BG Speed Demon 850 problems - carb experts please HELP

[Matt@RFR]

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]

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]

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.

[Matt@RFR]

Leave everything the same, but go up 10 jet sizes front and rear. This sucks long distance.

[dhardison]

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. 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]

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]

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.

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..

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

[dhardison]

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

LOL - I'm not dicking, I'm listening! 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]

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.

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]

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]

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/globa...ft-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]

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]

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]

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]

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]

Trying to, going to tow the Nova, run like hell, were getting busy launching again, we'll see what happens

[cad]

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]

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.

Hang in there!

[Skip Fix]

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]

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..................