Quote Originally Posted by inflight View Post
My rotax friend came by this morning (snip, snip)

The second problem arose when we did my first full power run up. Engine runs great at or below current ambient pressure (about 22 in/hg) but does not want to go above that and surges when I try. I think this may be due to leaky carb bowls. When the 3 way solenoid pressurizes the carbs the bowls leak and can't supply enough fuel to the motor. ?? I was planning on rebuilding the carbs but I have found new/unused 914 carbs on eBay that I am considering buying. It seems like a good deal but depending on there age from manufacture they still my need new gaskets. But for $350 for the pair it may be worth the gamble. Any previous experience/advice is welcome. Just keeping you guys up to date.
A 914 powered Highlander crashed once (I was there) with a very similar problem of "surging" during high throttle application and warm temps. The best we could tell after the crash (which was very severe both to the pilot and the aircraft) was that because the 914 gets all of its fuel from two electric pumps that continuously run fuel from the header tank to the engine , with all unused fuel going back to the header tank, that the fuel was getting hotter and hotter the more it was run on the ground, which caused fuel vaporization resulting in surging and eventually significant power loss right after takeoff. Lesson learned that day, run all unused fuel back to the wing tanks where there is a larger quantity of cooler fuel to mix with the hot fuel coming back from that engine compartment. Plus the air going over the wing also will help to cool the fuel, wheras the small header tank just lets the fuel get hotter the longer you run the engine.

I don't know for sure if that is why yours is surging, but the scenario sure sounds a lot like that Highlander. I'm guessing if it wasn't summer and if you weren't running it hard on the ground that maybe you wouldn't even know this was an issue. But now that you know that could be a possibility you might want to check out how yours is plumbed. The Highlanders were modified to return fuel to the wing tanks and many 914 Just Aircraft have since been operating successfully.

Knowledge is power...