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If it sometimes starts normally, then it sounds like your Bendix drive is hanging up. Explanation, in case you aren't familiar with a Bendix: The starter motor has a long straight shaft. Onto this shaft is slipped a Bendix drive, which is secured by a bolt at the tip of the shaft which goes through a loop on a coil spring. The spring is there to absorb shock. The other end of the coil spring turns a shaft with very coarse threads on it, on which spins the gear that engages the flywheel, held in place by a pin that rides in the coarse threads. How it works is this: When the starter motor starts to spin, simple inertia keeps the gear from starting to spin as quickly as the rest of the assembly. Thus the pin inside the gear rides in the coarse threads, moving the gear laterally up the assembly. This puts the gear in contact with the teeth on the flywheel. The starter motor spins the flywheel. When the engine starts, the flywheel immediately gets going faster than the starter gear, which causes the gear to ride back down the coarse threads to its "parked" position, where it stays until next time it's needed. What can happen? Well, there could be gunk in the coarse threads. It rides in engine oil. There are a couple of other things as well. One is a broken coil spring. Sometimes when that happens the spring parts catch occasionally and sometimes don't catch. Then there's the one you don't want. That is, teeth missing from the flywheel. If that's the problem, your indication is that when the starter misses (meaning it engages the part of the flywheel with no teeth), no amount of trying with the starter will make it catch, and the only fix is to turn the engine to a different position, usually by putting it in gear and rocking the tractor. Removing the starter to check and replace its parts is relatively simple, and the parts relatively cheap. Replacing the flywheel ring gear is more of a job. Let's hope you have a simple problem!!
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