The eneviable throttle "surge" spring for the above models always seems to be missing. Probably due to the spring steel rusting and the spring falling off after it breaks. If your Tractor is "hunting" or "surging" while idling, make sure the Govenor linkage is adjusted correctly FIRST according to your service manual. To help "balance" out the idle coming off of engine load, this spring will help to eliminate the "surge".  One end of this spring connects to the throttle linkage (hole in rod) and the other end hooks onto one of the exhaust manifold studs. If this spring is missing, replace it with one of our springs. This will give a nice smooth idle and eliminate the "surge".