a leak, or crud, thats my guess. I always fly with a filter, but you have to be careful with them also, cheapy filters can develop leaks. I like the types where I can screw them together real tight. I even have a filter in my bottle as I draw fuel out of it and into my tank. Double filtered before it even gets to the needle valve. Be very careful with your fuel bottle. Don't leave it open for extended periods and don't let dirt like grass particles get into it. Leaks can be anywhere, check all fuel lines, and bolts on the motor. If you have a loose backplate or head, you're definitely going to have those problems listed. Sometimes the best thing you can do is take motor out and put it on a test stand and just start eliminating problems, one by one. Frustrating as hell, but it should run correctly.