Timing or Carberator

Hello,
My son just purchased his first vehicle, a 1986 Toyota Pickup, 2.4 liter. The truck has been running good for the most part, except for dieseling just a little.
Yesterday, he moved the truck about 10 feet, and left it there for a few hours. Later he moved it back about 10 feet. It started up fine, and shut off fine. Late yesterday afternoon when he went to start it, it wouldn't start. The engine is turning over, and it sounds like it wants to start but it won't.
We checked all the plugs and distributor and everything is firing. We checked the carberator, and there is gas coming up into the carberator as it should. The plugs are black and wet, but do fire.
There were no unusual noises (knocks, bangs, squeels, etc..) the last time the truck was running.
We did not find any vacuum leaks. The motor oil and tranny oil was just changed, along with filters.
Oh, besides dieseling the truck did seem to idle a little fast, and we checked the gas millage, and it is only getting 18 mpg, not good at all, especially for a 4 cylendar.
We are thinking that the problem may be the timing belt/chain???? or carberator.
Please help. Thank you.