so i have a 1985 f150 with a 300 and a 4 speed with overdrive. ive rebuilt the carb, changed the intake/exhaust manifold gasket, replaced the spark plugs, wires, cap and rotor, changed the oil (not like it would affect it), reset the timing to 10 degrees btdc, and made sure it has no vacumme leaks. after all this i cant figure out why it misses and shakes at idle. it also passed california smog with flying colors. anyway i gotta tow a car weighing in at 1900 pounds with the trailer from uhaul which im guessin weighs around 1000 pounds its the full trailer not the car dolly for about 900 miles round trip. im already worried enough that i got a 3.08 gear ratio with the 8.8 and the crappy overdrive 4 speed (really wish i had a granny tranny and 3.73s), but now im worried that it isnt running good enough to do the job. the truck has great power and anything after idle runs smooth as hell just backfires a bit when u let off the throttle but ide like to figure out y it runs kind of rough at idle it doesnt die or lose any rpms it just misses and shakes. the only thing i can think of now is maybe a computer problem but i dont really know much about how it works or what exactly it does on this truck i couldnt imagine it does that much being how its an 85. any help would be greatly appricaited.

 

Check your distributor shaft. Is that loose? Go over the vaccum lines again and again till you are certain that there are no leaks. Check your brake booster fro leaks especialy as that is a giant vaccum can. Your distributors vaccum advance may also be faulty. Do you still have all that EEC-III smog junk on it? Then you can add more headaches. Did you hook the vaccum to the distributor to ported or manifold vaccum? Is your choke releasing?

These are ideas. This is most likely a simple problom but you need to find it first. Good luck as about 15 things can cause this issue.