My 87 did that when I had too big of a main in it and my header pipe was loose. I replaced gasket and tightened header and went down on jet size and it pretty much stopped misting fuel back out the carb. It was a thick mist spraying back out. But even after getting the carb tuned properly if I removed the air filter and revved it up it would still have a very fine mist of gas spray out. It ran good and the plug was chocolate colored after doing plug chops so I didn't worry about it.