Hope you don't mind that I'm adding to this, but I figured out the whole problem today. There is a little adapter thing that mounts to the driveshaft inside the pump, and the other end bolts to the timing gear. Makes it really easy to remove and replace the pump without worrying about the timing. Getting to the point, the bolt that holds this adapter to the pump had come loose and fallen out, and was rattling around the timing gear. I bought a new bolt, put everything back together, and presto, tractor runs great. I hope that this will help somebody somewhere down the road. I know that I get a lot of help from this site. The newest peice of machinery we have is a '78 j.d. baler. Needless to say, we do a lot of mechanic work.