Idle Issues

T

Tom in Michigan

Junior Member
Good morning and Happy Easter! After an engine rebuild this winter our "30 A Sport Coupe is back on the road. Having issues with the idle speed and idle mixture. Prior to the engine rebuild the carburetor and idle was fine (put a new kit and gaskets in it a few years ago). Prior to reinstalling the carb on the engine I cleaned it with solvent and air. Specifically, I set the idle speed and turn the mixture 1 1/2 turns of the seat as a starting point. Engine idles fine at this point, but engine stalls when stopping the vehicle....it does this continuously, but will restart just fine. I have discovered that the idle mixture screw has little affect at any setting. I have checked for air leaks around the carb and manifold with ether and found none. I have put a plug in the manifold on the boss for the vacuum wiper motor to eliminate any leaks in that system. Engine has power and accelerates fine. Suggestions please.
 

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Sounds like a leak in the intake. Take the carb off and make sure the flange of the carb and the manifold are flat. If not file them until they clean up. If that does not fix it, I would look for a leak in the manifold or it's attachment to the block.
 
Generally your problem is as Mitch said.

But, what carburetor are you using ? Tillotson's are noted for warping the flange and body. Filing does usually fix them.
 
Do not assume your carb "is still good", anything sitting on a shelve can have issues. It seems small ants, fly's, etc. can crawl into the worst locations and cause issues. Also you could have had a little dirt that when the carb was taken off found it's way into a spot it should not be. Yes blowing out with air may help, however depending what is in a passageway, a simple blast of air may not fix it. If you have a Zenith and you have high humidity it could have formed rust in one of the small passageways during storage also. The small idle air adjust screw is set after the car is warm, you then retard the spark and get the car to idle as slow as possible, then adjust the screw. If you have any air leaks at all you will not be able to hear any difference as you adjust this screw. Because you state that the idle screw does nothing (assuming you are setting it with the proper procedure) I would say you have a air leak someplace, a common one is between the carb and the intake manifold, do you have vacuum wipers? Is the line good?, Intake manifold gasket is also another point of problem. You can use a common unlit propane torch and blow propane around all the possible leak points to test for a leak, with the engine at idle you will hear a RPM increase when you find the leak. This is not a OSHA approved leak detection method, but it works, just be careful.
 
He said it idled FINE, only stalls on a quick stop. This is quite common with Zeniths, that's why the '32 Zeniths had the float chamber re-designed. When stopping QUICK, jist pull the throttle lever down a few knotches! When in goofy traffic, I just leave the throttle lever set at that position.
Bill W.
 
Yes Zenith will cause the engine to die on quick stops. That is not what I understood in the original post as he states it happens on all stops, and he can not get the idle mix adjustment to make any changes. So I went down that path. It also could be the float level, by adjusting the float level to factory specs may help, on the non-hard stops.
 
Float level seems to be off to me. I had a repo float and adjusted by the red book. Ran fine but died on every quick stop. Adjusted it with the level tube device from the venders and cured the problem. Repo floats tend to be heavier, I'm told. that throws the adjustment off if you don't know the fuel level.Therefore the little tube and fitting tool. Lets you see the fuel level when the carb is full.
 
Tom don't feel so bad. I did the same thing and it was a wonder it even ran.

I also forgot to tighten the drain plug on the bottom of the carb. I couldn't figure out why as we were going along I had to keep the choke out all the time. Made it home with a big puddle of gas on the floor when I stopped.
 
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