Is it time for a new top-end?

Hey guys,

I have an 04 cr125 that I've had for about 8 months. I bought it from the original owner who claimed that he only put around 20 hours on it, the bike was all original when I bought it. I've prolly put around 60-80hrs on it since I bought it. The bikes still all original and stock except for some power reeds that have around 10 hours on them.

Anyway, yesterday I went go out for a ride, first one in a couple weeks and it didn't want to start. Finally it kicked over. When I was riding it, it felt pretty sluggish, it lacked top-end and didn't feel crisp. Whenever I would shut it off, it would take a few kicks to start, when ussally it would go in one or two. I only ended up riding for about 15 mins because the bike just didn't feel right.

Is it time for a new-top end? I've never done one before and this is my first 2-stroke so I really don't know what to look for to know if it's time to replace it.

Thanks for any Help

get a compression tester and run a compression test.

If the compression is below 120 PSI you are going to need a top end

A 125 runs best with a new top end every 10 hrs. 20 hrs max. You are way beyond normal 125 top end maintenance time. It is easy. Do it yourself. Just keep everything clean and you'll be good.

A 125 runs best with a new top end every 10 hrs. 20 hrs max. You are way beyond normal 125 top end maintenance time. It is easy. Do it yourself. Just keep everything clean and you'll be good.

yea..if you race/ride the piss out of it...

do a comp test and see your compretion.

also, was the gas in the tank fresh or was it left in there for the few months you didnt ride it?

My CR250 was riding fine after 40 hours (I run an hour meter) and I still changed the top end. At the cost of a wiseco pro lite kit, you really want to chance what can go wrong without changing it?

Plus it's a 2 hour job.

Yes!!! You need a top end immediately! I know from experience, I had that same bike, rode it to about that same point, felt that same sluggish feeling, had trouble starting it, wound up shattering the piston. Had to take it to the shop, got it back 2 months later, new everything. If you want to, get a compression tester (free rental at auto zone), it should be about 120 minimum I think. But really, just do the top end. Its easy. Power valve can be a bit confusing on your first time, but it isn't hard to figure out.

To the post above, 20 hrs is ridiculous. It'll be 40 hours when it begins to lose compression unless you're really riding the piss out of it.

To the other post above, a 250 will outlast a 125 by twice as long.

I've parked the bike and gonna do one asap, do I have to bore the cylinder out or anything?

you shouldnt need to do anything to your cylinder if its not out of spec.

what size is the sparkplug hole in a 125? like 14mm or 18mm? because im about to get a comp tester. do they all work with each other?

on a 2t, assuming a tight head gasket and an in spec cylinder, compression is only going to tell you if the rings are worn. with a high revving 2 stroke, id be less worried about the rings and more worried about the fatigue that the piston develops. worn rings will result in blow by, which you can easily inspect for by just popping off the expansion chamber instead of going through the trouble of doing a compression test. add in that most people use cheap compression testers (wondering why their good running bike is only making 90psi), and compression numbers are almost meaningless to most folks. if you have no visible blow by, you're probably ok and your rings are still doing their job.

on that note, my last wiseco top end was ran for 53hours. absolutely no signs of blow by even with that much time on it. piston still looks good. i think, at least for a 250 2t, 40 hours, rering, another 40 hours and then replace, seems like a safe bet with a forged piston assuming desert use. racers may need to develope their own schedules based on measurements and experiences theyve had previously.

just my opinion

while it is apart take a look at the reeds, they can chip/crack and cause problems as well.

80-100 hrs. on a 125 2t topend is amazing that you were able to get that out if it. I normally will go about 50 hrs. on a 250 2t. Depending how hard I ride between topends I''ll do it sooner for cautious reasons (cheap insurance). The stock pistons are known for getting fatigue cracks on the skirts & breaking apart inside & taking out the cyl. & in some caseses also the bottom end. Wise choice by parking it & doing your topend. The newer Cr's have a nikasil coating inside the cyl. walls & won' need to be bored out but I would check for wear on the coating & replate if needed. I use warm soapy water with a red 3m scothbrite pad & clean the walls & measusre the specs of the cyl. before instalation. Also check your bottom end for play while your top is apart.

It's been awhile since I've been on TT, but I finally got sometime to work on the bike. I threw a compression tester on just because I was curious what it would read, and it showed 120psi. I then tore the bike apart and checked things out. I was surprised how easy everything came out, I only had alittle trouble getting the cylinder off. Cylinder Head was in spec, I didn't check the cylinder (because I do not know how to) but at 120psi could it be damaged? The rings had some play in them, are they supposed too, or should they just sit planted in the piston?

Does anyone have any tips to insure a smooth installation of the new parts?

80-100 hrs. on a 125 2t topend is amazing that you were able to get that out if it. I normally will go about 50 hrs. on a 250 2t. Depending how hard I ride between topends I''ll do it sooner for cautious reasons (cheap insurance). The stock pistons are known for getting fatigue cracks on the skirts & breaking apart inside & taking out the cyl. & in some caseses also the bottom end. Wise choice by parking it & doing your topend. The newer Cr's have a nikasil coating inside the cyl. walls & won' need to be bored out but I would check for wear on the coating & replate if needed. I use warm soapy water with a red 3m scothbrite pad & clean the walls & measusre the specs of the cyl. before instalation. Also check your bottom end for play while your top is apart.

About the Bottom End play. The rod has maybe 3mm of play side to side, is that too much?

About the Bottom End play. The rod has maybe 3mm of play side to side, is that too much?

I'm not familiar with the newer CR125's. If your planning on keeping the bike for some time I'd recomend getting a service manual. Maybe someone can chime in on the service specs for the cyl. & rod play for the 04 model.

As far as I'm aware sideways movement in the con rod is normal, my brand new one has about 2-3mm. It is up and down that shows wear.

As far as I'm aware sideways movement in the con rod is normal, my brand new one has about 2-3mm. It is up and down that shows wear.

What would be abnormal for up and down wear and how to I check it?

What would be abnormal for up and down wear and how to I check it?

If there is ANY up and down play your crankshaft will need to be replaced.

If there is ANY up and down play your crankshaft will need to be replaced.

Just checked it out, Felt nice and tight, no play:thumbsup:

I just threw the new piston in, the clips were a little hard but not too bad.

After working on the simplicity of a 2 stroke, I can't see why anyone would want a 4-stroke. There soo easy and cheap, anyone could do a rebuild on one of these.

Somewhat, you would be suprised at how many Ive fixed where they had put the piston on backwards and wrecked everything.

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