Speed Demon 850 Carb high idle

[193rdsow]

Carb expert needed- I have a new Speed Demon 850 carb on a newly built 440 stroked to 500". It's a 4 speed car. I am running a comp extreme energy 545 lift flat tapper hydralic cam. The engine has the Edelbrock performer RPM heads and intake. I just broke in the engine a couple of days ago.

The problem iI am having is getting the engine to idle below 1200 rpm. It runs fine above that rpm. When I attempt to back off the idle set screw the engine dies below 1200 rpm. The idle mixtures are out 1 3/4 turns and floats are set at top line.

I attempted to to back out the idle eze screw but that has no affect.

Anyone else have a similar experience or suggestions? I called tech support and they weren't much help other than to suggest drilling out the butterfly valve which I am a little reluctant to do to a new carb.

All advice appreciated!

 

[67 coronet]

first,check for vacuum leaks.sounds like you might have one.double check your timming as well.is carb new,or did it sit for a while?

 

[cwhubb]

I had a similar problem with a holley 950hp, it was a vacuum leak, the phenolic spacer had a small crack in it

 

[193rdsow]

Pretty sure there is no issue with vacuum leak. If I hold my Hands over the carb the motor stalls.

 

[Meep-Meep]

If you are sure of no vacuum leaks then it's possible the idle feed restrictors in the metering block are clogged. Maybe remove the idle mixture screws and blast some carb cleaner in there. Also might blow air through the idle air bleeds and all the idle specific passages. If you still have an issue then removing the metering block for further cleaning will be required. I use an ultrasonic cleaner to clean my carbs and it works quite well for loosening debris where you can't otherwise see. I know you have a new carb but don't assume because it's new there isn't a problem.

I just thought of another trick that has worked for me in the past. Before you do any of the above rev the engine hard then slam the choke valve closed. Sometimes the vacuum will pull crud out.

 

[67 coronet]

common stuff at that point would be to check firing order(recheck if done already).look for poss ignition drop of a cylinder.sometimes not noticable at higher rpm.is valve train adjustable?check that as well.