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Discussion Starter #1
I just removed my 86 K75C (53k miles) rear drive, swing arm, driveshaft to inspect and lube driveshaft splines and trans output shaft splines. They are actually all in good shape. I lubed all splines with BMW red lube (unfortunately had not yet found and read recommendations for Honda moly or others) and everything went back together without difficulty, including popping the front end of the driveshaft over the circlip to get it onto the trans output shaft. Test rode the bike and all is well, except - - Now over very small bumps and when gently bouncing down on the rear suspension at a standstill, I can hear and think I feel a rather minor "clunk". While trying to figure it out, it seemed like it was coming from under the seat but I could find nothing there. I'm almost sure it is coming from the front end of the driveshaft, apparently there is some slight play in the circlip/driveshaft mating. Just to be sure, I again removed the rear drive to check that the driveshaft is correctly mated to the trans output shaft - sure enough, can't pull it back, it's on as it should be. After reassembly, "clunk" is still present on gentle bounce of rear suspension. Is it possible that the my cleaning and lubing of the trans output shaft and the driveshaft front end have made that end of the driveshaft move so easily that the play in the circlip notch can now be felt? Perhaps after some miles the front end of the driveshaft will quit moving on the trans output shaft and the "clunk" will disappear? Anyone else ever experienced this? Thanks for the help.
 

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Given that you hear a clunk when the seat is raised, suggest that you verify the rear shock is correctly mounted and the shock bushing is intact. Grab the shock and shake it, listening and feeling for play there.

HTH
 

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Discussion Starter #3
That was my first move. It's mounted correctly, bushings are fine. I also increased the initial spring tension to make sure it wasn't the spring bumping against the upper stop. I'm confident it is not the shock/spring.

I also carefully examined all the mountings of the rear fender, the tail piece, etc,, nothing loose.

It takes a very small amount of swing arm movement with a light "bounce" on the seat to get the clunk. I can imagine the driveshaft moving forward just the slightest amount as I "bounce".
 
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