'80 KZ250 Shifting Mystery
Abstract: 1980 KZ250 suddenly developed erratic inability to shift properly, most notably down, and especially past third. Replaced gear shift lever, transmission, shift forks, shift drum, springs. Experienced variety of small improvements or retrogressions, however the problem persists.
Backstory: I bought it last summer, and at first it was a wonderful little around town bike. I needed a commuter bike, and I rode it to work daily for several months with no real issues. Sure, the previous owner had stripped its air system and put those godforsaken Air Pods on it so that I can't get the damned carb tuned right and so it sputters and cuts out once you're running it at 5k RPM's or 45mph, but I don't need to go faster than 40mph most days anyhow - if only it didn't soot up the spark plugs and cake up the exhaust with carbon, I'd just as soon forget about it, but that's an issue for another post! One day on my way to work, however, I was downshifting on an approach to a roundabout, and found that I couldn't get down into second. I pulled over and fussed for a bit, and found that I was able to, with some gentle caresses, get it back down into neutral. This didn't fix the problem though. I ended up limping home in 3rd and - as it was my only form of transportation at the time - this was followed by months of bicycling to work which I did not relish.
I am mechanically inclined and well supplied with metalworking tools (I'm a blacksmith by trade), but in no way knowledgable. Prior to this, the most complicated thing I've done is replace the rear main seal on a Jeep Cherokee. Thankfully the bike came with a manual, and I've relied on it, you guys, friends, and all but one of the motorcycle repair shops in my city for advice and support through this, and I've run out of locals to harangue about this, so I put it in your hands.
Process: The first diagnosis pointed to the gear shift lever, which showed some signs of wear, and there was some scoring where it travelled up against the right side of the crankcase. I started by tacking some weld onto the tips of the lever and then profiling them down to what looked like it might have been its original shape. This definitely changed the nature of the shifting, and seemed like a slight improvement, lending credence to the diagnosis.
I purchased a NOS gear shift lever off eBay, which ended up being slightly different than the original, but had the same part number and seemed to be identical where it mattered (i.e. where it came into contact with the bike in any way).
Unfortunately, this seemed to make the shifting worse. Local advice convinced me that the problem must lay deeper in that case, and that the shift lever was merely a symptom of a larger issue - likely the forks or the drum.
Thus I tore out the engine, stripped it down, split the crankcase, and got at the transmission. Here I found nothing that looked particularly amiss. There was a bit of wear on the forks, a bit on the gears, but nothing chipped, broken or otherwise. The drum was the most suspicious part - with the forks out, spinning it free on its bearing, there was a perceptible wobble to its oscillating when viewed from the right side of the crankcase. There was also a very very slight catch, a small amount of friction in its rotation at one point. Local advice said that might be it, or it might be that there was enough wear in the drum's grooves that it was overly difficult for the forks to travel over the humps.
In any case, the drum cost as much as the drum and transmission together, so I fetched them both and installed both hoping to avoid breaking back into the crankcase. The only thing of note here is that once again, the parts had the right numbers, but arrived very slightly different from the originals. The lobes on the drums looked ever so slightly rounder on the replacement drum, and the K was stamped on a different lobe. The drive shaft on the original had a long gear milled into the right-most end where the new one had a free-spinning collar which seemed to slot into the clutch assembly, if anything, more nicely than the original. The new output shaft had a whole extra gear at its right-most end which nestled into a gap that went unfilled on the old one. I wasn't sure what to make of all that, but I took it as a good sign that the crankcase at least seemed to be designed to accommodate these deviations.
I buttoned everything back up, taking the opportunity to replace the gobs of blue Permatex I encountered along the way with some paper gaskets, and got it running once more. I took it around the block and found that there hadn't been any measurable change in the problem.
Conclusion: The problem seems to have been restored to its original state, with the added bonus that I can't seem to convince it to leave 3rd at all now, and that the entire shift lever will sometimes skip of the shifting pawls and skate downward unchecked.
As far as I can tell, I've replaced every single piece of the transmission and its accoutrements except for the shift shaft and the shift peg. The shaft shows some signs of very light wear (the black coating is worn in a couple of places), but no damage. The shift peg is obviously not the problem.
The problem must be isolated to the shifting bits and/or the transmission itself. The problem can be observed with the clutch and clutch housing completely removed and the clutch cable disconnected.
I can shift up and down through all the gears by removing the gear shift lever and twisting the drum by hand by the pawls, though I don't know how to tell if it's taking too much force to make the forks travel - it certainly isn't easy, and I have to spin the rear wheel or the drive shaft to get the drum to click over fully.
I can't add links or pictures, but I've got a whole photo album of all my work so far, complete with all my note-to-self videos that I bet would be helpful to see.
Anyway, I'm completely stumped. I put it to you, dear reader. Many thanks for your thoughts!