I have a O.S 90, 4 stroke mounted inverted. When I throttle back from a manuever the engine backfires. Is the low end rich or lean??
O.S. F plug.

Sounds like its rich, the way I set my LSN is keep leaning it till it quits when u go from idle to full throttle. Then I richen until it stops quiting and transitions fast, adjust an 1/8th of a turn at a time.

i think is rich. Try to lean a bit and try again.....

I think it's lean. When rich it doesn't quit abruptly or backfire just sorta loads up and quits. At any rate youshould find out by making small adjustments one way or the other. This is done on ground not in air.

I have adjusted one for a member in our club same symptoms the engine was way rich on the bottom and the top. We ended up turning the bottom in 2 Full turns in and the top 3/4 of turn in, to lean the top and bottom. I strongly suggest you start with bottom end leaning first. One other problem with a four stroke engine running this rich, they will back fire while you are trying to start them and throw the prop off. An engine set this rich will also run very rough and cause high vibration throughout the airframe.
jerrysu29 Call Sign Wild Man

Mine did the same thing, when using heavy APC props, I adjusted the valves and it stopped.

I know the valves are adjusted well. It runs well on the ground and the throttle transition is right on and it has great power. I can hover jocking the throttle at mid point without it backfiring, except when performing regular aerobatics, it backfires at any given time when transiting the throttle??

I finally had a chance to tinker with the low end and found it was a tad lean, about 1/8 of a turn. It now runs like champ, thanks for your suggestions.