Jump to content

03 CR125 Top End Rebuild

Recommended Posts

Picked up a 2003 CR125 a couple of weeks ago for $1000. Bike is in decent shape with some new parts here and there. Guy I got it from was clueless as to how to work on them and he got frustrated because it didn't "run right" and cut his losses and sold it. Said he had about $1800 into it with what he'd done, new rear tire, chain, sprockets, bars, etc. Claims Precision Cycle here in Orlando resleeved it recently and it read 160 psi. Horse-hockey because I tested the compression last night and it only mustered 95psi. Yes, done correctly, throttle wide open etc. Didn't retest with a little oil because with a reading that low, I figured I wasn't going to gain much more psi adding it and retesting. Think I'll build a leakdown rig to have in the garage for future service checks.

 

Massive mid bog when I rode it a few times but pulled decent on the top end when it got on the pipe after stabbing the clutch to get past the bog. Plug was wet and tons of spooge from the pipe joints and silencer. Had the original TMX and I picked up a 38mm PWK A/S recently and put that on....50 pilot, 182 main 2 out on the air screw although it doesn't seem to respond much to adjusting it like the pilot is too fat? Bike starts first or second kick but no difference in performance. I took a look through the exhaust port and can see the cylinder wall looks like nothing has been done to it...tons of light vertical marks, no gouges, no sign of crosshatching from what I could see so far. RC Valves are nice and clean though with no sign of carbon buildup. I'm going to tear it down further after work tonight, pull the top end and get a better look inside the cylinder. Already had ordered new crank seals and collar because it's something I knew would eventually have to be serviced anyway. I'll check the crank bearings for end play etc when I change the seals....hoping they're still in good shape without having to split the cases. Considering a couple of options....

 

Buy a new cylinder and piston kit (Wiseco or Namura?) and start fresh.

Take this jug to Precision and have it measured and bored/honed if it is indeed a steel sleeve and order the proper piston/rings and go from there.

In either case, my plan is to put it back together with the PWK and being around 90* temps with high humidity at seal level, starting with a 45 pilot and 180 main based on reading the PWK swap threads as a starting point. I don't think I'll need to be as rich as a 50/180 in the end here in Florida. I have a late model RM NEDG needle going in and the slide is a 5.5. Carb came off a 99 CR250. Anyone else in a warm humid environment that can share some jetting info? I know, every bike is different, but just looking for suggestions on where I can start safely with a brand new top end. I have an assortment of pilots from 40-55 and mains from 170-182.

 

Millenium wants close to what I can buy a new cylinder for to replate when you factor in shipping, that's even if this jug isn't already pooched or has a cast sleeve.Thoughts or other suggestions?

Share this post


Link to post
Share on other sites

OEM cylinder. Wiseco piston. No idea on a PWK jetting for the bike but my 02 with TMX has bone stock jetting and it runs great. Stock exhaust too.

When I picked mine up in March, also for $1,000, the mid range was a little lean. Previous owner had leaned the needle by one clip so I moved it back down to stock position (third clip) and the mid range power was decent with good top end pull. My 2002 has a 36mm carburetor though (which came stock).

Share this post


Link to post
Share on other sites

Well....just took the cylinder off and this motor is toast. Complete rebuild is now needed. Big end rod bearing has up and down play, little end was extremely sloppy, scoring on the side walls of the bottom of the jug so this piston was flopping around all over the place. Has a 54.94 Namura in it so it's way over stock bore. Apparently it is sleeved? May be able to bore and hone it but at this point, probably rather just go with a new jug, piston and bottom end. I anticipated this anyway for as cheap as I got it. I'm surprised this turd even ran, lol. 

IMG_3481.JPG

IMG_3482.JPG

  • Like 1

Share this post


Link to post
Share on other sites

Rebuild it and ride it.  You can't put a dollar amount on fun.  The 03 is a great bike and can be a fun bike.  It may not have been rated as well as other years but you can still have a great time on the bike. 

