BG, the 455 is a chainsaw...2 cycle :)
It could be a couple things, there's a chance that the carburetor is just WAY out of tune, running way to rich. How does it idle? Is it extremely hard to start? It could also be the inlet needle in the carburetor not be sealing properly. This could be caused by either a worn needle, or dirt under the sealing tip. I would pick up a rebuild kit for the carburetor (should run $10-$15) and disassemble, clean with carb cleaner spray and install the rebuild kit. It's not very difficult to do if you take your time and make notes of how everything came apart, take pictures if necessary, and you can always post on here if you run into a problem. But if you don't feel comfortable digging into the carburetor, you'd probably want to take it to a dealer.