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Flathead Fever · 07-25-2010, 01:49 PM

A stock flathead will idle so smooth you can barely here it run. Some of the quietest smoothest running engines I have ever heard are flatheads. When they are right they are really impressive.

It sounds like the engine is running too lean. Closing the choke causes a higher vacuum under the choke plate which draws more fuel out of the bowl. This causes the engine to run richer which is why it is smoothing out. Check the float level, if it is too low it will cause all the other carburetor circuits to run lean. The float level is always checked first before making any other carburetor adjustments. Make sure the idle passages are not plugged by removing the screws (do not mix them up) shoot some carb spray through the holes while looking down the carb with the throttle open to see if it is spraying out the ports.

Lightly bottom out the A/F mixture screws and then back them out equal amounts, 1 or 1 1/2 turns as a starting point. I like to use the propane enrichment method for adjusting idle air fuel ratios. It was actually mandated by the State of California for smog inspection shops. Get yourself a little bottle of propane. Open it slightly and pass it over the carb. If the idle goes up the engine is too lean, if it goes down it is too rich. If the idle goes up with the propone, back both screws out another 1/4 turn and then pass the propane over the carb again. Keep evenly making adjustments until the propane has no effect on the idle speed. You can richen the mixture up a little more for all around driving. A carburetor will never run the same two days in a row do to atmospheric conditions so It is always a good idea to adjust it to the rich side.
If you cannot correct the idle A/F ratio with the screws then you have a vacuum leak, carb problem, timing problem or a mechanical problem.
The propane bottle is also good for finding vacuum leaks, run it around the base of the carb, throttle shaft and intake. The idle will change when the propane passes over the leak.
Dwell or point gap should be set first, followed by timing. Dwell effects timing but timing does not effect dwell that is why dwell is always set first. The idle air fuel mixture is always your final adjustment.

07-25-2010, 01:49 PM	#2
Flathead Fever Senior Member Join Date: May 2010 Location: Yucaipa, CA Posts: 1,094	Re: Tuning A Flathead A stock flathead will idle so smooth you can barely here it run. Some of the quietest smoothest running engines I have ever heard are flatheads. When they are right they are really impressive. It sounds like the engine is running too lean. Closing the choke causes a higher vacuum under the choke plate which draws more fuel out of the bowl. This causes the engine to run richer which is why it is smoothing out. Check the float level, if it is too low it will cause all the other carburetor circuits to run lean. The float level is always checked first before making any other carburetor adjustments. Make sure the idle passages are not plugged by removing the screws (do not mix them up) shoot some carb spray through the holes while looking down the carb with the throttle open to see if it is spraying out the ports. Lightly bottom out the A/F mixture screws and then back them out equal amounts, 1 or 1 1/2 turns as a starting point. I like to use the propane enrichment method for adjusting idle air fuel ratios. It was actually mandated by the State of California for smog inspection shops. Get yourself a little bottle of propane. Open it slightly and pass it over the carb. If the idle goes up the engine is too lean, if it goes down it is too rich. If the idle goes up with the propone, back both screws out another 1/4 turn and then pass the propane over the carb again. Keep evenly making adjustments until the propane has no effect on the idle speed. You can richen the mixture up a little more for all around driving. A carburetor will never run the same two days in a row do to atmospheric conditions so It is always a good idea to adjust it to the rich side. If you cannot correct the idle A/F ratio with the screws then you have a vacuum leak, carb problem, timing problem or a mechanical problem. The propane bottle is also good for finding vacuum leaks, run it around the base of the carb, throttle shaft and intake. The idle will change when the propane passes over the leak. Dwell or point gap should be set first, followed by timing. Dwell effects timing but timing does not effect dwell that is why dwell is always set first. The idle air fuel mixture is always your final adjustment.