CR125 Low/No Compression


I am new to this but not a complete novice, tinkered but never got this serious, heres hoping you experts can give me some valuable advice before I waste lots of money,
I recently bought a 1996 CR125, everything on it seems fine except the engine and rear brake which both need repair, I want to start with the engine so here goes... by the way I bought this cheap because of these problems..........

The facts:-
1.) The engine does not / did not run....
2.) kick start pushes down easily indicating little - no compression
3.) when kicked down rapidly there is a spark at the plug
4.) seems to leak water from the front of the cylinder head.
5.) drained Oil which was a mixture of oil and water.

What else have I done and found:-
1.) removed top from cylinder, piston goes up and down with kick start.
2.) noticed piston moves about 1mm side to side when pushed with finger in the cylinder bore (is this normal) piston looks brand new could have been replaced (not sure).
3.) removed exhaust and viewed piston through port, piston ring is in position.
4.) removed Carb, reeds seem OK all in tact.
5.) Noticed some droplets of water below the piston.

I have not yet removed the cylinder head, but have been told that the problem is probably something to do with gasket leakage, before I buy the gasket set for just a head replacement does this sound correct, is there anything else I could check, to see if this is something else, I have the workshop manual on order and did not want to remove the head before it arrives, Im also unsure of what is under the covers on the side of the head (one has HPP on it)can anyone enlighten me please?

Your help would be much appreciated, thanks in advance,




It would be wise for you to wait for your manual to arrive. Sit down and read the manual. Then, read it again. :cool:

Sounds like your engine needs to come apart. The water in the crank would indicate a bad head gasket. The water in the gearbox would be a bad water pump seal or possibly a hole in the right side cover behind the water pump due to corrosion. If the bike has sat for any amount of time with water in the crank, you should have the crank rebuilt.

The piston should rock a little bit at the top, but not 1mm. You need to remove the piston and mic it at the bottom of the skirt. You should only have somewhere around .003 inch clearance at the bottom of the piston to the cylinder wall. If you have 1mm clearance, that's about 10 times too much.

If the piston has the proper clearance and has not been seized or damaged in any way, you might just need new rings. When you get it apart, you can take the piston and cylinder to a shop and they can mic it for you if you don't have the proper tools to do this.

The cover that says HPP on it is your power valve linkage. There again, you will need the manual to set this up properly.

Good luck. :cool:


Thanks for the reply, I shall wait for the manual then get the head off and get it checked out, I am worried more bout the lack of compression than the water, I am guessing I can fix the water problem with a new gasket set, unless of course like you say there is some corrosion somewhere, the piston was apparently replaced by a shop a couple of months ago and does look brand new so I am guessing that it is correct, even with a wrongly sized piston I would have expected some compression, with everything put together I put my hand over the exhaust kick the bike but feel nothing, would you expect this or should there be some back pressure? Could there be a problem with the crank seals and if so are they difficult to change?

I am just trying to gather as much info as I can before getting started, if you can add anything else then that would be great, if not then no problem.

Thanks again,



Just in case anyone is interested, traced this problem after removing the head, the Piston is a standard sized piston while the bore is sleaved and bored to 1.5mm oversize!!! :yikes:

New piston coming on Thursday hoping to have this up and running by Saturday...... :cool: