So I took off the starter, opened it up, it looked fine, put it back together, and it bench tested it...where it spun up fine. I greased the gear-pinion assembly up good, put it back on the tractor, and she started right up. My guess is the the gear-pinion assembly was just sticking. I should've tried bench testing the starter before I took it apart though...just so I'd know if it was the gear-pinion sticking or if the starter was just bound (or something along those lines). I'm guessing it was the gear-pinion... Thanks for the suggestions...I'm sure I could still use a valve adjustment.

As suggested above need to know what engine? My CC RTZ42 has a cheap briggs on it and alot of them now have Kohlers. I would you take the spark plug back out and see if it turns over while watching the spark plug hole in the engine head. Do you see liquid gas coming out? (If yes then the carb. float is sticking/leaking and letting gas into the combustion chamber)Simple fix remove carb and clean it or at worse change the needle and seat if the float not leaking). If you don't see no gas spraying out then I would look at all the switches, this includes the elect blade clutch switch (make sure it pushed in)the steer handles (pushed all the way out so the activate the switches that will keep the engine from starting, the brake is set (switch has to activate with brake set for engine to start, and last the seat switch (your butt has to be in the seat for the switch to be activated. If it still don't crank then I would check the cables from the battery to the starter and from battery to ground. if thats OK, then I would get me a multimeter and start checking all the safety switches mentioned above (they are on and off switches very easy to check, either they alowing current or not). If it still don't start I would find somebody that's willing to look at it. some things I would do I'm sure others will have more suggestions.

The only maintenance to do with hydraulic lifters is to perform regular oil changes. Nothing to adjust regarding the actual hydraulic lifters or valves. Oil changes should be done at 50 hour intervals, or annually if less than 50 hours use in a season. Change the oil as the last thing you do at the end of the season, before winter layup. You should always have fresh, clean oil in the engine for storage over winter. After changing the oil, run the engine for 5 or 10 minutes, then shut down the engine and wait for spring before starting it again. It is possible that the ACR stuck in the run position the last time you used the engine, but I'm at a loss to say how it could have started working again.

OK. I opened it up and had a look. The exhaust valve rocker arm stud was backing out, and the rocker arm was consequently very loose. I think that all I need to do is to clean up the threads and hole, apply locking goop, and re-insert. The one thing that's puzzling me is that there doesn't seem to be a cap on the exhaust valve, and I don't see one flopping around anywhere. I was very careful when I took the valve cover off, and am sure that it didn't fall out then. And when I re-tightened the rocker arm stud, there didn't appear to be enough slop to accommodate the cap. Should the exhaust valve have one?