Sunday, July 15, 2012

CB125 Cylinder Leak

So even after finding a replacement dowel for the CB125 motor, it still is not up to par. In fact it seems worse now. I pulled and inspected the rings, reassembled and measured and they seemed fine and within specs but the cylinder is now showing a 12% (best reading) loss. That's about 3% worse than before. I can hear the hiss coming from the crankcase as the cylinder bleeds air. The only logical explanation is the piston and/or rings. Rather than screw around with it any longer I'm just going to have it bored .25mm over and go from there. I'm not even going reassemble it fully to do a compression test. On the upside the head seems fine and I guess I won't be needing those new valves I bought :-/


  1. Replies
    1. haha it's been considered, but a $50 bore and hone is a lot cheaper than having to replace a Chinese motor every couple of years.

  2. do you have facebook? ive been lurking you for the pass couple days (since ive googled you)

    I have a 79 CB125 right now, Im getting a knock or some sort near the head of the engine. your pictures are great.

    1. yep. facebook dot com slash charlesedwardhamiltoniii

      Is it a knock when running? Do you have a bent valve, piston slap, pre-det, cam chain loose? "A knock" is sort of vague.

  3. check out this video of the problem you can hear it at the very end of the video (i think.)