Weird Truck Problem
 

This can be moved if it's in the wrong section.

My friend has a 1991 S10 5speed 4cyl. when it gets a load, it backfires through the intake, at WOT it sdounds like popcorn, backfires, and bucks badly. now we're repalced the EGR, fuel preassure diaphram, TPS sensor, wires, valve cover, checked springs and stuff visually. compression test came back bad on all four cylinders. sounds fine at idle. can this be a bent valve?

I'd start with the basics, vacuum leak, check the timing.

How many miles on it? Those Iron Dukes are pretty solid engines, really don't think it's a bent valve. How far off is the compression? Could it have jumped timing by a tooth?

well, timing is dead on because it drives fine aside from the popping at WOT or when theres a load. now it has the nylon timing gears, maybe it jumped a took or smething, we'll have to take a look.