I broke an axle tonight at the track.:crying: I guess the 572 was too much for the stock 12-bolt axles. I did click off three runs in the low to mid 7.60s though (1/8 mile).

Madspeed
07-21-2006, 10:01 PM
Leave everything the same, but go up 10 jet sizes front and rear. This sucks long distance. :)

Matt Once again Hits the nail on the head
Stop dicking around and listen to the man, change the jets allready =) your too lean
the power valve is a way for the holley to *cough* get better economy when you are at cruise it closes of a passage and leans out the A/F then when you acellerate and need power it opens and lets more Fuel in IE making the A/F (iff set right) more in the 12.1 a/f
So what you want is to have it open at a lower vac reading than cruise I would think 5 or 4.5 should be close

Anyways MAt is right listen to the man =)
GL bud

Jims78elky
07-22-2006, 01:40 PM
Dan,

Sorry to hear the 572 has some carb probs,though iam just
blabbing on,are you sure there is no vac leak? Sure sounds or
acts like one.well you could always put on the ol' dual carbs
off the old LS7 454 and see if it runs better.:dunno:

Any big difference in the old and new engine,minus the carb
issues? Such a monster engine,you would think it would be
a no problem deal...i hope it all works out for you man.

I got so tired of dealing with carbs and there nightmare running/setting issues..:screwy:

Not that EFI applications are hassle free..just easy for dum-dums like me! lol.

dhardison
07-22-2006, 05:43 PM
Anyways MAt is right listen to the man =)
GL bud
LOL - I'm not dicking, I'm listening! :lol: I'm just trying to take small steps so I'll know what fixed it. Once the axle is replaced I'll bump the jets up a drastic number and see what happens.

dhardison
07-22-2006, 05:49 PM
Sorry to hear the 572 has some carb probs,though iam just blabbing on,are you sure there is no vac leak?
Yeah it sucks, this and any other 'crate' motor shouldn't have these problems in my opinion. Heck I've easily spent an additional $900-$1000 dollars on new carb, distributor, wires, plugs, etc. just to try and cancel out certain areas as the problem. So far the only thing getting 'fixed' has been my wallet. :hand:

I sprayed carb cleaner all around the base of the intake, between the intake and heads and on both ends of the intake where it meets the block but the idle didn't drop a bit. I also sprayed around the base of the carb, nothing. Is there some other place I need to check maybe?

Everyone keeps screaming lean, and I'll definitely check that route as well, but I know 2-3 other people with the same motor and they've had zero issues like this. Now granted they live in other parts of the country and their carb set up would be different due to local air/elevation differences.

Steve68
07-22-2006, 05:59 PM
Steve, in my opinion, stock Holley carburetors trick people into thinking they know how to tune any carburetor. The reality is that stock Holleys don't run as well as others, but they don't run well equally on most motors, which equates to very little tuning time until it's as good as it's going to get. Yes, Demons are extremely touchy, but if you know what you're doing, you'll be glad they are. IE 1/8 turn on the idle mixture screws actually does something! They are a lot of work to get right though.

If you ever want to see how a carburetor should run, have one built professionally. Then you'll understand how bad stock Holleys really are.

I understand that, so if you turn the Deamon screws it makes a difference, damn, I might have to try one, I've been perfecting my craft lately on Jap carbs, I can say I'm tuning OK, I know Hollys are like putting a cardboard box on the intake, but it's all I have at the moment, I wanted to buy the carb in the FS section but moneys tight/er now that gas has been kicken my butt for months!!

Dan wanna sell the 950HP?? Moser axles!!! I have a C-clip eliminator if your interested!! but you might not be,

dhardison
07-22-2006, 06:47 PM
I understand that, so if you turn the Deamon screws it makes a difference, damn, I might have to try one, I've been perfecting my craft lately on Jap carbs, I can say I'm tuning OK, I know Hollys are like putting a cardboard box on the intake, but it's all I have at the moment, I wanted to buy the carb in the FS section but moneys tight/er now that gas has been kicken my butt for months!! This is my first Demon as well. All I can say is it kicks the 950 Holley's but when measured on the ass-dyno. Of course it was jetted quite a bit little lower as well. Barry Grant tech support claims the Demon 850 "wet" flows 1080cfm. I dunno.........


Dan wanna sell the 950HP?? Moser axles!!! I have a C-clip eliminator if your interested!! but you might not be, I saw those Holley's in the F/S forum as well and wish I had seen them before I bought the 950. One of them would've been mine! Right now I'm not interested in selling the 950 and actually it's a QuickFuel Pro Series carb (P-950). It sure is 'purdy'!

http://static.summitracing.com/global/images/prod/large/qft-q-950_w.jpg

I'll be selling one of the other depending which I get to work correctly first.

