You might have a build up of gunk in the engine. I always get good results by flushing the engine. I use a couple cans of B12-Chemtool in the tank and a spray can of B-12 down the intake. For the spray, I get the engine up to temp, get the revs up, and spray down the intake until it starts to die, get the revs back up, wait a minute and start again until the can is gone. It will smoke quite a bit while it is burning off the gunk. Always helps to stay away from cheap gas also.