Here's one way to see if the upright mounting is acting silly....

....get the plane up high enough, and push it into an outside loop. Push
a lot of down into the control, and listen to the plane. If it richens way
up, and goes into a four stroke....it's your upright system doing the bad
things. If that is the case, you can either live with it....or mount the engine
sideways to eliminate those pesky fuel problems.

FBD.