Floats tend to stick most right after the carb has been dry.
Did you have the cams out or the timing chains off? Anything to do with the valve timing or mixing the cams up?
A compression test would show if it's compressing with intake valves open. You just get a low reading, if any. An air test would define it without a doubt, similar to a leakdown test, just without gauge readings.
You really can't alter the ignition timing any, not without re-engineering the pickup coils or the electronics. But ignition timing wouldn't send air back out the intake.
Intake valve stuck open is a possibility, but a huge stroke of bad luck to have it on two cylinders. All the exhaust mixes inside the goat belly, so just checking tailpipes isn't a good indicator of which cylinder has the problem.
With gas pouring out of the pipes, I doubt it could backfire on the flooded cylinder. Not saying it can't, but it's a lot less likely, and I'd want a fire extinguisher close just in case.
Mine would always start up on one cylinder, the idle was down by almost half when it did.
Last edited by Spockster; 10-18-2017 at 10:22 PM.