Is The regulator screw set flush to the housing. I was having the same problem with my YS 63FZ in a funtana. My friend who has several YS engines suggested setting up the engine with the regulator screw set flush. I had the ability to run the engine 3/4 turn leaner with the reg screw set flush. Also I was having problems with air bubbles. Are you running the engine with a fuel filter? Is the filter screwed in tight?