When all else fails, make sure the pressure line nipple on the muffler is tight.

Have a Supertigre S75 with a Slimline muffler in a Pica FW-190D that runs like a top, until one day when I'd go full throttle it would run for a few seconds and then just go Waaaaaaaa and quit, as if someone pinched the fuel line shut. Changed plugs, fuel, you name it....finally found the pressure nipple in the muffler to be loose.