[Big Kev-O]

I had the same issues and thought it must be the HEI so I replaced my cheap MSD StreetFire HEI unit with the more expensive MSD Pro Billet HEI. I changed the plugs at the same time thinking they were fouled up from lack of ignition. Truck ran fine after the HEI & plug swap but the truck started hesitating and back firing from the tailpipe a couple days later. Turned out the intake manifold gasket was leaking and allowing air & oil to be sucked into a couple of the cylinders. I replaced the intake manifold gaskets and the problem was solved.

Turned out the intermittent hesitation and backfiring was caused when oil would foul the plug causing a cylinder to miss. The non-combusted fuel would then ignite in the exhaust causing a loud backfire out the tailpipe.

This is a common issue with vortec heads & performance intake manifolds. The paper gasket Edlebrock recommends does not seal well. The steel framed rubber gasket made by Fel-pro solved the issue.