  • Like 2

Share this post


Link to post
Share on other sites

I agree but it's definitely getting rebuilt...for the simple fact that it isn't worth squat with a roasted motor. I'm not the type of person that would try to pass this onto someone else after band aiding it. It's a dirt bike...I plan on trail riding it with my kid and not MXing it so I don't put much value in a magazine rating that pitted it against the YZ and RM that year. Many suns have set since the days I used to be able to ride one of these things to it's fullest potential anyway, lol. New top and bottom end and it'll make a nice little bike to go out and have some fun with the kid....like frdbtr said, that's priceless to me. 

  • Like 2

Share this post


Link to post
Share on other sites

Well fortunately for me the cylinder is salvageable.....dropped it off this morning and ordered the new oversized piston. Once that's in, he'll bore it to spec and set my ring gap. 

  • Like 1

Share this post


Link to post
Share on other sites

Going in !!! Let the fun begin....made a little template to track the case bolts and taking pictures as I go in case I get stuck along the way ?

image.jpg

  • Like 1

Share this post


Link to post
Share on other sites

Just a little update on my progress....everything went smooth splitting the cases and rebuilding the motor. Hot Rods crank looks like a quality unit vs the tin can Wiseco crank. Stole a page out of Doug Colemans book on the head because I liked the color he used on various parts on his 250. Coming along nicely and it'll be time to fire it up later this week. 

IMG_3488.JPG

IMG_3489.JPG

IMG_3491.JPG

IMG_3502.JPG

IMG_3503.JPG

IMG_3510.JPG

  • Like 1

Share this post


Link to post
Share on other sites

Judgement day has cometh, lol. Fired right up....doing my heat cycles and making sure everything is good to go before I....Braaapppp :ride:

 

IMG_3523.JPG

  • Like 1

Share this post


Link to post
Share on other sites

Wow, what a difference. First ride out, took it fairly easy...half throttle or so, perhaps a smidge more:rolleyes:

A little loaded up at first but I expected that and it takes throttle well and doesn't bog like before the rebuild. Rode it up and down some power line trails down the street from me revving it just enough to get it into its midrange pull and coming onto the pipe to start seating the rings in. Nasty downpour came and it's still raining. I went with what I'm thinking is a little fat on the jetting, 50 pilot, 180 main on the PWK for this central Florida 95 degrees and humid climate. Rather be safe right now during the break in. 

 

No fluid leaks, rechecked the base and head bolts torque so looks like tomorrow I'll go back out and rip it a bit harder and go from there. Overall very happy with how everything went with the rebuild and turned out. Should make a fun little bike to go out and have some fun on with the kid. 

Share this post


Link to post
Share on other sites

Quick ride report:

Nice early morning start and this thing absolutely rips. Dialed in the jetting and I'm satisfied with how it runs throughout the rpm range. Ended up at a 48 pilot, 178 main, 1.5 out on the AS with an RM 250 NEDG needle in the middle clip. 88 degrees and sopping with humidity here in central Florida. Hopefully this may help someone else considering the PWK swap on choosing needles and jets. If you're in a cooler drier environment, a 50/180 may be the way to go...that was my starting point.  This Airstriker is tits to tune. Pulls strong now :thumbsup:

IMG_3528.JPG

IMG_3530.JPG

  • Like 1

Share this post


Link to post
Share on other sites

Just trail riding it, lol. I'm far too old to be racing. This one is actually now going to the kid and I'm looking to get a four stroke for me. He wants to start MXing it with his friends instead at our local track in Bithlo. I thought it was a little too big for him (5-4", 110 lbs) but he's been riding it and can kick it on his own so....he has no fear. He's about outgrown his Yamaha TTR so it makes sense to let him grow into this one instead. It's got a FWW on it to tame the hit for now...as he get's more used to the power, I'm going to remove it if MX'ing becomes his primary use for it. We'll see.

Share this post


Link to post
Share on other sites

Sick job on that rebuild.  That bike looks nice.

 

What size piston did you end up at?

  • Like 1

Share this post


Link to post
Share on other sites

56mm Wiseco in a sleeved cylinder....that's pretty much the limit I can go for available pistons. I'll have to go back to a new oem cylinder or keep my eye out on ebay for one in great shape that can be replated next time I go to rebuild it if this one is out of spec. This cylinder won't last forever unfortunately. Ended up with a mid bore at 134cc.

  • Like 1

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×