ORIGINAL: bigtim
as long as you tune your inverted 2st engine correctly, not overly rich, they run great the biggest mistake is running to rich and having the fuel tank positioned incorrectly making sure the fuel lines line up with the carb nipple, and not to high, so the fuel siphons out of the carb.
the biggest mistake made is flipping a good running engine over and expecting it to run well without re-tuning seems like some 2st engines run better upside down better than others .
as for 4st engines all mine are inverted and they work great
AMEN!
I have a bunch of these 40S Ultimates all now running just fine with inverted .46's.
Any problems I had turned out to be, not having the engine(s) tuned properly or fully broken in.
Now that I know how to set up the engine, I'm running quite a few inverted mount engines on other planes as well also w/o problems.
The Ultimate 40S' tank is not that far out of line that you have to worry about the stopper level.
The plane sits nose up when idling, which puts the stopper at the right level even when the engine is inverted.
If you are worried however you can merely invert the tank to lower the level a bit. (Adjust the fuel tubes too!).
Remember that if your engine doesn't seem to run inverted and you mount it upright or side mount, you haven't necessarily eliminated the problem this way. At low engine speeds in a tight bank or aerobatics, etc. you WILL be running the engine inverted and you may or will get deadsticks. Found this out the hardware after listening to some (mis)advice here about turning the engine on it's side... not needed.
It is best to tune the engine with 1/3 tank of fuel in an inverted position then work backwards to eliminate the problems. Make sure that you get consistent RPM's at low and high throttle speeds this way... then work on the mid range.
That is how I got mine running RELIABLY w/o having to resort to a hotter plug... which didn't really help.