La318 cylinder head question
Which seems odd with the backfire through the carb, but I’ll see for sure once I get the head off.
Not snarky.
If a cylinder has a leaking exhaust valve, then;
On the intake stroke, with the carb at idle, that cylinder may find it easier to pull air in from the exhaust manifold, than from the plenum. So it idles rough cuz that cylinder does not have a combustible mixture in it.
If you whack the throttle open, then atmpspheric pressure, being higher than exhaust pressure for at least part of the induction cycle, stuffs some intake charge into the cylinder.
But The Log manifolds are now full of hot expanding gasses and not all of them have finished burning.
If the pressure in the log is higher than the pressure in the cylinder, then the fire can enter the cylinder with the bad valve, set that charge on fire before the intake closes, and Blam, you get a "backfire" in the intake.
If you have witness marks on the piston then yes the head has to come off, and the reason for it to be found and eliminated.
If you find witness marks in other cylinders, my guess is that your spring pressure is inadequate to keep the valve train together, and that the lifters pumped up, trying to do what they're supposed to do.
But if ONLY one valve hit a piston, then there could be other reasons, such as;
a bad lifter
a bad rod bearing,
A jumped timing chain,
A pushrod not sitting in the lifter-cup,
or, yes, a valve stuck open in the guide.