Franks is out. Rough guestimate is he still had 3/4 of his travel to go (changing oil real often may have unseen benefits). An Idea Follows.
In the XS650 Yammi, of which I had several, and built several motors for, a screw assembly tensioned a strong spring which acted against the plunger that pressed the chain guide for chain tension. The chain was correctly tensioned when you could see the right end of that plunger, which was exposed when a cap was removed, just barely moving in and out of the housing. In other words, some slack/no slack was the best. The spring would allow for this, while keeping it to a minimum. I notice that the FJR's allows no movement other than forward and against the guide, constantly tightening, but never slacking. Not sure about all this, but I'm thinking that rather than a solid tension against the guide, a stiff compression spring acting against it would be better, and allow for the tight/loose action that naturally occurs with a chain. Note, however, a XS650 cam is single, and acting on 2 cylinders, while the FJR's is a double, and working 4 cylinders worth of valves.
I'm gonna work on an idea, since 4" of snow has Frank sittin on his dead *** anyways.
The problem with doing this on a timing chain tensioner is that the spring is free to compress and allow the chain to develop slack at high rpms. To prevent that you would either have to have a very stout spring, which would probably put too much tension on the chain causing the guides to wear prematurely, or use a relatively weak compression spring with a ratcheting mechanism to prevent retraction. This is a proven technique in autos and other bikes.
I think the trick to finding a better automatic tensioner will be to try and find one from some other application that can be adapted to fit in the FJR. Or just go straight manual adjuster and make adjustments at every other oil change? If it was easy enough to do, it would be no big deal.
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