I'm going to stick with c-clip axles for now. My experience with c-clip eliminators on a street car has been a 'leaky' one at best.

Steve68
07-23-2006, 06:17 AM
Keep me in mind, I have the Strange ones, never leaked on the 68, but you have to cut the rear end where the bearings go, not good!!

Those carbs are pretty, talked to the owner last year,

Did I tell you I was looking for you in orlando during the P/T

lil zl1
07-24-2006, 06:06 PM
It appears to me that you have a fuel delivery problem that may not be associated with carb. Take a look at your fuel pump, the lines and the filter being used. A vacuum leak is also a possibility. Or the distributor is not holding its advance. Hope these suggestions help.

dhardison
07-24-2006, 06:33 PM
Thanks zl1. I've already tested for vacuum leaks, unless it's internal to the intake (if that even makes sense). I've tried another distributor (new), but it didn't help. A couple other people have also mentioned fuel delivery, but it runs fine when on the gas, so maybe it's a fuel VOLUME problem, dunno. I have a 1/2" line from the tank to the engine compartment and #8AN lines from there to the regulator, pump and carb. I have a new filter installed as well. I'll hook up a fuel pressure gauge so I can view it when driving. I hope it's that simple. Thanks again!

dhardison
07-24-2006, 06:34 PM
Did I tell you I was looking for you in orlando during the P/T

Sorry I didn't get the chance to meet you on the PT. I missed meeting a lot of the PT guys. Are you going to the Pigeon Forge event?

Steve68
07-24-2006, 06:37 PM
Trying to, going to tow the Nova, run like hell, were getting busy launching again, we'll see what happens

cad
07-28-2006, 04:27 PM
I have had problems with my 750 brand new mighty demon recently, also. Problem was that no matter what jet we put in, we couldn't get the plugs to color up at all....and 1/8th mile performance did not change a hundreth..we had way big jets in them. Put my little holley 650/proform ctr on it, and it ran just as good...albeit, poorly. Then, I borrowed a friend's 90s era 750 BG (built while he worked at BG) that flows 950cfm. With little jets, the plugs were extremely brown. And the car picked up 3 mph and 2 tenths. Needless to say, the mighty demon is back at BG for a booster swap (2 of the annulars were crooked) and reflow/calibration check. It's been 2 solid weeks, and Mike in tech didnt have any details on their shop's progress. That was a little disappointing. Anyways, mine is a 383 that has run 7.73. The mighty demon slowed to 8.08s....not pleased with the Demon, yet. I have a full new MSD setup, and checked the fuel delivery, which is right at 110gph. Go figure. Anyways, keep posting until it's cured.
Clark

Jims78elky
07-28-2006, 06:06 PM
Dan,

I know this might sound redundant,but have you checked your
coil? I was reading a post at another site and a guy had a
similar engine prob like yours,he worked for weeks trying to
figure out what the problem was,changed carb(s) new fuel
lines,jettings,yada yada,..even a new intake,but to no help.

He is running a 540

Then he states that after all that he thought he would check
the coil,says that it wasn't giving a full spark,so not being to
sure he ran down to the local parts store and purchased a
stock GM coil,wired it up and..!!! fire in the hole!! He is a
happy happy dude right now.

Are you running a MSD coil? or the stock HEI that came on the
572? Have you checked to see if there is spark? or enough?
could be a bad HEI from the factory? or a bad connection?
Iam just ramblin on here,but to have a massive cool engine
like a 572 i would go nuts if it didin't start/run and make me
poop my pants on command.

Just seems like the carb issue is not what it is,i don't know
but iam so bummed about your whole deal right now..lol,
man i wish you the best on it.Its not like its a "cheapie"
engine to own.I'll keep following this thread,iam curious to
find out what it is or was.:dunno:

Hang in there!

:cheers:

Skip Fix
07-28-2006, 06:30 PM
You need to find someone with a wideband O2 meter to see where the heck you carb really is. Another thing to rmember that was brought up in the Chevy Hiperf article tunig a Demon -the idle circuit is active and affects more than the main jets up to almost 3000rpm!!! I have found the same thing using the LM-1. Best investment I've made for saving time tuning.Many of the chassis dyno shops have them.

I chased an off idle hesitation and cruising surge on my Holley 850-4781 DP with squirters etc. Jetting up helped the surge but not ht eoff idle. I had set idle mixture for best vacuum in park. Guess what with a tighter street convertor(had a race convertor in it before) in gear it went from a 12.5 AFR to a 16:1 AFR way lean. Adjusted the mixtures richer and no more off idle hesitation and I could jet leaner and no surge.

