Cr125 compression and top end noise

Maybe a broken piston skirt??? I’ve also seen them partially seize the big end bearing on the rod, or the wrist pins and make knocking noises... it’s a two stroke; Best advice I can give you is to take apart the top end and check it out... gasket kits are cheap enough... plus, it gives you a little peace of mind...

Give the top end a look. Might as well if you are rebuilding.

What was compression when fresh?

I haven't torn it down yet to reply any further on the condition, going to tear it apart either later today or tomorrow. I bought it used and my buddy said he brought it to a shop which said everything was good (which I know how some shops are). He also sold it to me with it leaking gas with the petcock turned on so obviously the shop didn't do too hot of a job. Ill post some pics once torn down.

Very low compression for a fresh 125.
Bad fit of the piston to cylinder will cause noise.

Paw Paw

Took the cylinder and piston out today. They both look good with no markings or anything on them. The piston reads 53.7mm and the cylinder read 53.8mm. I can’t find a 53.7mm piston online though. Also noticed some play in the crankshaft, approximately 1/4 inch of play side to side but virtually nothing forward and back.

I believe the std bore on a CR is 54.00mm so I don’t think those numbers can be correct...Also the jaws on those calipers don’t look deep enough to get a descent reading on the piston... Piston and bore measurements should really be taken with a micrometer and a dial bore gauge; or at least a set of snap gauges that you can use an outside micrometer with to get a more accurate reading (there’s enough articles out there on measuring pistons and bores)... The only thing calipers will show you is if the bore or piston is severely worn, they will not show you any taper in the cylinder... Also can you feel any material change on those marks leading up from the bridges on the transfer ports??? Those would be my only area of concern... I don’t see any plating issues occurring which is good... Finally, There is a part number stamped into the piston dome I would look that up and that should at least give you a rough idea of what MM the piston was/is...

Paul_Pitzonka wrote:

I believe the std bore on a CR is 54.00mm so I don’t think those numbers can be correct...Also the jaws on those calipers don’t look deep enough to get a descent reading on the piston... Piston and bore measurements should really be taken with a micrometer and a dial bore gauge; or at least a set of snap gauges that you can use an outside micrometer with to get a more accurate reading (there’s enough articles out there on measuring pistons and bores)... The only thing calipers will show you is if the bore or piston is severely worn, they will not show you any taper in the cylinder... Also can you feel any material change on those marks leading up from the bridges on the transfer ports??? Those would be my only area of concern... I don’t see any plating issues occurring which is good... Finally, There is a part number stamped into the piston dome I would look that up and that should at least give you a rough idea of what MM the piston was/is...

Yes I can feel those scratches by those ports. What does that indicate?

That cylinder shows a lot of wear.

Paw Paw

if you can catch the marks with a fingernail odds are the cylinders worn out and needs a replate...more than likely that is the cause of your low compression... also see if you can feel a ridge at the top of the cylinder where the rings/ringland stop this is a very quick way to see if a bore has an excess amount of wear... there’s a lot of companies out there that do cylinder replating (do some research)...I’ve had a lot of luck with millenium technologies and the price is way cheaper than a new cylinder... They’ll even finish hone the cylinder to the corresponding piston size

Okay looks like ill have to start looking into that. Good thing this is a project bike and my kx250f is my main track bike. I also need to do the front forks and rear shock rebuilds and a bunch of other little things. Learning process as I go and luckily this forum has a ton of knowledgeable people on it!

Changed out the piston today and it has a worlds of difference in compression and the rattling has gone way down from what it used to be. Still smokes pretty badly mainly when revved. Not too bad at idle. Do you guys think it could be the reeds? I really don’t see how it could be the jetting.