I have a zero turn mower that surges when NOT under load. It started doing this when the mower had about 20 hours on it. Now has 90 hours on it.As soon as I engage the blades, it runs like a champ. Dealer said this problem is not covered under warranty, and he sold me some overpriced B&S additive which did nothing. I assume it is a carb problem, but why does it run great when under load and surges when the blades are disengaged? There is no problem running it this way, but sure is agravating. Any ideas?