I had a lot of problems flying an ASP 120 size 4 stroke inverted. The best I could do was to get the fuel tank (centre-line) about 3 inches above the carby. The engine had hard starting because of fuel locking and wouldn't run anymore than a few minutes (it would slowly load up with fuel and finally quit in a haze of smoke). Leaning did not make much of a difference.

I didn't want to go to the expense of a fuel pump or regulator and initially tried an extra length of narrower guage fuel line looped over the top of the fuselage with an inline filter. This worked in that it gave me a reliable running. The engine still tends to load up if idled more than a minute or two, but clears (relatively) quickly and runs clean WOT.

My theory is that the extra length of fuel tube, narrower guage and inline filter added enough resistance to slow the fuel flow so the engine could handle it. I still get fuel lock/hard starting on the occasions I don't get the engine running on the first few turns, but I can live with this.