I starter to work on the 318 today and one of the first things on the agenda was to sharpen and balance the blades and replace the belts plus I had to weld up some cracks on the mule drive. Got the cracks welded no problem and finally got the blades balanced, boy were they bad. The new deck drive belt of 90" fit perfect but the problem is the deck belt. The John Deere parts site calls for a 1/2" x 98" deck belt and the new 98" belt is WAY, WAY to long. Does anyone know the correct length of the blade belt on the 50" deck?
Also I did take time to take the top off the carb, a good shot of air from the compressor though the ports and jets took care of the surging. Nice and smooth now I just think it is idling to high but I will address that later.
There should be a tension spring stop on the deck. Not sure but maybe this could be your problem. Just looks like a piece of steel bolted to the deck that limits travel of the tensioner pulley. I've had to adjust them in the past.