Cold start thermal switch?
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Cross-Country
Joined: May 2015
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Cold start thermal switch?
400 in my 77 f150. On cold starts, push the pedal once, it starts right up but idles around 600 on the fast idle cam. If I try to mess with it I have to fight with it till its warm enough. If I start it and just let it idle for about 10min, its like someone flips a switch, the idle jumps up and it runs how it should. I've been checking everything I can think of. Choke, pull off, timing, mixture ect but I don't see how it can be any of that since its such an instant change even though I touched nothing. The only thermal switch I'm aware of is on the water neck and afaik its for the egr and switching vacuum source for the advance. Lately I've just been starting it and walking away until I hear the rpm jump but that's not much of a solution
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Cross-Country
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That video isn't a motorcraft carb but I liked his method for setting the choke cap. My repop cap doesn't even have a mark on it. I should shoot a video so you guys can hear what I'm talking about
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Joined: Dec 2019
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From: Colorado
Maybe check your timing on a cold start, my 1978 400 has full vacuum advance on a cold start until it reaches temp and a temperature activated vacuum switch changes state, then it will run at base timing at idle. With full vacuum advance, mine will run a couple three hundred RPM faster, when the vacuum switch changes state it drops down to set RPM at idle. If the vacuum advance diaphragm is shot, it won't advance and also cause a vacuum leak.
Also there may be vacuum tree on the back side of the intake (follow the brake booster vacuum line), there are probably 3 or more unused vacuum ports. Make sure the rubber caps on them are not rotted away causing vacuum leaks.
Also there may be vacuum tree on the back side of the intake (follow the brake booster vacuum line), there are probably 3 or more unused vacuum ports. Make sure the rubber caps on them are not rotted away causing vacuum leaks.
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