ORIGINAL: JohnBuckner
Very common problem with inverted mounts if the fuel drips and syphons there is only one solution Lower the Tank as already has been noted. The same relative tank center/carb jet relationship must be maintained with an upright or an inverted mount or there will be runing problems.
John
This is a very very common problem and it is unique to UCD and like designs. For example my Phoenix Topstar which is just like the UCD. I too mounted a 75ax inverted on it. What followed was a full season of engine problems, frustration, deadstick landings etc. Keep in mind these are 3D airplanes and I wanted to hover while it wouldnt even run for 2 mins no matter what we did.
Finally, I got frustrated and swapped the 75 for a FS91SI- my first 4c much against the warnings of my fellow fliers. That however was not the clincher. When I installed the 91, I sealed off the stock firewall hole and made a new one lower so that the tank centreline was in line with the carb spray bar. I changed absolutely nothing else. That did the trick. Over the last season the Topstar was the only reliably running airplane I have and even after having several near crashes (trying to do spectacular things with the new found relaibility) it still is.
Drop the tank lower, you will enjoy this airplane
Ameyam