Just recently purchased a mid 90's model Ingersoll 4020 Onan powered. Used it to plow some this winter and have used it to cut grass a few times this year. The last time I noticed the engine starting to bog down. Figured it was just heavy grass I was cutting but continued to get worse. Disengaged the mower deck and found the bogging to occur when attempting to drive. Forward or reverse and high/low range. Didn't do it as bad in reverse as it did in forward. Makes a strange noise the more you move the travel lever forward. Wheels spin freely when in neutral. Whats weird is the engine still bogs when in neutral when moving the travel lever and again gets worse the farther the lever travels. Any ideas on what could be causing this?