My Demon 850 and Holley 850 dynoed identical. Their jettings are way different, the Demon larger, because of differences in the bleeds.The Demon air bleeds are way off for the fuel curve for my current motor on the street whereas the Holley is right on.

If you are comparing Holley HP series cfm with Demon Speed cfm you are way off. Holley omn their HP series(and Demon on their race series) use a different test pressure. A 950 HP is actually a 750 body with 850 throttles, a 1000 cfm HP is the same as a "regular" Holley 850 DP and a Speed Demon. You have to look at the venturi and throttle blades to directly compare.

dhardison
07-29-2006, 07:57 AM
Thanks for the continued input on my problem guys! I'll definitely keep posting the things I've tried and hopefully (soon) a solution to the problem. Right now I'm waiting on replacement axles and also installing an exhaust system so the neighbors will stop calling the cops on me (j/j). Hopefully that'll all be completed in the next couple weeks and I can once again start back on the surge problem.

Clark:
I hope you get your Mighty Demon back soon and right this time. That's a pretty amazing E.T. difference between your Demon and your friend's. I have a Speed Demon and so far the plugs look good all around. I tried the Holley swap and also saw a noticable decrease in power. But to be fair the Holley was straight out of the box w/o any tuning. I just wanted to see if it would cure my surge at cruise problem, but no such luck.

Jim:
I've also talked with another GM 572 owner that had coil problems. I have a nearly brand new Pertronix billet distibutor and coil that I stabbed in but it didn't make a difference. So I'm back with the HEI that came with the engine (which is really an unbranded MSD). I've basically swapped everything I can swap with the exception of the fuel pump (which holds enough pressure/volume to run out clean on the top end). Right now I'm going to go from 85s in the primary to 95s. According to the vacuum gauge, the surge cleans out when the power valve opens so it appears to be a lean condition, but damn 95s seems extreme in the primary...... If that doesn't work, I dunno.... The only way GM will warranty the engine is if I take the car to a GM Performance Parts Dealer and let them look at it. But I'm not too keen on having some GM tech leaning over my fenders and driving my car up and down the road. If it comes to that, I'll be there the whole time "supervising". I sincerely appreciate your concern and you're right it does SUCK that this problem is occuring.

Skip Fix:
I've considered investing in a wideband O2 meter, but I've spent so much EXTRA already it's just not in the budget. Thanks for the info on the Holley/Demon size comparision, I often wondered about all that. If increasing the jets don't pan out I'll play with the idle circuit. Funny that in the 5-6 times I've talked with Barry Grant tech support they've never mentioned that at all.

It really shouldn't be this hard.................. :banghead:

07-29-2006, 08:25 AM
Right now I'm going to go from 85s in the primary to 95s. According to the vacuum gauge, the surge cleans out when the power valve opens so it appears to be a lean condition, but damn 95s seems extreme in the primary Four jet sizes is a pretty standard change, and yours may be so far off, you need to make a big change to prove if jetting is the problem or not. If 95's run fat and solve the surge problem, then atleast you know the solution is between 85's and 95's somewhere and you can forget about all the other theories. (Not saying jetting is the problem, but you have to start somewhere.)


If increasing the jets don't pan out I'll play with the idle circuit. Funny that in the 5-6 times I've talked with Barry Grant tech support they've never mentioned that at all. Did you ever mention the idle circuit? You didn't mention it here, which means people assume it's good. Yes, the idle circuit on Demons are active while at cruise, but if your idle quality is right, then don't even think about messing with the idle circuit to tune a cruise problem. All you'll do is confuse yourself and mess up your idle. "Skip Fix" had a cruise problem AND an idle problem, which is why he saw improvment in both when he tuned the idle circuit.

When working with artillery, making a huge change is called bracketing. One shot you hit way short, the next shot you hit way long. You plot those two points, and now you know exactly where the enemy is. This is what you're doing with the jetting. If it makes no change to the surging problem, that's still a good result because now you can be positive about one possible cause. So far you're only positive about one thing. Same thing with changing the power valve...it didn't solve the problem, but you learned about the problem.

Skip Fix
07-29-2006, 08:27 AM
I had one BG tech argue with me about tuning with them. he said "just read the plugs for WOT jetting and don't worry about the rest!"

