Installed a new 351 Bill Blue cylinder yesterday. It momentarily fired off, backfired extremely loud, then lost all compression. Took it back down and found the valve keeps off on 3 valves. No damage done. It’s back together and about 70 miles on it. It has me a little on edge. Has anyone ever heard of this before?

 

No, but it would make sense that the backfire could entirely possibly push the valves back enough to have the keepers come off. It might just have gotten enough of a load of fuel for the misfire, I'd only be concerned if there is future backfiring.

 

Prior to the 351 bb, I did the mcm mod and deactivated the kacr by removing the spring. During the big bore install I set the timing back to Original timing but forgot to reinstall the KACR spring.
After much deliberation I now believe leaving the spring off added to this almost disaster.
It was reinstalled😎
and….6th gear is now stinking AMAZING!!