Paramotor: Short stop midair

In my last flight, I had two issues with my PAP Top 80: the engine died on takeoff and I experienced short, sudden stops mid-air.

I think I might need to richen the mixture. I started with 1.25 turns on the low screw, then increased it to around 1.5–1.6 turns, after which it worked fine/better. As I richened it, the idle RPM dropped slightly.

While in the air after about 20 minutes, the engine misfired three times at around 3/4 steady throttle. It would cut out for about half a second before running normally again. Is this normal, and what could be the cause?

My first guess is air in the system. I can see some small bubbles forming in the fuel line, in the curve above the carburetor—maybe around 1/4 of the diameter of the pipe.