F250 acceleration problems

I have a 1989 F250 which has been parked for a long time. It will start and idles fine. It will not accelerate either under load or in neutral without bogging to a stop. Both sensors on throttle body are fine, (switched with known good sensors). Also will backfire through intake under load. Exhaust seems to flow ok and there are no obstructions in intake. Any ideas? TIA

I don't know if this will work but my truck is having the same problem and i know an automachinic and he told me to change the gas filter then check to make sure that the vaccum lines hooked up to the egr valve and map sensor have good suction through them when the truck is running.

check the timing. i'd go ahead and replace the plugs/wires/cap/rotor, fuel filter. these are cheap and wear out. check for vacuum leaks around the upper intake plenum gasket, carb cleaner or wd-40 works good just spray it around while running the idle will change if you find one. if this doesn't cure it check all the sensors with a scan tool cts(coolant temp sensor) and map(manifold absolute pressure) usually causes the most problems i assume you swapped out the tps, did you adjust it?. a compression check should also be done.

Take a fuel pressure check, usually backfiring is a condition on the engine not getting fuel. Timing could be the culprit too. Change the fuel filter, run some seafoam injector cleaner through it, mix the seafoam with about 5 gallons of gas and it will clean that engine out in a hurry. Check the MAP sensor, thats pretty much how the engine sees how much throttle you are giving it. Becuase if there is a bad vacume leak, it will think you are still at idle, and will not richen the mixture. Also is the check engine light on?

I have '89 250 w/ 351 W been fighting this same prob. about 4 weeks now and SEAFOAM is the way I went! So far so good! Thank you 97chevyman it is improving every time I drive it and . I save alot of money by trying that first . LOL thanks I will let you know how it turns out in a day or two or 300 miles which every comes first.