I have an 85 f-150 with a 302 and carb. the truck runs fine when the weather is warm, but when the temps drop to about 50F. the truck will still start fine and run ok for a few minutes, then slowly the RPMS will drop until the motor stalls. I can keep the engine running if I keep the RPM about 2000 but as soon as I let my foot off the gas the engine stalls. If I start the engine and begin to drive cold the truck will run as long as the revs. stay up, as soon as the engine slows for stops or turns it will stall no matter how long it has been running! I drove for 5 hours one time and the truck stalled as I came off the freeway. Also, I can tell when the truck is going to do this while I am driving because the gas pedal will feel stiffer than usual and when I let off the gas I hear and feel a "Clunk" sound. Again, I only have this problem in cold weather. Any ideas?
It sounds like the carb is not being heated enough. The racing guys cringe at the thought of heating the carb, but on the street in cold weather it is a must for driveablity. There are two systems the factory uses together to warm the carb. They have a heat passage that runs through the center of the intake manifold under the carb. Exhaust flows through this passage and it can sometimes get carboned up and block the flow. Most of the time you can put your hand on either side of the carb in the center of the manifold and it will be very hot(enough to burn the paint off).
The other system is the original aircleaner with a air control flapper in the snorkel. It draws warm air from a shroud around the exhaust manifold up into the carb intake. There is a thermostatically controlled vacuum valve mounted in the aircleaner which controls the flapper that keeps the incoming air around 100f.
Without these heating systems, frost can actually build-up around the throttle valves and make the throttle stick, and mess up the fuel metering of the carb. The vaporization of the fuel has a cooling effect, which is how when it is above freezing, frost can still build up inside the carb throat.
Ive seen alot of throttle plate shafts wear and cause the same problem. What happens is the throttle plate will close tighter when worn, causing the engine to stall, and sometimes they will close correctly.
thanks for all the ideas. I have a feeling my problem lies in one of the vacume lines or sensors. some of the lines are broken and missing. can the hard plastic vacume lines be replaced with soft rubber lines? this is all I can find at the auto shops.
do any of you California guys know how difficult it would be to put a newer/larger engine into a smog controlled vehicle? I would really like to have the power of a 460 but I don't know if it is worth the trouble to get it registered.