Hi Dave -
No. The fan was not running when the generator was running. Only during the flare.
The exception was right at the end. We turn the fan on before turning the generator off, so that flames don't shoot out the intake port of the gasifier. You can hear this right at the end, when it sounds like the generator is about to quit because the gas has become richer from turning the fan on. Then, after the generator is off and the valves are closed, we turn the fan off.
I will think about what you guys have said about moving the air mixing valve closer to the engine.
But honestly, I think you are on the right track with your diagnosis of valve issues.
When this generator was new, it started easier than any generator of this size I ever had.
But that didn't last more than a season. It then became the hardest generator to start.
The backfires and hard starting are routine with this generator, and the fact that it behaves similarly with woodgas tends to eliminate a carb. problem.
It's crazy how smoothly it runs once it's going though.