so the engine runs good and makes power, but it just doesn't have any "Umpf" to go or pull?
i would pull the spark plug out and take not of the color of the electrode end, Is it Black, White, or tan?
If the plug doesn't appear to be the culprit, as the 610 has plug issues, i would look at the drive belt next. your engine may be putting out plenty of power, but if your belt is worn and slipping, that power cant make it to your wheels.