84 cr 125 low compression PLEASE HELP

dave Olejnik

May 8, 2007
I bought my brother a 84 cr125 for his first bike. The motor was saposeably rebuilt 1-2 years ago. The problem were having is it is almost impossible to start. we have to pullstart it forever to get it running. I checked everything it said to check in the repair book. I found The motor has low compression. It is only at 70 psi The honda dealer said it should be at least 130-150. It doesnt look like there is a problem with the gasket so Im guessing The motor needs new rings and possibly a piston. My question is How hard is this to do? Ive never rebuilt a 2 stroke engine. I have the manual that tells you step by step how to do this. But what do you think. Should I atempt it? If I do should I take the piston, and the cylnder to the dealer to make sure I get the right piston,and rings. And get the cylnder honed. I believe the motor is already 040 over. Is there anything else I should check before tearing it apart? My brother did overfill the clutch and transmission case with oil. I thought maybe he blew a seal and that was causing the low compression. But now that I think of it It always did seam to have low compression,and was always hard to start. What do you think I should do? I cant aford a 4-5 hundred dollar repair bill.


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.


Apr 25, 2000
Did you check the compression with the throttle wide open? That's the only way to get a real accurate check. 70psi will not start and barely run. If that's the way you checked it and then got 70psi. SELL IT if you can't afford a $400-500 repair bill because that's what it going to cost in the near future. Sounds like it's worn out in more places than the piston/cylinder area. Crank seals and bearings come to mind too!


Jan 27, 2000
I wouldn't suggest trying to sell it. It's a little hard to sell a bike that you have to pull to get started. It's possible you just may need new rings. Rings are not very expensive and while you have the cylinder off you could have your dealer check and measure your piston and cylinder for excessive wear. Buy a manual and do the labor yourself.

Check out lasleeve.com for a ring or ring and piston kit and aftermarket gasket kit. :cool:


Oct 21, 2006
you should have a lot of compression if its been bored out -
tear it apart-check the piston ,and rings- see if it needs rings, or a piston kit-
or by some crazy chance the piston might be in it backwards- yea this has happened to someone in this forum- someone put the piston in with the window on the exhaust side-

if you need a new kit get it from dennis kirk- ,a wiseco piston kit,and vesrah gaskets
hone out the cylinder lightly- to get rid of the oil build up- or sand it with some sand paper- 360 grit if your hand is small enough

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