I've noticed from my work on these tractors, that a lot of vibration perceived as coming from the engine. It can be calmed down A LOT by going thru chassis and getting rid of all of the "slop" in linkages and such, like mounts, flex disc joints, clutch linkage, etc. Add the Cradle modification while you're in there.
I have been thru 2, 1200 in recent years. I tried solid mounts in 1st one, and it about rattled my teeth out. I had done the clutch with a new driveshaft and solid mounts. Those solid mounts stayed in the machine about 10 minutes. I put sway bar link bushings in it from a car for mounts and it was much better but not completely so. That made it "tolerable", didn't know about Cradle mod then.
On the 2nd 1200, which got a complete engine overhaul besides the clutch job, also got those solid mounts that were "too much" for the 1st 1200, put into it. . A lot better almost from the get go, but not totally smooth. Tractor owner (same guy owns both, and no/ its not me) was not very nice to this one when he got it back. I went thru the clutch in short order (he likes to run 3rd gear in shin-high grass) and more blade drive belts than he should have eaten up in that amount of time, as well went with HD "pulling" pressure plates and driver, right off the bat. and the cradle mod.
When I went thru the clutch the 2nd time, I replaced the pressure spring with a "blue" one, replaced everything in the driveline between the 2 clutch jobs but the drive shaft itself, it was still fine. (surprisingly didn't show any wear where it goes into hub on flywheel, which was where the wear was on the 1st tractor)
I reused the "pulling" pressure plates and driver that I put in on the initial clutch job. I remember having to replace the driveshaft in the 1st 1200, when I had done that one a few years prior.
I found slop in frame on 1200 #2 where clutch arm shaft goes thru there, clutch fork needed some re-welding and re-drilling plus added some stiffening. I welded some steel bushings to clutch hanger and clutch fork at the pivot, and a few other spots I never really thought to look. The clutch fork pivot shaft was wallowed and worn down too. I tightened up all of the worn areas, reassembled with those same solid mounts that he wasn't happy with on the 1st tractor, changed out the engine oil pan to cast iron, and WOW I was amazed, what a difference between the 1st time thru the clutch, and the 2nd time thru it all. The difference between the 2 1200s is huge. A lot less vibration in the most recent clutch job on the 2d 1200. I didn't even think to check that stuff out, when I did the repairs on the 1st 1200, back then.
Remember the 1x8s and 1x9s have the engine solidly mounted from the get go, and don't generally shake so much. So there is more to vibration reduction than just the engine itself.
I have a 12hp version, a 14hp version, and a 16hp version, in a competing brand of tractor, that is likewise shaft driven/ all Kohler K series engine powered like these Cubs. All engines solidly mounted (Ariens) and none of them vibrate like that 1st 1200 did, when I tried mounting that engine solid. Yet the 2nd 1200 never was as bad when mounted solid, with either iteration of the clutch job. Same tractors, same model engine same everything.
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