Well, after much delay, due to numerous unforeseen circumstances (and general laziness), I finally got the chance to tear into the HT23. It had been stranded in the driveway since January, when it died moving snow. Earlier in the mowing season, I had been using the 1253 for that role, until that engine finally gave up the ghost. The 1253 had been burning a lot of oil and smoking quite a bit. Luckily, I am blessed with a couple of great neighbors who took on the mowing duties while I was mowing-disabled.
So, a month ago, I pulled the HT23 engine and got it into the garage, so that I could work on it out of the weather. After four weekends of labor, I finally got it fixed and back into service.
The problem turned out to be exactly what my most recent suspicion had been. That infamous plastic cam timing gear had roughly 25% of the teeth sheared completely off. The rest of the teeth were chewed up pretty badly, too. The cam gear was so mangled that it actually caused damage to the plastic governor gear, as well. Before I even pulled the engine, I had purchased a steel cam gear off of eBay, thinking that it was the culprit, so I was prepared. After seeing the damage to the governor gear, I was lucky enough to find an older governor on eBay which had a steel gear on it. I didn't even know that the governor gear was steel at one time. What was Kohler thinking when they switched to plastic gears?! Maybe it was a cost-saving attempt, due to the lower-cost competition from China.
While I had the engine out, I did a number of other things, too. I removed the heads and decarbonized everything, replaced the points, removed and cleaned all the plastics bits out of the oil pan, adjusted the valves (the clearance on both intake valves was a few thou over) and painted all the engine tins. When the engine was replaced 15 years ago by the PO, they left the engine in it's original primer gray. I used Rustoleum Dark Hunter Green. I think a PO had repainted certain areas of the tractor and maybe used the same color, so it looked OK. Maybe some day I'll do a full restoration and use a more original-looking color. Right now, I'll settle for this. It definitely looks better than it did. This tractor has that working "patina".
While I had the heads off, I inspected the condition of the cylinder bores. Happily, they look nearly new! The crosshatch marks from the original honing is still visible.
While scraping the old gaskets off the heads, I noticed that the areas near the exhaust valves were a little harder to remove and looked somewhat dark, so I checked the flatness on a granite surface plate and determined that sure enough, both heads were slightly warped. After about a half hour of wet-sanding with 320 and 400-grit paper (on the surface plate), I had two perfectly flat heads. I probably removed no more than .002" from each. After I got the engine installed back in the tractor, I did a compression check and got 125lbs on cylinder #1 and 120lbs on cylinder #2.
Luckily, I remembered to mark the position of the governor before I removed it, hoping that the timing would be close when I reinstalled it. I installed the plugs and then it was the moment of truth. After the fourth try, it started and after a little twiddling with the carb, it ran quite well. I don't think it even needs any fine adjustment of the timing, but I'll probably check it anyway.
Last night I started working on the mower deck to get it prepared for the rest of the mowing season. I got the deck cleaned and the blades sharpened and balanced. Today I got the deck mounted, greased and lubed, but it rained before I could mow the lawn.
All I need now is some dry weather, so I can put this beast to the test.