Vintage Dirt Bike Q & A

Hey all,
I took my cr125 out to the trails last weekend and made it down the first stretch and the bike died when I decelerated. Could not get it started again and had to be towed back to the truck. Did a compression test when I got home and it topped out at 33 lbs pressure Tore it down to discover a cracked ring on the air intake side, right over the port. No obvious damage to the cylinder (need to take a real close look later). The piston and head have some moderate ping damage from debris. The piston sides are completely undamaged. Just a small portion of the ring is missing.

I bought the bike 4 years ago and it was a low hours bike that some kid must have gotten new and ridden to he crashed and bent the handlebars. I am the third owner with the second guy intending to restore but never got around to it. Only ridden around the yard a few times. I did a top end and cylinder re-plate. The crank was in excellent shape. Put on a new Mikuni vm32mm carb, tires, handle bars and rear shocks. It was running great for the last few years. I put on about 25-35 hours since the break down. The one new variable is I used premium gas mixed with Blendzall Gold Label instead of the Blendzall Racing Castor I normally use. I ran this fuel on the ride previous to the incident and during the final (short) ride. There was a noticeable gain in engine performance with this fuel - not crazy but noticeable. I'm not sure if this had anything to do with it, but in this old bike, I won't be running that fuel mixture again. I will stick to my castor or maybe go modern. I didn't expect the problem to happen this soon. I was planning to change my rings every 50 hours or so. Maybe I need to do this more often??? I trail ride with open flat areas for givin' it some gas.

I think it's time to tear down the lower end and contemplate sending the crank to Mr. Crankshaft for a rod conversion and rebuild. I have a new piston and rings and could just clean out the crank well and give it a go. Not sure, what do you guys think?

I am also thinking about blasting the engine and doing a repaint. I've enjoyed having the bike in original condition and pretty good at that for a 33 year old bike. It is only original once!

I am leaning toward doing the complete job on the engine but there is some temptation to clean things up and put on the new piston. ?????

What a bummer at the beginning of riding season!!!

nola_fiddy

I'd pull it down and find that missing ring piece. I didn't do so on a Bultaco Persang and had a needle bearing destroy a new piston & cylinder.

Just do the basic rebuild, clean everything well & replace any seals/bearings that are old. Sounds like the cylinder is alright, but really look through the bearings in the bottom end. As you said, only original once.

If everything works out ok, don't paint it, clean & polish up all the parts, power wash the rolling frame (tear it down & regrease everything) then ride it! Or I can give you my address and just send it to me, I'll be sure to ride it for you

Jim

35 hours is a lot on 125 rings. Time plus decelleration. When you decellerate, you have lack of oil to top end. Wouldn't blame your mixture, but I'd also stick with the mixture you are most comforable with. I use 927 castor blend

Mitchie, I guess I learned my lesson the hard way (could have been worse though). This is my first 2-stroke bike and it was hard for me to believe the fact that I had to do ring maintenance so often. The manual recommends much more frequently than 35 hours and for now on, I will do the maintenance timely. I love riding the bike and plan to keep it as my main rider. I'm the third owner, with the second owner not putting any hours on the bike at all. I would guess the bike has less than 100 hours total over all these years. It still had one of the old single ring oem pistons when I got her. I will have to decide if I should get the crank serviced after I take a look at it.

thanks,
nola_fiddy