Check the fuel lines and filter. The lines split after awhile. Make sure the fuel is fresh and of good quality. We tell all our customers at the shop to use 89 octane or higher. Don't use that E-85 crap !! If everything else is ok ie; spark arrestor screen, exhaust port, compression, ignition etc, then you most likely will need to thoroughly clean the carb and install a kit. I've had some carbs so bad that they had to be replaced completely. One other thing that can cause your problem but is rare, is a crankcase air leak. This can be a worn seal, loose cylinder, leaking gaskets, etc. It will cause the air/fuel mix to lean out and cause the symptoms you are describing.