If what you are seeing is gas spitting back out of the carb, it may not be flooding. Instead a valve may be sticking or set too close allowing combustion/compression pressure to leak back into the intake and out the carb.
If you don't know when it was last done, I recommend starting with a valve adjustment and decarboning. Pull the heads, intake and valve covers. Clean everything well, adjust the valves and put it back together.
Before starting that though, check the intake manifold for leaks. They sometimes leak air at the intake port or between the manifold halves. If so, checking before removal will allow you to fix that while it's apart.
Good luck, Onans are great engines!