Idle speed too fast

[Gschwab]

FIXED

I just purchased a rebuilt zenith carb for my 1930 coupe, installed new intake and exhaust gaskets to very clean flat surfaces. The engine starts and runs really good but the idle speed is too fast, I have the idle speed screw backed out all the way. Even with the throttle linkage disconnected to make sure the throttle plate is fully closed I can't get the idle any slower.

[Jim Brierley]

Check all gaskets for leaks, could be it is sucking air from somewhere other than the carb. You also might remove the carb and look at the butterfly to see if it is closing as it should, and maybe try a different carb.

[Marshall V. Daut]

Separate the two halves, hold the upper half up to the sky or a strong light. With the throttle closed and the idle screw backed way off, you should not a see any light around the circular throttle plate, or if you do see light, it should be a consistent NARROW gap all the way around the circumference of the throttle plate. If one side shows more light than the other, the throttle plate is misaligned. If you do see too much light, then loosen the two throttle plate screws and wiggle the plate within its shaft until you reach a place where the light doesn't shine past the plate or it is consistently narrow all the way around. Of course, ideal would be no light. Once the plate is centered, tighten the screws and recheck that the plate didn't shift while tightening the screws. Sometimes when rebuilding the carburetors, guys don't spend enough time centering the throttle plate in its shaft.
Also, open the throttle plate all the way and examine the bevel on visible side of the plate. It has to close with the bevel sealing against the carb's throat. It's possible to put that plate in upside down so that it doesn't seal, allowing too much air to pass through the throat. This will also affect the idle.
Marshall

[Rob Doe]

X2 all the above.

Don't forget the wiper connections all the way to the wiper motor. It's gotta be getting air from somewhere???

[johnbuckley]

All of above. Also 1) Sometimes the idle air mix screw does not seal very well- the tapered end of the screw gets worn as does the the orifice in the carb where it seats. ... that could be making the idle mix weak and 2) check the spring and cap of the idle mix screw are not preventing it from being screwed in sufficiently ...( I have had to shorten these on several carb rebuilds)

[Gschwab]

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Thank You all,
Sure enough the throttle plate was off center and not closing. She runs and idles great now!

Thanks again for the help

[Marshall V. Daut]

Marshall

[GPierce]

Originally Posted by Marshall V. Daut

Separate the two halves, hold the upper half up to the sky or a strong light. With the throttle closed and the idle screw backed way off, you should not a see any light around the circular throttle plate, or if you do see light, it should be a consistent NARROW gap all the way around the circumference of the throttle plate. If one side shows more light than the other, the throttle plate is misaligned. If you do see too much light, then loosen the two throttle plate screws and wiggle the plate within its shaft until you reach a place where the light doesn't shine past the plate or it is consistently narrow all the way around. Of course, ideal would be no light. Once the plate is centered, tighten the screws and recheck that the plate didn't shift while tightening the screws. Sometimes when rebuilding the carburetors, guys don't spend enough time centering the throttle plate in its shaft.
Also, open the throttle plate all the way and examine the bevel on visible side of the plate. It has to close with the bevel sealing against the carb's throat. It's possible to put that plate in upside down so that it doesn't seal, allowing too much air to pass through the throat. This will also affect the idle.
Marshall

Great post. I learned a lot from it. I’ve got one that idles too fast. Never occurred to me to me the throttle plate could be misaligned.

[katy]

Originally Posted by Gschwab

Sure enough the throttle plate was off center and not closing. She runs and idles great now! Thanks again for the help

Doesn't say good things for the rebuilder.