Well, admittedly any number of things could be wrong, and out of morbid curiosity I'd like to know this particular engine's problem and what caused it, but I strongly suspect it'll be time-consuming to repair, and unfortunately, I'm the repair guy.
And actually, even the repair wouldn't be all that bad, except that it's a marine application engine, and there's plenty of rust on more than a few fasteners.
Anyway, as for the problem at hand:
The primary issue is the fact we have no compression whatsoever. A hole in the piston is possible, but unlikely. The engine was running fine when it was put away, and I've been present for virtually all attempts to start it this year. There have been no catastrophic events that might have led to holing a piston.
That means the valves- IE, one or the other valve is not closing. I strongly suspect it's the exhaust, as it sticking open would- theoretically- produce the popping back up the carb, as the intake opens and exhaust from the manifolds is blown through.
However, if that's true- at this point it's only a Sherlockian logical deduction- then why would two cylinders suffer the same fate?
If it were one cylinder, I'd suspect a broken valve spring. Two failing simultaneously is statistically highly unlikely. That suggests an external cause, and the water was certainly such a thing.
Except I can't see the water rusting a valve so badly it sticks open, but not leaving behind any detectable trace (as in, I saw no evidence of rust or residue on the plugs or in any of the expelled water vapor.)
Also, it's certainly possible it's the intake valve(s) instead- that'd also give us the pressure signal to blow fuel droplets out the carb, and the popping as the exhaust valve opens too.
However, I can virtually guarantee there wasn't any water in the intake, let alone enough to rust a valve open.
A flat cam lobe, collapsed lifter, bent pushrod or broken rocker would keep the valve from *opening*, not prevent it from closing. A broken valvespring would keep it from closing, or at least closing well, but again- the same failure on two cylinders at the same time is highly unlikely. Not impossible, but unlikely.
If neither valve were opening, there'd still be a pressure signal on the compression gauge- at least enough to bump the needle. I get nothing- the needle doesn't budge.
It's an odd one all right. If I learn more, I'll give an update.