If you haven't read the Chevy Hiperf article it is a good one. I think they have it on the LM-1 site.
www.innovatemotorsports.com (http://www.innovatemotorsports.com)

If you "paid" yourself shop time for chasing carb tuning (jet changes, PV changes, etc.) I'd bet you could easily pay of the LM-1!

dhardison
07-29-2006, 10:54 AM
I never mentioned the idle circuit here for the exact reason you stated, the idle is perfect. I turned all four idle screws in to seat, then back out 3/4 turn, and adjusted from there to get the best idle quality. Now thinking back, I did mention the idle circuit to BG which is where I got the 3/4 turn out, because normally on my Holleys I turn out 1.5 as a base.

Thanks for the link Skip Fix! I think I have that issue on the back of the "throne".

07-29-2006, 11:54 AM
I never mentioned the idle circuit here for the exact reason you stated, the idle is perfect. Excellent! Don't screw with it. :)

Skip Fix
07-29-2006, 08:43 PM
dhardison and Matt you are incorrect in the fact the idle circuit tuning being correct because "it feels right" that it is right on with a correct AFR. A few times with wide bands on different motors that are supposedly "right" you see they are WAY off, even using the "correct" jetting and plug reading in all different circuits.

My idle also felt fine,in fact perfect, had great vacuum in park and in gear, but being that lean (not knowing that until the O2 sensor)the accelerator circuit couldn't make up fuel delivery quick enough off a stop sign and a hesitation. It had a very slight surge as it cruised lower rpm(2000) and the load increased because it was already so lean with only a slight load, cruising adds more load. Way fat jetting did help that by feel but was stil llean but it did not help the off idle hesitation due to the fact it was a lean idle circuit. That's why following the carb tech guys I jacked with squirters until I finally switched the wide band on it and took 2 minutes to accurately correct it.Now its back to stock squirters and is very responsive. Throw a different idle bleed on and the whole picture changes.

In tuning other carbs and motors when the idle is fat or lean WITHOUT surging you can see the AFR stay the same well up the rpm range the same direction, Chevy Hiperf found the same thing! My surge was "cured" by jetting but it really was just masking the real problem, it was the lean idle circuit with some light load.

Your surge can still be the idle circuit as when cruising your motor now sees load(see the Chevy Hiperf cruising at less than 3000 is mainly on idle and transfer slots that also work off idle mixture screws), whereas at idle in neutral there is no load. Main jets have barely tipped in. Like my auto tranny did dropping it in gear it added load and going from 12.8-16.1 with the SAME idle mixture setting. I had to reset it for having some load. So its a little fatter at idle(jumped from 12.8 to 12.4) not in gear to get it right in gear ,12.8-13.1 for this motor. With a stick it is harder to add just a little load to check the idle circuit-you can't put it in gear with someone on the brake like an auto.


Back the screws out 1/4-1/2 turn and see what it does, simple test and no expense! Worse case it doesn't help and you blew 5 minutes of time.

07-29-2006, 10:37 PM
Back the screws out 1/4-1/2 turn and see what it does, simple test and no expense! Worse case it doesn't help and you blew 5 minutes of time. I can dig it.

dhardison
07-30-2006, 03:11 AM
I can dig it. Same here. I wish I had the info on the LM-1 before I bought that 950 Holley carb. Hell I could've saved $300. :hammer:

Sounds like the LM-1 would be a good tool for diagnosing this problem and for future carbs as well. I wonder if the O2 sensor has the same thread type as my current evacuation bung? If so, would it work in the collector, or does it need to be mounted somewhere more specifc?

Rick Dorion
07-30-2006, 01:08 PM
Standard bung. Will work fine in the collector. Here's another vote for the LM-1. Good tool!

Skip Fix
07-30-2006, 02:29 PM
The only thing they recommend on the bung is NOT putting it in the bottom of the pipe. Water condensation will hit is and can kill it.

You might can tell I'm a little prejudiced FOR the LM-1. It really tells you where the heck your mixture is. No more guessing. It easily swaps between cars. I think it is standard with Mustang Chassis Dynoes also that alot of the EFI crowd use.

Pontiac guys like Q jets and have a suggested primary jet/rod combinations they say should work. My LM-1 has found them to be way off. You can also see you air bleeds leaning or riching up the mixture as you increase cruising rpm. Good if you have a carb taht has interchangeable bleeds.

It also can help if you have to pass emissions, letting you know where the car is.

dhardison
07-30-2006, 02:50 PM
Any tips on vendors for the LM-1. Obviously looking for the best deal possible....

Rick Dorion
07-30-2006, 03:29 PM
I think Summit carries them

Steve68
07-30-2006, 05:32 PM
Dan, I have a painless bung NIB, it's your for the cause if you want !!!!


painless bung, hehehehe, he said painless bung,hehehehehe

dhardison
07-30-2006, 06:15 PM
Dan, I have a painless bung NIB, it's your for the cause if you want !!!! painless bung, hehehehe, he said painless bung,hehehehehe

LOL - thanks Steve. Hopefully my current evacuation bung (LOL!) will work. If not I'll give you a shout.

Beegs
07-31-2006, 04:11 AM
I wonder if the O2 sensor has the same thread type as my current evacuation bung?

My evacuation bung doesn't have any threads.:lol:

Sorry, couldn't resist.

Steve68
07-31-2006, 05:34 AM
Just let me know, and I'll send it off to you!

all the bung humor, couldn't resist!lol

Skip Fix
08-01-2006, 03:40 PM
I believe threads are the same as an 18mm spark plug.

Summit does carry them, about the same price as direct. Not sure if there are any E bay discounts out there.

dhardison
08-01-2006, 04:55 PM
I picked one up with a free case for $350 off of eBay.

cad
08-01-2006, 10:52 PM
I hope you have success with the LM1. My dad has built a circuit board that reads voltage from a wideband o2 sensor. We will be installing that when the carb comes back from BG. I called last Thursday and Mike/tech had no idea what the status on the carb was.
Anyways...http://www.innovatemotorsports.com/forums/search.php?searchid=142728...search on demon and follow along at what these guys go through to get the perfect AFR curve....quite impressive and great support with the log files.
BTW, a local dyno-guru once dyno'd a friend's drag corvette that ran in the low 11s. They set the car up with an across the board 12.8 AFR. When the car went to the track, it slowed down .7 (tenths) of a second. When they leaned it down, it picked back up. So, what the LM1 hopefully will do is show you where you are getting that stumble/surge, but not necessarily the best drag performance.

Skip Fix
08-02-2006, 05:38 AM
You are correct you never know what AFR each motor will like for track times and HP. You really need to do track AFR logging and changes to tell for sure(or chassis dyno).

For my uncalibrated "saet of the pants" rear it helps me see where the carb is fat or lean. My son's 81 TA with a pretty stock 400 felt great and responsive but would cruise at 16.1! It would get leaner as cruise rpm increased, showing too big and air bleed.

Tech @ BG
08-02-2006, 09:23 AM
Dan,

Sorry to hear youíre having all of these problems. The baseline calibrations on the carburetors for the GMPP 572Ē Crate engines are actually on the safe ďrichĒ side, so this is not a normal situation in most cars. Now that you have an Innovate unit it should make the tuning a good bit simpler.

Check to see what the AFR is at cruise when youíre having your surge. You may be able to help it somewhat by going to a higher number power valve so that it opens sooner. You mentioned earlier in the post that you were going to try this, but I didnít see where you did. Another thing to try would be coming out on your idle mixture screws to the point the idle is rich. This will let you know if the engine is wanting more fuel at your part throttle RPM. Once youíve determined that we can figure out the best course of action.

Does it only do this if youíre cruising in a specific RPM range? Meaning if you down shift and run at the same MPH, but higher RPM will it also surge then?

dhardison
08-02-2006, 11:27 AM
Thanks for your reply. I'm running right at 11 inches of vacuum at steady cruising rpm (when the stumble/surging is occuring). I've tried a 6.5 power valve and a 4.5 power valve. In both cases when the vacuum, reaches the power valve rating (and all the way to 0), the surge clears up. If I let off the gas and the vacuum falls above the power valve rating (i.e. the pv closes), the surge returns. The surge is not tied to a certain rpm, gear, etc. It will surge in ANY gear at ANY rpm as long as you hold the accelerator steady and vacuum is numerically ABOVE the power valve rating.

ANYTHING you can do would be be GREATLY appreciated. I've spoken with Eric on your tech line 4-5 different times and so far have tried everything he's suggested. He's been very helpful and we've tried small changes so far. His last suggestion (which I have yet to try) is to go way fat on the primary jets 85s-95s to see if it alleviates the surge. We don't want to leave it that way, just want to see if that will clear it up and work from there. He also suggested that I work to possibly gain more timing advance down low.

Thanks again!

08-02-2006, 12:27 PM
I've spoken with Eric on your tech line 4-5 different times and so far have tried everything he's suggested. He's been very helpful and we've tried small changes so far. His last suggestion (which I have yet to try) is to go way fat on the primary jets 85s-95s to see if it alleviates the surge. Sounds familiar. Have you done this yet? Have you fattened up the idle yet?

