Just bought 98 cr125, have questions about compression

I just picked up a 98 cr125 today with low hours. When the owner started it up it sounded fine (don't know much about 2 strokes haha) and I was told all it needed was a rejetting. Well long story short I get home and try to start the thing and the compression is so bad I can push the kick starter down all the way to the foot peg with my hand, it starts fine however. Does it need a new top end? Any help would be appreciated

125's can be started by hand, so I wouldn't use that to determine your compression.  Get a compression tester.  You can pull the exhaust and look through the port at the piston to see how it looks.  Or, for the cost of some gaskets, you could pull the top end and inspect it all to see how it is looking inside the cylinder.

I'll definitely be getting a compression gauge. When you say you could start a 125 by hand do you mean you mean you could easily? Because I can literally put my foot on the kick start without applying any weight and it goes all the way down, my xr100 is noticeably harder to kick if that helps.

pull the exhaust and have a look inside..take a pic of the piston half way up so you can see the top, rings and some piston.


and get a compression tester...pretty cheap and you will use it more than you think.


you want to see 180- 190 if the top end is fairly new...the lower you go the older it is.


as a side note always listen for a ting ting ting sound in the exhaust, that signals something inside slapping around. 


at that point your usually overdue for a new piston, keep riding it and it will grenade (if it starts doing it out on the trails baby it back). 

I'll take a picture and post it when I get home. Is it for sure something that is in the top end, like I won't have to do a full rebuild?

hard to say, as long as you dont hear any noises (clunking or thumping) the bottom end might/should be ok.


its best if you have to do a piston to have a look down inside, grab the connecting rod and see if you feel any up/down movement, also bend it left right.....it should be snug with no movement...aside from it sliding left and right.


top ends are easy to do and relatively cheap, about 150 ish all in.


get a quality piston, I am returning to mx after a few years off so if someone can recommend a good piston kit. wiseco has always been good.


measure the bore of the cylinder not the piston, measure a few different spots to make sure you get a proper reading. or take it to a machinist / local shop to get measured...prob charge you $20..  well worth it.  if you get a shop to measure it..prob best to order it from him.


you will need a torque wrench to put it back together.

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