I have a 1998 chevy c3500 with the 7.4 liter

Have a 1996 Chevy 3500 truck with a 350 vortex motor. Truck started running rough and doesn't have too much power. Have changed about all the emission parts along with electrical parts that book can mention plus checked fuel pressure but nothing seems to help. Cut into catletic convertors because of broken catlis inside but only one convertor had anything inside. The other one was totally empty but no evidence that it had been cut into before. Don't know what else to try.

You can't change the timing in this engine nor can you adjust the valves. The timing is adjusted throught the computer therefore there is no mechanical way to change this. You might be able to with some kind of programmer or computer program.