Well I'm not sure how this happens, but I have an OS53FS mounted inverted in a Cub, and it ran happily for months....until last week. It has me thinking that there must be another reason. It quit twice in flight.

I also flew a World Models Sports 40 for the first time last week with an inverted OS52FS, and it doesn't seem to run very well at all. It didn't quit, but just didn't put out the power it should have.

I'm willing to look at the fuel tank possibility, but can there be anything else?