Help with Backfire
.020 were milled off the heads. I did install new valve springs.
OK, I'd guess you used the Hughes 1110 springs. That pushing close to 2 times the open valve spring pressure with that cam's lift. And the peak valve lift is high enough to push the retainers into or very close to the stock valve guide tops. You have enough spring pressure and valve lift now that you can't be causal about this and take anything for granted.
The milling will drop the rocker cups down towards the lifter and take away some of the range of adjustment of the lifter's piston. (Depends on the head gasket too.) And with valve work, the valves may be set deeper in the heads and that will also rotate the pushrod cups towards the lifters and take away more of the range of adjustment in the lifter (if the builder did not 'tip' the valves to shorten them). Eventually, you can run out of adjustment range in the lifter; then the valve cannot close. That by itself would not push the pushrod through the cup and since you engine ran OK initially, that does not seem to be the issue. But all that possible stack-up of things shortening the pushrod area ought to be checked out, as has been said.
You may just have had 2 issues:
- A weak pocket broke out with the heavier springs, and/or hitting the ratainer on the guide.
- A valve seized in the guide, or a weak valve stem gave way with the higher spring pressures