Tech @ BG
08-02-2006, 01:37 PM
Thanks for your reply. I'm running right at 11 inches of vacuum at steady cruising rpm (when the stumble/surging is occuring). I've tried a 6.5 power valve and a 4.5 power valve. In both cases when the vacuum, reaches the power valve rating (and all the way to 0), the surge clears up. If I let off the gas and the vacuum falls above the power valve rating (i.e. the pv closes), the surge returns. The surge is not tied to a certain rpm, gear, etc. It will surge in ANY gear at ANY rpm as long as you hold the accelerator steady and vacuum is numerically ABOVE the power valve rating.

ANYTHING you can do would be be GREATLY appreciated. I've spoken with Eric on your tech line 4-5 different times and so far have tried everything he's suggested. He's been very helpful and we've tried small changes so far. His last suggestion (which I have yet to try) is to go way fat on the primary jets 85s-95s to see if it alleviates the surge. We don't want to leave it that way, just want to see if that will clear it up and work from there. He also suggested that I work to possibly gain more timing advance down low.

Thanks again!

Dan,

The first thing I'd do would be to get some Lambda readings since you've already got the Innovate system. From their it will be a matter of seeing exactly what your specific combination wants. Not sure that I'd change the main jets that far just yet. First step would be to come out on the mixture screws to see if we can get enough fuel through the idle circuit. We can walk you through making changes there if need be. The next step I'd try would be raising the primary float levels up in the window. This will get the main circuit to start pulling fuel sooner, and may help slightly. From there I'd try a higher number power valve something in the 8.5 to 9.5 range.

dhardison
08-02-2006, 06:16 PM
Sounds familiar. Have you done this yet? Have you fattened up the idle yet?

No, I'm still trying to get the rear end back together from the broken axle (hopefully this weekend). I'm taking excellent notes though!!!

dhardison
08-02-2006, 06:26 PM
The first thing I'd do would be to get some Lambda readings since you've already got the Innovate system. From their it will be a matter of seeing exactly what your specific combination wants. Not sure that I'd change the main jets that far just yet. Ok, I'll hook up the LM-1 and see what it tells us before changing anything.


First step would be to come out on the mixture screws to see if we can get enough fuel through the idle circuit. We can walk you through making changes there if need be. How far should I back them out? Right now they're around 3/4 turn out all around.


The next step I'd try would be raising the primary float levels up in the window. This will get the main circuit to start pulling fuel sooner, and may help slightly. How far should the floats be raised on the primary circuit? Right now they're right below 1/2


From there I'd try a higher number power valve something in the 8.5 to 9.5 range.
I understand that by going to a numerically higher power valve it will open sooner, but won't this cause the power valve to start opening during steady rpm cruising? It holds an average of 11 inches of vacuum, but sometimes it's 10 or so. My concern with a numerically higher power valve is that the power valve will open-close, open-close during steady rpm cruising.

Don't get me wrong, I'm open to anything at this point.

Thanks again,
Dan

Tech @ BG
08-03-2006, 07:20 AM
Dan,

As far as the mixture screws go thereís not going to be a set amount to try. All we want to do at this point is to affect the situation youíre running into. The Lambda will really be able to help you dial in from there. Iíd probably start by coming out a Ĺ turn or so on them. Itís going to ruin the idle (for now) again; all we want to do is see what your combination is wanting.

Same scenario with the float levels, Iíd go at least ĺ of the window. Just make sure you donít go so high that the boosters drip fuel at idle.

Yes, going to a higher number PV would get it to open under your steady RPM cruise. This will hurt your fuel economy, but if the engine wants the fuel, which is what it sounds like from the testing, youíve done than this should help.

dhardison
08-21-2006, 11:37 AM
Dan,

As far as the mixture screws go thereís not going to be a set amount to try. All we want to do at this point is to affect the situation youíre running into. The Lambda will really be able to help you dial in from there. Iíd probably start by coming out a Ĺ turn or so on them. Itís going to ruin the idle (for now) again; all we want to do is see what your combination is wanting.

Same scenario with the float levels, Iíd go at least ĺ of the window. Just make sure you donít go so high that the boosters drip fuel at idle.

Yes, going to a higher number PV would get it to open under your steady RPM cruise. This will hurt your fuel economy, but if the engine wants the fuel, which is what it sounds like from the testing, youíve done than this should help.
Finally got my new axles in and had a chance to try some of the recommended changes. Here's are the changes I tried:

I turned out the mixture screws an additonal 1/2 turn and adjusted the float levels to 3/4 window. No change.

I then plugged the power valve and squared the jets all around at 94 (originally 85/93). So essentially I gave the primaries 9 additional jet sizes of fuel all the time. This did nothing to improve the surge/stuble but made the carb load up quite nicely under light throttle.

