87 camaro iroc 350 TPI has been sitting for a while. Replaced the MAF and fuel filter as car would not start right. With new MAF starts without issue, but if I press on the gas, it starts hesitating and will not rev, almost sounds like its backfiring... but if I lightly press on gas pedal revs build like normal.

What could it be? I tested the fuel pressure and its about 30-35psi.