dave Olejnik

May 8, 2007
I bought my brother a 84 cr125 for his first bike. The motor was saposably rebuilt 1-2 years ago. My brother overfilled the clutch,trans oil resivor. He fouled a plug. I relized what he did, I drained the oil put a new plug in and it ran good. But the bike has been getting harder, and harder to start. I checked everything and relized it has really low compression. 70psi The honda dealer said it should be 130-160. I dont think there is a seal or gasket out. The bike has always been hard to start. Even before he overfilled it. He always had to poush start it. Im thinking it needs new rings,and possibly a piston. My question is HOW HARD IS THIS TO DO? Should I even atempt it. I have never rebuilt an engine before. The manual I have shows you step, by step how to do this. But Im still a little apprehensive on doing it. What do you think? I really cant afford a $400 repair bill. If I do decide to do it should I take the piston and the cylnder to the dealer to make sure I get the right parts, and to get the cylnder honed out? I think it is already 040 over. Also is it possible to have the head machined down a little to get a little more compression out of the engine? Any help would greatly be apreciated!!!! Thanks Dave.


Oct 19, 2006
It's pretty easy to do. With compression numbers that low, I would suggest you pull the cylinder and take it to your dirt bike mechanic/machine shop and have it bored to the next oversize and use the corresponding oversized piston. You can pull the pipe and carb/reeds and do a preliminary inspection of the piston/cylinder through the intake and exhaust ports.


Apr 8, 2007
ive got a 84 cr125 as well. if your running 70PSI you most likely need a new top end. Im running around 180PSI with a top end with about 5 hours on it. Doing a top end on a 2 stroke is very easy, i mean very, very easy. As long as you follow the manual's instructions you will be fine. I would reccomend taking it to the dealer to make sure you get the right size piston and get it honed as well. If its a Wiseco piston, you can match up the numbers on the head of the piston so that way you dont have to take it to the dealer to get it sized.

There are a cupple things you should check when taking the 84 apart. you will know what i mean but when you take the cylinder off, you can see a linkage part that sticks out of the crankcase and into the cylinder. These parts tend to break and honda no longer makes them. make sure it ok and that it moves freely without and binding. Also when your finished and your about to put everything back together you might want to check the ATAC chamber and make sure it working properly. if the clutch cover is on, the butterfly valve in the chamber should be completely open, if the cover is off the valve should be completely closed.
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