I have an appointment tomorrow at a GM Performance Parts dealership to show them first hand what it's doing. I'm BEYOND tired of screwing with it. :banghead::banghead::banghead::banghead:

mac71ss
09-06-2006, 05:21 PM
the the dealership ever help you out??

dhardison
09-07-2006, 03:24 AM
the the dealership ever help you out??

Sort of..... I'm pretty much still getting the run around.

See this thread:
https://www.pro-touring.com/forum/showthread.php?t=21941

Skip Fix
10-13-2006, 02:26 PM
Where are you at on GM and warranty.

dhardison
10-13-2006, 05:59 PM
This is the latest. Hope to have the new TKO in this weekend or next.

https://www.pro-touring.com/forum/showthread.php?t=23218

NOVA
10-13-2006, 06:28 PM
did you ever get your Demon carb issues resolved?

dhardison
10-13-2006, 07:38 PM
did you ever get your Demon carb issues resolved?

Nope. Neither me nor the GM engineers ever came to a conclusion that it was or wasn't a Demon carb issue. We were getting ready to swap the demon and distributor out of their 572 test truck when my tranny let go. End of test session.............

Skip Fix
10-14-2006, 04:27 PM
You need to borrow a wide band for your cruise to see if its lean or richening when it does that. If the AFR is stable I would think it would rule out the carb.

dhardison
11-03-2006, 10:59 AM
Well I FINALLY have a resolution to my ZZ572 crate engine issues.

Last week GMPP sent my local GM dealership a new distributor and BG carb to swap on to my 572. So, yesterday I made the trip to the dealership to perform the swaps. On the way there I kept an eye on fuel pressure and the AFR. The fuel pressure held a steady 6psi, even under moderate throttle, but the AFR was around 10.8 at steady speed, showing an obvious rich condition. Just as before, when I would accelerate, the stumble would disappear and the AFR would hit ~13.5 for a few seconds before falling back to the mid-10s.

When I arrived at the dealer the service guy and I first started by swapping the distributor. We took the car for a test drive, but the surge/stumble was still present.

Next we swapped the carb. The 572 fired right up and sounded noticeably different, even at idle. We took the car for a test drive and the surge/stumble was all but gone. As the engine was brought up through the gears there was no longer any stumble/missing sound from the engine/exhaust. It sounded smooth, crisp and the engine was more responsive. The AFR was a steady 14.0 across the board showing the rich condition to be a thing of the past.

The new carb was a 95% improvement! :headbang: This was a surprise to me given the fact that I had already swapped in a brand new Holley 950HP, and it didn't help one bit. There is still a slight stumble at low rpm (below 1800rpm), but I think that can be attributed to the camshaft. The 572's camshaft definitely does NOT like being "lugged" down below 1800 rpm or so, but when cruising at 75mph (2000 rpm) there was NO stumble/surge felt at all. I think the remaining stumble can be reduced/eradicated with new plugs (current plugs were black as tar), light carb tuning and possibly a little steeper gear. It appears that my 3.42 gear may be a little low (numerically) for the 572's power band, a 3.73 or 4.11 might be in order. The remaining stumble would probably not even be noticed with an automatic transmission due to converter slippage, but with the direct drive of a manual it is noticeable at lower rpms.

Even though the carb did the trick, we decided to leave the new distributor installed. When we pulled the original distributor we noticed that the firing marks in the cap were on the extreme leading edge of each terminal, and appeared not to be firing properly (burn marks). After the drive home I was pleased to pull a light brown spark plug instead of the previous tar-black plug.

I still need to change the plugs, double-check the timing and put 25-30 miles on the new carb. Hopefully this (plus maybe a gear change) will completely take care of the issues. It's been a long and tedious four months, but I can finally see light at the end of the tunnel. Thanks to all that offered suggestions!

4MuscleMachines
11-03-2006, 12:36 PM
Congrats on finally getting it fixed. I would have thought GMPP would have sent you a measly carb much sooner than that to prevent you from having to go through all this crap of removing the engine, etc. Not many people would have gone through all of this and stayed with them until the very end. Again, congrats on getting er done!

Neil B
11-03-2006, 01:10 PM
"The fuel pressure held a steady 6psi, even under moderate throttle, but the AFR was around 10.8 at steady speed, showing an obvious lean condition. Just as before, when I would accelerate, the stumble would disappear and the AFR would hit ~13.5 for a few seconds before falling back to the mid-10s."


Just curious, your cruise AFR was mid-10's? That's very rich (not lean), correct?

dhardison
11-03-2006, 01:22 PM
Just curious, your cruise AFR was mid-10's? That's very rich (not lean), correct?Yep, you're correct, rich not lean. My bad. :pat:

Steve68
11-04-2006, 05:42 AM
What was wrong with the carb??? BG build a bad carb???

