Hello,

I recently replaced head gaskets, valve stem seals, intake gaskets, fuel pump and filter, distributor cap and rotor, and now having backfiring problems. 40-60mph is 3rd gear overdrive with low RPMs and moderate throttle. I've adjusted the valves 4 times now. I can now hear them clapping so I'm starting to move away from that. Any ideas?

Also, codes say rich mixture in both banks. Chevy dealer troubleshot and can't figure it out. All sensors and tests showed normal.

thanks