i have a 1988 jeep comanche when i put the gas all the way to the floor it runs like a bat out of hell but if i try to go a steady speed like 40or50or even 30 it spits spudders and backfires

What is your question?

i just put a used motor in have checked timeing rep plugs wires cap and button changed o2 sen tpi .

What makes this problem better or worse?

really putting your foot down makes it run a lot better for a few mn

How long has your 1990 Jeep Comanche had this problem?

since i put this motor in(this is the 2nd motor first ran great but had a rod tapping)