WOndering if anyone here can help. i bought an 86 F150 awhile back with the l6 300 with the 4speed stick. well it just started idling really high. when i first start it up it runs quiet and smooth and as it heats up its all down hill. its not overheating!!! normal opperating temp it just idles crazy. when its in gear it runs fine no smoking knocking or anything but once i push the clutch in or shift to neutral the idle goes way up and it sounds like it gonna ratle apart... Any sugestions? i have check all my vacume lines they all look good except this one copper or brass (not sure) tube that comes off the carb its about 12 inches long and broken at the end and i cannot find were it broke off of but it does suck in air.also i was told it could be intake manifold gasket which is on order and should be here soon

 

It's not the intake gasket, and that's a nightmare to replace anyway, so don't.

The broken tube is the choke heater tube, and it goes to a fitting on the exhaust manifold, but it doesn't really work even when it's working, so I'd just cap it off. It's supposed to pull fresh air from a breather in the air filter housing or on the top of the carb down thru a rubber (usually orange) hose to a steel fitting on the bottom of the ex.man., thru a thin steel tube that passes thru the ex.man. (but it always rusts & leaks exhaust), up thru that hard line to the choke housing to a manifold vacuum port built into the carb.

The high idle is most likely the gasket at the base of the carb, where it's held on by only 2 nuts. The gasket compresses over time, and the carb rocks back & forth, creating a vacuum leak. Use a mechanic's stethoscope (or 3' of garden hose) to listen around the base of the carb while the engine idles. Any vacuum leak will be a distinct hiss.

If it's not that, examine the throttle linkage carefully for binding, and listen to the throttle shaft - they wear and open up a vacuum leak. If it's that, either swap to an EFI engine (the best move) or buy a reman carb.

 

thanks alot. i replaced that carb gasket and also replaced the manifold gasket since i already had ordered it and it certainly was a pain. also capped off that peice of tube and all seems good now. thanks again for the advice

 

I don't recall what year they started but does yours have a throttle position sensor? If so try changing it out.

 

Why would he blindly swap parts? It's already fixed.