Hi guys,
I am new to petrol engines so please excuse what might be silly questions to some.

This mvvs 45cc petrol engine was running a bit rich on the low end and high end during break in and has not changed settings ever since, the engine has had about 20 flights if not less and three times now i've had a certain problem. When the throttle is advanced more than 50% there is a gunshot like sound(this happened both flights today) and the engine wants to die out but it didn't. After the first flight we asked someone at the field he said to richen it up a bit because the engine had run in and is going leaner so we opened it up a bit more and this time it happened worse. It was like one or two gunshot sounds (maybe backfire) in a row.

My question is, do backfires happen on petrol engines when its lean and rich ?

This engine is spinning a mezlik prop 22/8 at approx 7.5k rpm, is that too rich?

Thank you for any advice you have to offer, i won't get insulted as im new so feel free to explain it like im clueless.