Rough Idle/Stalling

T

Theo

Senior Member
Hi Guys. I have been having some problems with my model a this week. It is having trouble idling. It keeps stalling out over an over again. It will start then idle real nice for about 20 seconds then it will randomly stall out. I have located the mixture adjustment on the carb, but could not find the idle. Is there a key to fine tuning the carb so it idles ok? I know everything else is fine, so it looks like its the carb. Thanks guys
 
Not quite sure what you mean by ' could not find the idle'.. What does that mean? You can find the idle mixture screw, idle speed screw ? The speed screw is a stop screw on the carburetor housing that limits the throttle shaft closing, just follow the linkage to the carb throttle shaft.. The idle mix screw is on the carb throat[it should have a little spring wrapped around it] and may be brass. Turn this screw in to very gently seat it and back it out 1 1/2 turns.. The choke and high speed mixture is the rod that passes thru the firewall into the cab.. Gently seat that rod by turning it CW and then back it out 1/2 turn.. Once the engine has been warmed a bit, adjust the idle speed, adjust the idle mix speed screw 1/8 turn at a time CW and CCW until the highest speed is noticed and the engine idles the smoothest, readjust the idle speed screw..
 
My on and off battle with a smooth idle came down to perserverence like all my issues with the coupe. It was a combo of fixes.
- leaky manifold gasket, changed it
- dist advance not touching from side to side, rotated steering column
- just getting lucky with the air screw mixture adjustment and drilling out the carb jets to the correct specs, they were way off
- correct float height, 2 clear neoprene gaskets and a new vitron float valve
She curb idles at 425 knock on wood
 
If you can't find the idle screw, then my guess would be that you don't have one?? It is easy to see as part of the throttle linkage (the same part where your throttle linkate attaches to the carb. I'm bettin that might cause the car's idle RPMs to be way too low..
 
See Bratton's 2011 parts catalog page 62 for an exploded view of the carb. #11 is the IDLE SPEED ADJUSTMENT SCREW. This screw should be turned in enough to keep the throttle plate from binding in the bore, and enough to let the engine idle at about 500 RPM.

#5 is the IDLE MIXTURE SCREW, and this screw controls the amount of AIR that mixes with the gas at idle. This is opposite how most other carbs operate. Most other carbs use this screw to control the flow of gas at idle. So, on the Model A turning this screw OUT will admit more AIR and make the mixture LEANER.
 
I found the other screw to adjust the idle speed. I am going to try adjusting the two when I put my new radiator this weekend. Hopefully that fixes it or I guess I will be taking off the carb.
 
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