I have a brand new crate GMC 350 I just put in my 1979 GMC 4WD Pickup. The first time I started it I adjusted the valves and it ran great. It was late and my baby's bedtime, so I shut it down immediately after the adjusting. The next morning I started it back up and it was backfiring terribly out of the exhaust. I pulled the #1 cylinder spark plug and it stopped, so I new it was that one. I messed with the exhaust valve, but the only thing that stopped the backfiring was loosening the rocker all the way to bad tapping, or oddly, at correct adjustment when it was backfiringand I tightened it a little extra it would stop but only for a few seconds before it started backfiring again. I could then tighten it a quarter turn again, then it would do the same thing:stop backfiring and then a few seconds later, start the backfiring again. I can't think of what is wrong. Please help!

 

Is the rocker lose when you tighten it or is just as tight as you left it? Do you have the right torque specks?

 

No, the rocker actually tightens down; it doesn't stay loose. Yes, I torque it to the right specs, but still backfires out the exhaust. If I tighten it to where it just stops tapping, it backfires. If I tighten it a quarter turn, it then stops backfiring but after a few seconds starts backfiring again. If I then tighten it another quarter turn, the same exact thing happens. That actually happens all the way up to torque specs (and a quarter turn farther as well). Hope that makes sense...