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07-19-2006 #1
BG Speed Demon 850 problems - carb experts please HELP
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!
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07-19-2006 #2
Running lean!!
Sounds like it's either too lean or need to change the power valve to a lower vacuum!!
MarkAbility is not being able to do something once or twice.Ability is being able to perform upon demand!!
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07-19-2006 #3
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?-Matt
Welders: The only people that think a co-worker catching on fire is funny.
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07-19-2006 #4
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/fram...ckV8s/572.html
Thanks!
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07-19-2006 #5Originally Posted by dhardison
-Matt
Welders: The only people that think a co-worker catching on fire is funny.
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07-20-2006 #6I 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/sh...ad.php?t=20927
If you have any suggesstions I'm all ears!
07-20-2006 #7What do the plugs look like?
-Matt
Welders: The only people that think a co-worker catching on fire is funny.
07-20-2006 #8
07-20-2006 #9Plugging 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.-Matt
Welders: The only people that think a co-worker catching on fire is funny.
07-20-2006 #10a 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.
07-20-2006 #11Originally Posted by Matt@Lateral Dynamics
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?
07-20-2006 #12
07-20-2006 #13When 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.
-Matt
Welders: The only people that think a co-worker catching on fire is funny.
07-21-2006 #14I 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 #15I 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.-Matt
Welders: The only people that think a co-worker catching on fire is funny.
07-21-2006 #16I 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'.
07-21-2006 #17I 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.-Matt
Welders: The only people that think a co-worker catching on fire is funny.
07-21-2006 #18Originally Posted by dhardison
If I wasnt working on that dang barn I would come watch. You going next weekend too??
07-21-2006 #19
07-21-2006 #20