That engine will run fine inverted, sideways, or straight up - as long as the tank centerline and carb are the same. As that's a taildragger, prop up the tailwheel to keep the fuselage level. Four-strokes like nitro, I never run less than 10%.
Kochj's tuning is spot-on.
I'd suspect carb/tank height, or a leak in the fuel plumbing or intake.
What prop are you running, and what's the highest rpm it holds?
Last, test for overheating (lean run, water in fuel, too much prop). Run it for 2-3 minutes at high rpm, then shut down. Lick your (least favorite lol) finger, then touch the fins. If you can touch lightly for a couple seconds, it's good. If not, it's running too hot.