My 2000 ZX-12R with 13,000 miles started jumping out of 2nd gear a few weeks ago and got worse and worse until 2nd was basically useless. I rode it too long like that.....At first I thought it was ignition. Cutting out or missing, but it was only in 2nd gear. Took me a while to figure that the problem was with the transmission. Anyway, I tore into it this weekend and wanted to share pictures and what I've learned.
First, I dug into the oil pan from underneath. You must remove the radiator, exhaust, blah blah and then the oil pan. The Kawasaki manual is actually really good about giving step by step intructions. Buying one is a must if you plan to do this. You can change the shift forks and shift drum from underneath pretty easily by going through the oil pan. I was hoping that would be the only problem and I'd be done with it. Not the case.
I got the oil pan off and saw lots of debris. Turns out most of it was gasket type material, but there was also lots of pieces of steel too. Not good. Shift forks and gears are made of steel, by the way. Here's a link to some big digital pictures. They're all about 1 Mb, so I didn't post them here. http://www.imagestation.com/album/?id=4289679617
I pulled out the shift drum and shift forks. There's one shift fork on the input shaft and 2 on the output shaft. I was skeptical of the fork that engages 2nd gear, and when I took a look at it, sure enough, one side was badly worn. It was ground down and blued from heat. The way this works is that this shift fork actually moves 5th gear side to side. When you want second gear, this shift fork moves 5th gear towards second gear. Each gear has dogs on the side and they lock together. Since this fork was worn down, it didn't push the gears together all the way, and they'd move apart and jump out of gear.
So, now that I knew this fork was in terrible shape, I knew at least 5th gear was in bad shape too. Both pieces had to be badly torn up since they rub on each other. So I knew I had to change at least one gear. To change transmission gears, you need to split the crank case, and to split the crankcase, you have to pull the engine.
So I pull the engine step by step like the manual say. Again the manual was great. It was a little tedious, but much easier than I imagined. I used my home rigged engine stand and flipped the engine upside down. I actually put an old pillow on my work bench, flipped the motor upside down by hand, and set the motor down very gently on the valve cover. Then I straped it in place so it wouldn't fall over and went to work.
I split the crankcase and finally could see all the damage. 2nd gear and 5th gear are toast. The dogs on each gear were rounded off and you could see where they slipped past each other when it jumped out of gear. See the photos in the link above. Also, as I guessed, 5th gear was badly worn where the shift fork rubbed against it. I feel lucky no dogs broke off and took out the rest of the engine.
So far, I have 1 bad shift fork and two bad gears. 2nd and 5th on the ouput shaft. That's where I'm at now. I'll probably replace all 3 shift forks just for fun. I'm also going in for closer inspection of all the gears and bearing and then ordering parts. It's actually not that bad of a job. Lots of little steps so it gets long though.
Question, do I also need to change the 2nd and 5th gears on the input shaft also? They look fine, but putting a new gear in that meshes with a used gear seems like a bad idea.