I had an impromptu transmission overhaul session this afternoon, that upon 6 miles of testing, yielded some fruit that may get me closer to my current goal of 80mpg.
I had initially opened the rear cover merely to change out the very old, (19,500 miles), final drive oil. I ended with no sludge, but dark grey oil that was obviously done. Flushed the transmission and refilled with Royal Purple gear oil at 75w90. (For those that aren't familiar with the Burgman 400, Suzuki calls for 10w40 for the final drive). This final drive also has a vent tube, that for no reason I can figure goes into a small cubby hole on the transmission cover. It has no protection from the elements, and probably accounts for most if not all of the gunk in the oil. I placed a K&N style filter on the end of this hose, and I intend in the future to replace it with a sealed pressure relief system.
Then I figured, since I was already there, I'd go ahead and pop the cage off and inspect the belt. After getting that off, I figured, why not? I'll pop off the pulley's and have a look. A few hours later, I had the analog scale out, measuring the roller weights, which came out between 18.7 and 18.9 grams each. I glued 7 gram tire weights into the center of 4 of these to increase their weights to 26 grams, and now my speed/RPM ratio is much better.
In addition to the lower RPM's, the transmission resists "downshifting" more when applying throttle, so I can accelerate with a wider throttle opening at lower RPM's than previously, which is what was giving me some grief on a few hills.
I ended up putting the old belt back on, as it's not cracked, shows no indications that it's going to break soon, and even if it does, I've got a new one ready to go in at a moments notice.
All in all, I have a slightly more-drawn out clutch lockup, (the clutch exists between the transmission and final drive), but significantly lower RPM's at most speeds, a 1mph higher clutch release under DFCO, and a noticeably, but not measured, coasting distance due to the new final drive oil. With only 4mpg remaining to my goal, I feel confident that my next fillup will yield the magic 80mpg.