Skip Fix
11-04-2006, 08:10 AM
It can be alot of things that can do that. The idle circuit does kick in until over 3000 rpm by way of the transfer slots as well as the main jets.Eventhough you have the AFR great for idle it adds in on cruise along with the jets. Goose it a little(not enough to kick the power valve open) and the extra load drops the AFR. The "bad carb" may have some extra issues in those circuits.

My current Holley does the same thing 11.9 cruise, 14 goose it. 12.3-12.8 WOT. A Pontiac buddy has been adding replaceable jets in the tranistion circuit to help with that type issue. He has a Demon 850 of mine doing that right now.

Auto trannies give a whole new set of carb tuning issues off idle as you can't slip the clutch to get them through them.

Since you had a rich reading I assume you are using a wide band and not a narrow band meter. Which one?

dhardison
11-04-2006, 08:37 AM
What was wrong with the carb??? BG build a bad carb???Don't really know and at this point I don't really care. :) I'm just glad it's fixed!

dhardison
11-04-2006, 08:38 AM
Since you had a rich reading I assume you are using a wide band and not a narrow band meter. Which one? Innovate wideband LM-1

Skip Fix
11-04-2006, 05:05 PM
Same on I have. I've got the 5 channel AUX box that has a MAP so I can see vacuum at the same time.It's a GREAT unit.Helped me out on several cars and carbs.

Skip Fix
11-04-2006, 05:07 PM
Here's one of the logs of my recent 11.0 1/4 mile run.

Steve68
11-04-2006, 07:58 PM
I'm glad your glad!! but it's just strange, new carb with tons of problems,

Jims78elky
11-05-2006, 05:14 AM
Dan,

Awesome news buddy! Well at least your not going too have to
tear down more into the 572,what a long and pain in the rear
time you have had.Iam very happy you got all the probs worked
out on it,now maybe you can enjoy it a little before the big snow
hits real soon.

Now..i would like to see a EFI set-up on you ride next summer!:seizure:


Great news Dan!

:twothumbs


-Jim

Skip Fix
11-05-2006, 06:50 AM
From what I hear the BG carbs often have "issues" and their customer service is not always up to par. If you look Summit doesn't carry them anymore.

I personally have one that worked GREAT out of the box. Others have found metal shavings in them etc. I recently bought a remanufactured 850 from Jegs as a core for some major carb mods.Not sure if reman from BG or another source. Often manufacturers take parts that don't pass initial quality checks and reman them themselves. I expected a carb that had some issues that were corrected in the remanufacturing process. Just a quick check showed air bleeds one that was drilled off center and were different sizes. Took 2 minutes with some drill bits to see they were incorrect. No visible shavings anywhere.

My current new Holley 850 had no shavings in the bowls but did have them embedded in the gaskets. When I got my Q jet back from The Carb Shop from mods years ago it came with metal shavings in it.

So quality checks from alot of places have fallen off in recent years. I've had big issues with stuff from Comp Cams also. wrong SINGLE springs in the correct marked box for a set of roller cam springs, cam journal too big to fit in bearings. Most recent my Crower rocker shaft system for Pontiac Edelbrock heads was WAY off. I had to send it back and juggle offset rockers to get it right(I ate the shipping). I have heard BG is trying to improve in this area. You just have to double check everything for a good build.

LS6 Tommy
11-09-2006, 08:07 AM
Summit doesn't carry BG's anymore because Barry pulled the line from Summit. Summit was selling the carbs for less than the licensed BG dealers and wouldn't listen to reason about pricing. Barry's dealers were bitchin' up a storm.

Tommy

Tech @ BG
11-10-2006, 10:20 AM
Dan,

Glad to hear they got you fixed up. What dealer did it go through? Just want to try and get the carburetor back to determine what was causing the problem.

Technical Support,

Barry Grant, Inc.

www.barrygrant.com

dhardison
11-10-2006, 10:23 AM
Dan,

Glad to hear they got you fixed up. What dealer did it go through? Just want to try and get the carburetor back to determine what was causing the problem.

Technical Support,

Barry Grant, Inc.

www.barrygrant.com (http://www.barrygrant.com)

The dealer I went through was Roby Auto Group in Marysville, OH. However the dealership service manager has returned the carb back to GMPP. If you'd like my GMPP contact please send me PM.

Tech @ BG
11-10-2006, 10:28 AM
The dealer I went through was Roby Auto Group in Marysville, OH. However the dealership service manager has returned the carb back to GMPP. If you'd like my GMPP contact please send me PM.

You've got mail:twothumbs