70's Ford F100 truck idles at 25 mph!

302 V8. Autolite 2150 carb. Auto-tranny. My vintage Ford truck which I love has developed a problem where it idles too fast for too long after warm-up. What I mean is that it is normal when it works correctly for it to idle fast for a few minutes until it warms up, at which point the idle speed slows down to normal. The way it works when it is working correctly, after a few minutes to warm up, I step on the gas pedal briefly, and the idle speed to kicks down. That’s the way it is supposed to work. It seems actually like there are actually three idle speeds when it is working correctly. Fast, not quite as fast, and normal, all accessed via the kick down operation. Working correctly, it takes maybe three minutes to kick down the two steps to the normal idle speed. Does that sound right?

Anyway, the problem is that now it stays at the highest idle speed for 15 minutes. Eventually it kicks down to normal. But the high idle speed is too much for 15 minutes. Plus as it warms up the high idle speed becomes even higher since it doesn’t kick down like it should. So it becomes difficult to keep the speed below 25 mph even w/out pressing on the gas pedal. It idles at about 25 mph for 15 minutes as it warms up!

I looked at my Chilton’s manual – better than nothing, but barely – and it mentions something about a vacuum choke kickdown, a heat riser gadget, and the auto-choke.

Any ideas on how to test each one to find what the problem is?


I’ll bet the carb linkage is dirty and sticking. Try cleaning the outside of the carburetor with your favorite carb cleaner, and work the linkages to get the cleaner into the shaft bores. If you don’t have a favorite, let me suggest Gum-Out. Back in the day, when I had a car with a carb, and the throttle and choke was sticky, cleaning it this way fixed it. Less that $10 and less than 1/2 hour of trouble.

Remove the air cleaner. With the engine cold, overnight cold, open the throttle and the choke plate should snap shut. Sitting on the side of the carb connected to the choke shaft is a vacuum pull-off. Start the engine and it should pull the choke open about 1/3 of the way in a few seconds…The choke itself is probably electrically heated, look for a wire connected to the black plastic choke housing…In a few minutes the choke should open all the way and the idle speed drop to normal. There is a white plastic “fast idle cam” connected to the choke linkage that controls idle speed during warm-up. As the choke opens, the cam drops out of the way and idle speed returns to normal…

If the electric choke heater is burned out, finding a replacement may be difficult…Same with the pull-off…

I would look at replacing the manual choke spring controller. It is a disk shaped object that screws onto the side of the carb. Put in a manual choke control is an option.

If San Jose is home, the choke could be adjusted totally open and made virtually useless, patting the accelerator several times to start the engine and holding the engine speed up to 1,500 +/- rpms for 30 seconds would likely get things going. Fuel mileage would likely improve and engine life would be greatly extended by doing so. An old mechanic familiar with carburetors would have no trouble correcting the problem but there aren’t many left.