Originally Posted by
Flying Fiz
I thought I'd better test out the engine some more before attempting flight and it became immediately apparent that under load the engine would fail instantly. This was because (despite what the HH reps advised me) this engine (Evolution 15cc) does need muffler pressure. The vent line to the atmosphere is not sufficient. So I took the presure nipple off my stock muffler and tapped a new hole in the Pitts style wrap around muffler, reworked the vent line to that and bingo! All Good. Previously as soon as I held the plane striaght up at near full throttle the engine died. Now it was good.
Crazy that the HH reps would tell you that muffler pressure was not necessary as, so far as I have been told, the carburetor design absolutely requires muffler pressure to function properly. I have a 20cc Evolution that I could not get to run reliably and so I decided to try a fuel pump on it. Well that was a dismal failure as you might imagine since the carb HAS to have pressure in order to meter fuel properly. I finally sent the engine back to Horizon and they have assured me that it is good to go now but I have not had a chance to get it on a test stand since getting it back. I am cautiously optimistic that it will run good now but I have a feeling there were many issues with the design which is why they were all re-designed with pumped carburetors.