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Old 07-25-2010, 12:15 PM   #1
Joe KCMO
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Default Tuning A Flathead

I am working on a 24 stud flathead. The block has been machined and all new internals installed. The point gap has been set at 0.14". The description:

The engine starts instantly by just touching the starter button.
The engine pulls 21" of vacuum at a fast idle and runs very smooth at about half choke. Oil pressure is about 30" when warm and runs all day at 170 deg. The engine has what appears all the power you want after idle thru top end.

Problem(seeking perfection without diagnostics tools)

The engine I would describe has difficulty idling at low rpms which flatheads are famous for idling very smooth at idle. It seems to run best at full advance with the choke partially closed. Open the choke - poor idle, low vacuum. The mixture screws do not seem to make a difference unless all the way closed. With one closed still runs. Close them both she stalls.

My question is timing, carburetor or something else giving me a tough time idling here.

Last edited by Joe KCMO; 07-25-2010 at 03:12 PM.
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Old 07-25-2010, 01:49 PM   #2
Flathead Fever
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Default 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.
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Old 07-25-2010, 03:51 PM   #3
Russ/40
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Default Re: Tuning A Flathead

Thanks for your post. I have learned something from it. Never did know what the Propane enrichment process entailed until now. It sounds much simpler than I had envisioned.
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Old 07-25-2010, 05:01 PM   #4
Joe KCMO
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Default Re: Tuning A Flathead

The engine is 239 cu in with a stock 221 type carburetor. Could this be part of the problem.
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Old 07-25-2010, 07:03 PM   #5
G.M.
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Default Re: Tuning A Flathead

Sounds like a vacume leak. The shaft on the throatle plates or the bushing the shaft go's through wears and sucks air and the adjustment screws have no effect. G.M.
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Old 07-26-2010, 12:03 PM   #6
Bruce Lancaster
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Default Re: Tuning A Flathead

221's and 239's used the same carbs in a given year.
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Old 07-26-2010, 12:51 PM   #7
jdl
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Default Re: Tuning A Flathead

Great info from Flathead Fever...now I know what to do with those old propane bottles I have laying around.
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Old 07-26-2010, 02:03 PM   #8
fordy_nine
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Default Re: Tuning A Flathead

Thanks fr the info re: propane enrichment...Bob L
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Old 07-26-2010, 02:40 PM   #9
Mike in AZ
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Default Re: Tuning A Flathead

sounds like your idle circuit is blocked...or partially blocked....if screwing in the idle mixture screws doesn't make any difference....if they can both be screwed in completely and it doesn't die at idle, that is where i would look first for the problem...my best guess.... Mike
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Old 07-26-2010, 02:59 PM   #10
Joe KCMO
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Default Re: Tuning A Flathead

How would one describe the procedure clearing the idle circuit. blow air thru, soak, disassemble jets?
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Old 07-26-2010, 03:01 PM   #11
hbleadsled
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Default Re: Tuning A Flathead

thats good info on the propane method it seems like clean easy way to check for a leak. thanks for sharing. Harold central coast Ca.
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