Vintage Dirt Bike Q & A

Greetings,

I dug into my '79 CR125 today, and got the engine taken apart.

A bit of background: I quit riding this bike about five years ago when I had a bad third gear shortly after I began riding it. It shifted into third fine, but would lurch and clank and I thought I had a few stripped teeth. Basically, it sounded like the gearbox was going to explode... I attributed it at the time to my own clumsy shifting that caused a gear to break.

So, the bike got set aside as I didn't have time/money to fix it and had other bikes to ride.

Back to today's problem: Upon taking everything apart (and spending lots of time doing so), I expected to see metal in the oil and scored/damaged gears. All the gears look perfectly fine. When meshed together, I don't see how it would be physically possible for the 3rd gears to slip. No chipped teeth, and none that looked excessively worn.

What else could contribute to this? What should I be looking for? Again, when it was running it shifted into and out of 3rd fine. Could it be a bad drum? I inspected that and didn't really see any excessive wear, plus I wouldn't assume it's a high wear item. Furthermore, it ran through all other gears fine. I rode it for a couple hours and just skipped 3rd (not too fun) before I finally put it in storage. The only think I can think is that it was assembled wrong by the guy who had it before me or something.

I have three other engines, two that are almost complete as donors. I also plan to build up another engine out of them. Any suggestions before I go taking three others apart? Any help is greatly appreciated... I never thought I'd be DISAPPOINTED to take apart case and not see damaged gears...

Thanks a lot,
-Pat-

Look closely at the cogs on the sides of the gears. These usually round off some and cause the gears to pop out/pop in...