Idle Problem
 

Hi All,

Ive got a 81 F250 4.9L, T18 trans, and a 3.33:1 differential. Ive been having an idle problem for a few months now that I just can't figure out, so thanks in advance for the help.

When I start up the truck, after sitting on the gas for a second (I have no choke) the truck idles well. It continues to idle well until it warms up to around 160 F, then the engine starts to miss and shake. Once the truck fully warms up to 190 the idle returns to being smooth. After I'm driving around the city, staying in second and third gear, it continues to idle well when I come to a stop. However, after driving on the highway for a few miles and when I get off and come to a stop, the engine idles rough and starts to miss again. If I let it idle for a bit after taking it off the highway or drive for a few blocks, eventually, it returns to being smooth. The problem is consistent, but the truck runs great otherwise.

Here are a few more details: I have endlessly tried to adjust both the timing and idle mixture screw, but that doesn't seem to help the problem. Ive replaced all of the spark plugs, which still look clean and are gapped correctly. I've replaced the carb, spark plug wires, the coil, and the distributor (although maybe the timing curve is wrong?- I don't know enough). Ive got the vacuum advance hooked up to the vacuum port on the carb itself, not on the tree. Also know that I've deleted all smog components and have made sure to seal the potential vacuum leaks. I have made sure that all other hoses are sealed and not producing a vacuum leak. I get just above 20 inches of vacuum. The vacuum gauge needle, however, does move up and down slightly- within a quarter, maybe half, inch of vacuum- when it idling at its best. When it idles poorly, It moves a bit more and it drops in time with the misses.

Also, should the idle mixture screw be sealed in the carb? I preformed the starter fluid spray test and the idle jumped up when I sprayed the screw. Is this normal? The threads on the screw and the carb are in good condition. Could this be my problem?

Thanks again guys for any help. This problem has been driving me crazy! Let me know if you need anymore information.

Kirk

The screws should not leak. When you are spraying in that area, it's possible it's being sucked in somewhere close by, like the gasket area under the carb?

Does this problem happen summer or winter?

I live in San Francisco, so the summer temp and winter temps are pretty much the same. The problem happens in all temperatures, but I think it may be a bit less severe in warmer temps.

It almost sounds like carb icing. But that should not happen above 50 degrees or so. And it should be a lot worse in damp foggy or rainy weather when it's cool outside.