At low speeds and low throttle, the engine on my 2000 Foreman runs fine. However, when I hit the throttle, the engine will "bog down", lose power, and the exhaust noise will be muted or muffled. After letting off the throttle, the engine will recover to normal power and the exhaust sounds normal again. Any ideas what causes this? The atv had not bee used for about 6 weeks before this problem showed up. Even before this problem, the exhaust would emit a loud sharp sound when the throttle is released. Are the problems related? Help!