Perry pumps are not that great on the BGX. These engines are a bit slow to spool and the pump provides a lot of fuel as soon as the throttle is opened making it sputter a lot. Fuel draw is directly related to rpm, a BGX needs about 8000 with muffler pressure to be really good. Your 20-6 prop should tach more than that. I found that 15% nitro will broaden the mixture range enough to cover the effects of big props or big tanks. Dont lean these engines much beyond a four cycle setting because they tend to get hot and lose power in flight. The stock muffler will eventually shake itself apart. The BGX is somewhat unique, most peoples expectations of it are too high.