I put a new head on my 94 pickup. When the engine accelerates it hesitates a little. When the engine accelerates while the vechicle is on the road the problem becomes more severe. It hesitates bad and backfires. It's not a loud backfire, but you can hear it in the engine. It seems as if it's not getting the right spark or there is fuel problem. The new head is a little more high performance and included a different cam than the stock. I set valves at a different lash as instructed in camshaft installation. I put the new head & timing on because it blew a headgasket. The catalytic converter melted during the blown headgasket fiasco.