That was going to be my next question.....how does it run with no air cleaner at all.

I do think you're onto something though, a lean condition will cause a stumble. I know on the dyno, we have an air system that dumps straight into the carb via a big duct hose. Taking that duct hose off and letting the carb suck dyno room air causes it to go a little richer.

The lid of that air cleaner may be causing a lean condition. If you wanted to try, you could jet up and see what happens. More air and more fuel = more power.