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Old 04-04-2008, 08:03 AM
Sixtyninemercury Sixtyninemercury is offline
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Lean problems with BG Speed Demon carb

I just bought a new Demon carb - first time buyer always had Holley's & Edelbrock's - and these past two days I've been working and finding time to tune my new carb to perform well with my car. Crazy thing is that after about ten minutes of driving my A/F meter reads big-time lean! I have years of experience tuning my past Holley's and Edelbrocks but I couldn't figure out what was wrong with this new Demon carb. A call to BG's tech line gave me a question about what sort of manifold do I have. After I told the guy RPM Air-Gap he asked if I had checked to make sure that the two vacuum lines on the passenger side in the front of the carb were seated and sealed onto the manifold and not just only sandwiched on the gasket. I took off my Demon carb and noticed how the carb's vacuum lines passages on the underside of the carb where actually awfully close to the edge - unlike any Holley carb. Then I noticed that the top part of the manifold on the passenger side has less metal on that side making a "wall" than on the driver side "wall" of the manifold. Being that the manifold is aluminum and the Demon carb's base is too I guess that the BG tech guy is saying that maybe after the metals get hot they move which allows a "air-leak" into the carb down there giving me the very lean A/F meter readings after ten minutes of driving.

I've never heard of this before. The annular boosters in this 750 Speed Demon carb for low end throttle response are something else!!! I just ordered a 3/8" thick aluminum open spacer to cover up my manifold irregularity on the passenger side which is right under the Demon carb's vacuum line passageways. The carb spacers have the same amount of metal on both sides unlike my RPM Air-Gap. I'm hoping that will fix the problem.

I would think that two aluminum materials - the manifold and carb base would get bigger and would seal each other as the hotter they get! The telephoned tech guy at BG didn't seem too helpful. Maybe he thought that I was a carb beginner? He asked me what type of manifold I had and after I told him he said to make certain that the underside of the BG carb was completely covered by metal and not just the over-hanging gasket. I asked him about any other advice and he said that I'd have to start the carb over at it's baseline. Well the EZ-Idle screw I have bottomed out clockwise so it shouldn't be effecting the motor. All four of the idle mixture screws are turned 1.5 turns counterclockwise. Both float levels are good - visible through the huge glass sights. I had to richen up the jets to get it to run properly which fixed it - until the motor gets up to operating temperature. I haven't modified anything else on the carb. The accelerator circuit - untouched - seems to be working fine.

After about a minute of warming up the cold motor the car pulls away and runs very well. It idles well at the light and takes off from idle to cruise while accelerating well also. Then when the motor and carb get to operating temperature I can sit at the light and watch my A/F meter go from the green lights (where I like it) move to the yellow lights and then within say thirty seconds it works it's way into the lean red lights! (Please remember that if I switch back to my tuned Holley 750cfm VS carb it runs like I like it the entire time - so I know it's not my current motor but something on the BG carb.)

I wonder if when the BG carb's base heats up it is warping? Hopefully this weekend I can put the spacer on the manifold and try the Demon carb again. I will post my findings. Has anyone ever experienced something like this before?

Last edited by Sixtyninemercury; 04-04-2008 at 09:36 AM.
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