I have been fighting a miss/backfire out the exhaust for 3 weeks now. It's on my 351 Cleveland in my 1955 tbird. It was freshened up with rings and a reground crank before I installed it.Heads gone through too. I even tried a new distributor and coil, it got a new 3310 Holley when I installed the engine.Checked the fuel pressure, timing. Then it was getting much worse, and I could hear the lifter rattling. So I removed the valve covers and found the number 4 exhaust valve rocker arm was very loose. I installed the elgin cam and the lifters that came in the kit. I drained the old oil and filter, valvoline vr1 10w40. I coated the elgin cam and lifters with the assembly lube that came with the cam. Ran the engine 20 minutes like the instructions said. It's a mild cam , 204&214 duration, .485&.510 lift. I even used the correct springs.so what is going on with this engine??? I have bought a new Crower cam and lifters kit, I sure don't want to go through this again!