I had the exact same thing...

I did two modifications at the same time so I'm not sure which was the fix.

#1 mod: I covered the 1 inch hole on the firewall directly opposite to the carb intake with thin balsa. (solution to airflow problem as it enters carb normal versus inverted flight)

#2 mod: I soldered a 1/2 inch long copper pipe (I believe it was 1/8 inch size?) onto the carb plate where that little breather hole is, put a piece of glow fuel line on it and put it through a little hole in the thin balsa to the fuselage. (solution to airflow problem which leads to changes of air pressure inside the cowl for normal versus inverted flight. Instead you get a more steady air pressure senced) This is similar to the "balsa box" mod that many have tried successfully. For me, that mod did not work.

Also, you will find that after break-in the engine will be little more smooth and generate more power. My engine peaked at 7000 rpm using a bambula 22-8 at around 5 gallons using 100:1 synthetic. (all this in Calgary at around 4000 feet above sea level) Inexplicably my rpm has dropped to 6800 rpm after richening it to about 80:1 and it hasn't improved since going back to 100:1.

Any thoughts folks? Spark plug perhaps?