The problem has been going on for a long time. Mostly it is just an occasional misfire and a puffing sound come from a cylinder. I did a compression test and all came back mostly even around 115psi hot. I added oil and got around 120psi for all of them cold. I replaced the valve guide seals and that helped a little bit. The car also loves to stall when put in gear and when coming off idle.
This all started after easter, it ran fine then, the next day it was backfiring like crazy and would barely move. If I retard the timing the cylinder stops puffing but it won't run above idle. When I clean the spark plugs or put new ones in it runs great with crisp throttle response until I shut it down. It is a 400 with a 670cfm holley and headers. I have replaced the cap, rotor, plugs, coil, wires, put fresh fuel in it, used seafoam, MMO, and no vacuum leaks.
