There are mixes you can spray into your motor that loosen up old, built up carbon and flush it out. Carbon builds up on the piston domes, exhaust passages, rings, and ring grooves. Excessive carbon buildup can block off the exhaust side, rob compression due to stuck rings, and cause miss-fires due to hot spots on the piston dome. Sea Foam Deep Kreep, Yamaha Ring Free, Quicksilver Power Tune, etc. are available all over the place. Old motors like yours that grew up on a steady diet of old school, heavy ratios of 30-wt. are often found with considerable amounts of carbon inside, so it's a good thing to do, regardless.