Flathead V8 idle problem
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Flathead V8 idle problem
I have a '50 F5 with a fresh rebuild. Before the rebuild, I had a "surging" idle once the engine was warm. After the rebuild, it is still there. It cycles from normal idle speed to slow, normal to slow, every five seconds or so until it finally dies. Any speed other than idle it runs great. Fiddling with the idle mixture adjustment screws helps, but only for a few minutes and then they have to be readjusted. Any suggestions?
Fleet Owner
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From: NM
No, it just dumps gas into the engine without going thru the proper jets etc. The bottom side of it has a passage to intake manifold vacuum and if it leaks, you get a very rich mixture.
Try taking the carb off, and take the upper body off it. Fill the bowl with gas and leave it overnight with the lid on loosely. If the gas has leaked out, there ya go.
Are you using the stock fuel pump or electric?
Try taking the carb off, and take the upper body off it. Fill the bowl with gas and leave it overnight with the lid on loosely. If the gas has leaked out, there ya go.
Are you using the stock fuel pump or electric?
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It has a 6 volt pump. I have noticed, after driving several miles, a "whoosh" of air when I take off the tank cap. I'm so used to it on my modern cars I haven't been concerned; but, there should never be any vacuum in this age truck tank, right? If that was a factor, though, wouldn't it affect it at all engine speeds? I do lose fuel out the bowl, but it has always appeared to be leaking from the jet access plug. Maybe it is leaking at the power valve, too.
Fleet Owner
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From: NM
Do you have a pressure regulator on the line to the carb? Holley 94's don't like high pressure at all, 2 - 2.5 psi is the most. Holley sells a very nice regulator just for older carb'd vehicles, model 12-804. Autozone sells it as do Summit, etc. Holley Fuel Pressure Regulators 12-804 - SummitRacing.com
No, you shouldn't hear any "whoosh" at all. It may be pressure (not vacuum) in which case it adds pressure to whatever the pump puts out.
No, you shouldn't hear any "whoosh" at all. It may be pressure (not vacuum) in which case it adds pressure to whatever the pump puts out.
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Post Fiend
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From: Island Southeast Alaska
Try running with the fuel cap loose and see what she does. Your cap should be vented and sounds like this one is not.
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Tuned
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Check the plugs to see if it's running lean or rich.
Also check the throttle shaft, as it can be worn allowing air to leak by the shaft and cast-iron body. As the CI heats up gap gets larger, hence lean when warm.
If it's loose, you can solder the brass shaft and sand it back within spec. Many carbs get rebuilt, but not many have the shaft tolerances checked or rely on felt or rubber seal.
Also check the throttle shaft, as it can be worn allowing air to leak by the shaft and cast-iron body. As the CI heats up gap gets larger, hence lean when warm.
If it's loose, you can solder the brass shaft and sand it back within spec. Many carbs get rebuilt, but not many have the shaft tolerances checked or rely on felt or rubber seal.
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Thank you ALBUQ F-1! I've tried many things (found out the "vented" aftermarket gas cap was not vented at all; I put my rusty but functional original back on) but the fuel pressure regulator nailed it. I had to readjust the float level higher (a sure sign it was overfilling before) and not only is it idling beautifully, I am seeing an increase in acceleration in top gear and the gas mileage is now over 10mpg where it had been down around 7.5. Even though the plugs looked OK, it had to have been running rich. Thanks again for the suggestion. This is a great forum.
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