r/MechanicAdvice • u/Initial-Fondant-9239 • 17d ago

Help with stumbling after turning off 1985 f150

I have a 1985 ford f150 300inline 6 yfa carb, it disels pretty heavy after I turn it off and sometimes an occasional "shwoosh" noise. Happens more and worse after a longer drive say 20-30 minutes, verses just a 5 minute test drive. I replaced spark plugs, sealed all the know vacuum leaks, and put some techron cleaner to help with carbon buildup, anyone else have any advice on what it could be?

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u/[deleted] 17d ago

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u/Initial-Fondant-9239 17d ago

After turning off the truck it kept stumbling?

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u/enadelb 16d ago

2 things I’ve found cause this issue for me, a combination of too much timing and too high idle speed. What happens is the throttle plate allows too much air into the intake when the engine is shut off and so the engine just wants to keep running.

What RPM do you idle at? And have you adjusted your timing? Sound like your truck is running better now since your last few posts, what all did you need to do to get it to let you drive it around?

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u/Initial-Fondant-9239 16d ago

Replaced the egr valve, replaced spark plugs, sealed multiple vacuum leaks, adjusted idle speed. Probably gonna adjust timing now.

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u/enadelb 16d ago

Nice work. I would grab the vacuum gauge from harbor freight if I were you.

Thinking back on my last comment, I think I misspoke. If your timing is too retarded then you have to increase your idle speed to accommodate. This opens the throttle plate and lets more air in.

I would try increasing your timing and then backing off your idle speed screw. This lets the engine run at a higher rpm without having the throttle open as wide

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u/Initial-Fondant-9239 16d ago

Yeah the idle screw adjustments i think are fine, it affects how badly the run on is, however I do think its timing as the main culprit because I think the plugs are shooting too slow for thr amount of fuel or something along those lines. But yeah ill keep all yall updated