hi I'm the new owner of a 2002 mule with the 610 cc engine it is 4x4 and I went for my first ride with it over the past weekend and found it has severe trouble climbing up a hill. I tried it in high range and my wife was able to walk past me climbing up the hill so I tried it again in low range and it did make it up the hill but only at a snail spped. I have had the plugs changed and a new drive belt put on it since I purchased the mule and have less than a hr of run time.is this normal operation or what can I do to fix it and pass my wife walking up the hill? help please!