I have a 1965 1850 diesel. The rpms will surge but so far they only surge up about 200-400 rpms or so for 5-10 seconds then come back down to where you have the throttle lever set at. For example, I'll be out discing at 2000 rpm and out of nowhere it will jump up to 2400 rpm on its own and hold steady there for 10 seconds or so. Then it will come back down to 2000 rpm where you have it set at and run there for a while. This happens at all rpms and at a load or under no load. It seems to be increasing in both frequency and duration and makes it frustrating as the operator. Both fuel filters were changed last year and the tractor does not see a lot of hours each year. I always maintain at least half a tank of fuel in it, too, since the fuel gauge doesn't work. Any ideas on what could be causing the rpms to jump like that on its own?