carb help
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Senior User
Joined: Jul 2010
Posts: 272
Likes: 0
From: Parksville BC Canada
carb help
recently me and my dad got our old '76 f100 started after sitting for 12 years, it runs great dosnt smoke or anything, but it wont idle. i cleaned out the carb and now its getting gas but just wont idle. i was wondering if the mixture/ idle screw was set wrong and i cant find any info on were to set them. the motor is a stock 302 with what i think is a stock motorcraft, im not sure if '76's had electronic choke
Thanks
Thanks
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
How was the carburetor cleaned out? Often times, if it was a simple spray-down, this creates more problems than originally were there. The proper way to clean a carburetor is to fully disassemble it, then soak the metal parts in chemical solution, spray them down, and blow it all the internal passages with compressed air. If this wasn't done, then you could be chasing your tail solving the problem. Spraying down a carburetor can just move varnish from one area to another (like an internal passage).
If it was rebuilt properly, and it's still having issues, then poor idle could be a vacuum leak or incorrect mixture. To eliminate the mixture issue, with the truck off, gently run both mixture screws all the way in, then back out 2 turns each. This will be in the ball park. To dial it in, you can use a vacuum gauge. If it still won't idle, then does it only happen when the engine is cold? Can you get it to run long enough to warm up, and if so, does it still happen once the engine is warm?
If it was rebuilt properly, and it's still having issues, then poor idle could be a vacuum leak or incorrect mixture. To eliminate the mixture issue, with the truck off, gently run both mixture screws all the way in, then back out 2 turns each. This will be in the ball park. To dial it in, you can use a vacuum gauge. If it still won't idle, then does it only happen when the engine is cold? Can you get it to run long enough to warm up, and if so, does it still happen once the engine is warm?
Thread Starter
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Senior User
Joined: Jul 2010
Posts: 272
Likes: 0
From: Parksville BC Canada
i took the carb apart and let it soak and i blew out all the passages, and i can let the engine get warm buy holding the gas down a bit and it still wont idle. but im pretty sure i have a vacum leak some were though
Senior User
Joined: May 2011
Posts: 207
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From: Erin, TN
If the truck was sitting for that long without being started, chances are you have a lot of cracked/dry-rotting vacuum lines in the engine bay. I would start by replacing the vac. lines one by one (so as not to forget which one you're replacing - maybe it's just me but sometimes I forget what goes where 
) until you've got them licked then give it another go and see if it will idle.
) until you've got them licked then give it another go and see if it will idle.
