Don and Morgan are right.
The pre-88 Marathons used a Mikuni BV18 carburetor, while the 1988 model used a BV24. That's 18mm vs. 24mm, or a 33% LARGER venturi on the 88. That, paired with the 1988 only exhaust which is MUCH better, gets you some very noticeable power gains on an 88 over the older models. One post I read stated the early models were rated at 9hp, while the 88 was rated at 11hp.