ORIGINAL: Rcpilet
It's the carb diaphram. When your inverted, the air flowing over and around the cowl is different. Thats creating pressure on ther diaphram--on the jet side-- and causing it to go rich.
Do a search for "balsa box" and "brass tube carb plate" in this forum. That will tell you how to fix it.
I agree with Rcpilet on this one. I have experience with 3 different FPE 4.2 engines..... I believe these use the same carb as the 5.8 and are very similar in all respects otherwise. All of these 4.2s had pretty much the same problem. Would cut out in different attitudes, at higher airspeeds, etc. They didn't like any direct airflow from the prop or slipstream around the carb area. One of these engines was cured by simply putting a baffle in front of the carb, the other by using a balsa box, the other was cured by selling it!
FPE recommends using an angle cut velocity stack also .... angled with the short part of the stack in the front. This helped one of the 4.2s and was provided free by FPE ... they might be standard equipment now, I don't know. I always thought these carbs might be helped some by changing the pop off pressure too but I was not aware of how to do that when fiddling with these engines a few years back. Overall, pretty good engines though when you get the bugs worked out. Pretty much a Brisson or similar clone.
Roger S.