I have an 81' that I need some advice on.When I'm ridding as long as I'm accelerating or at kind of a high RPM it runs ok.But when It is just coasting or at a lower range it sputters. Also when revving the motor it seems like it takes a long time for the RPMs to go back down..I'm a freakin noob to the bike but not to wrenches.Can some one please give some advice