Interesting point, we were just playing with this on the hangout last night. I had the gasifier warmed up and we were trying various startup settings. With the gasifier wide open and the air valves flooded I was able to crank up just fine - but only with the air filter removed. It seems that the slight drag from the air filter makes the motor pull from the woodgas instead, flooding it. This is more or less what you're saying.
I then tried closing down the gasifier without much luck - it fired but immediately stalled. I think with the gasifier closed down, there's a good mixture for cranking but there's not enough gas to support actually running.
I think the best startup results are going to be a dedicated airtight valve with no filter, that can be closed off while actually running. This in addition to the normal filtered air to mix with gas for normal running.
As far as the woodgas backing up into the air filters, that is a problem because it will eventually soot the inside of the filters, and they will clog prematurely. Peter should be OK because his valves are air tight, and he can close them off during shutdown.