Juan:

You didn’t say how much run-time you had on the engine; however, I too have a 3W75US mounted inverted inside a rather tight cowl. Initially, I also had nearly the identical issues as you describe, so I tried several different locations for the static pressure vent tube and various needle settings—nothing worked. Finally, I resorted to doing away with the remote vent pickup—just the short nipple coming out of the carb. I figured I would just have to live with this crazy behavior.

The good news. As the run-time on the engine increased (about 10 gallons later) and after continued fine adjustments to the carburetor, the issue went away! I’ve now burned nearly 20 gallons and, with no other changes, the engine runs solid through all flight regimes without any noticeable changes in mixture—rolls, knife edge, snaps inverted, it just doesn’t care.

The bad news. I recently found that the engine would momentarily cut out completely during some maneuvers! Although these occurrences were only a second or less, they were very disconcerting. I won’t go into all the details of the time wasted trouble shooting this one—instead here is the bottom line. I utilize an electronic kill switch and, while I had correctly set “off” parameters to positively kill the engine, I accidentally had set the “on” parameters in the transmitter set so that the electronic kill was just barely “on.” All it took was just a little noise while in flight or a slight reduction in radio signal and the kill switch would do its thing—kill the engine!!

All is right with the world now! The 3W74US starts every single time on the second flip, never misses a beat and continues running like a Swiss watch. Hope this helps.

Ed