I'm trying to accomplish a pneumatic sync on my 912 ULS Bing carb and it is running so rough when I hook up my carb mate it doesn't let me make any progress toward syncing because the vibration overcomes the float needle valve and fuel vents out the carb (left carb only). This was part of another previous forum discussion and I thought I had it resolved when I lowered the float levels in both carbs, which worked fine until I hooked up my carb mate. I'm attempting the p-sync at 3000 rpm. When I removed the carbs to do another internal cleaning I noticed a thin film of some dark sticky residue on the internal carb surfaces downstream from the butterfly valve. It came off with a quick squirt of carb cleaner but I'm wondering where it came from? It seems like there's only two possibilities: 1) Something coming in from the air cleaner or 2) something coming in from the fuel tanks or lines. The engine only has 2.6 hours total run time. During my gas tank install into my wings last year I did coat the inside of my tanks with the Bill Hirch gas tank sealer that some on the forum suggested would protect my older fiberglass tanks from ethanol. It appears to be surviving fine when I look into and probe the internal surfaces of the gas tanks. I did put a few gallons of ethanol gas into the tanks a couple of months ago when I did my first fuel flow test but I drained it all out after the test and have only used 91 octane and no ethanol gas ever since. The fuel lines I'm using are all the thicker black rubber fuel lines that were supplied by Kitfox. I have a clear polyurethane header vent tube. Has anyone seen this type of residue buildup inside their carbs? If so, is this normal?