Re-jet, idle mixture, and clankety-clank on deceleration

[charlie55]

As I had previously posted, I did the upjet from 102 to 105 and didn't notice much, if any difference. LMC had also previously stated in response to another post that there might be a need to re-do the mixture and idle speed thereafter. Such was the case with mine. Nothing major, but it took a while to get everything tweaked to my satisfaction.

In the course of playing with the adjustments, I was regularly running it (on the center stand, of course) up to between 6-8K RPM, then snapping the throttle off to see if it would stutter/clank as it wound down. Sure enough, it did. This was with the idle speed adjusted to about 2K RPM (via the Blur's tach, since I don't have anything else available). Just for the hell of it, I picked the idle up to about 2250, and, voila, wind-down was flawless.

Coupla things/theories I gleaned from this:

1) No surprise, but the Blur's tach is probably off appreciably.

2) I may be off base here, but assuming that there's some sort of logic/programming within the CDI, it appears as if it doesn't handle deceleration as well as acceleration. With the idle set to 2K, snapping the throttle shut resulted in the engine actually dipping quite a bit below the 2K mark. Set to about 2250, there seemed to be a proportional dip on deceleration, but, since the nominal idle speed was higher, the dip never reached the "stutter/clank" threshold.

3) Assuming no shortcomings with the CDI, I'm pretty sure that the carb has some sort of deceleration circuit (path), that is supposed to prevent lean burns. I believe this situation occurs when you close down the throttle, but the engine's still cranking at high RPMs due to the clutch still being engaged and the rear wheel, in effect, "driving" the engine. Perhaps this feature isn't optimized.

Personally, I don't believe the clanking to be mechanical, rather it sounds a bit to me like knocking/pinging. I also think it ties into one or both of the theories I stated above. I could very well be wrong, but my ears tell me differently.

If (as I've seen alluded to in other posts) you're getting that metallic clank on deceleration, you might want to try raising your idle RPM a bit to see if it helps.