Hello folks.
A bit of a prelude. I'm an old guy - well, I suppose relatively older. Been retired for several years now. Once in a while I'll take-on a project just to keep busy and also as a help to family and friends alike. Due diligent research landed me here, as well as a couple of other venues - anyhow: Here goes;
A close family friend who is a bit of a tomboy asked if I'd be willing to help her with a 2003 CR125 which she purchased new shortly after winning somekind of BMX series back in the early 2000's. Evidently, right after purchase and by recommendation of one of her friends she had a pro-circuit pipe and spark arrestor fitted, with somebody supposedly rejetting it accordingly. Everything else is absolutely OEM stock.
Having time on my hands as well as all of us being rather fond of her, of course I said I would and gladly. The bike had sat dormant under a cover in a fully insulated garage in Reno, Nevada, unridden for well over a decade. She said the last time it was ridden was by a friend several years ago who, to her horror, and before she could get outside to stop him, kicked it over and immediately revved it up, lofting the wheel and ripping up the street without any warm-up time. According to her, she laid into him with a few choice words when he came back and that was that. Time passed and one day she decided to drain and refill the tank with fresh fuel and take it for a spin when she noticed it easily kicked through its stroke when attempting to start it and "something just isn't right". The bike went back into the garage and there it stayed until just recently.... and here we are.
Upon hearing this calamitous tale, I was relatively certain of what to expect. Evidence of a cold seizure..... and bingo:
Cylinder is being replated to OEM specification with a new Wossner forging. I will be drilling the exhaust bridge holes for additional lubrication/cooling. The primary focus on this refurb is not for MX use. She specifically asked me to set it up primarily for trail and lower-speed desert plonking. What surprised me is just how pristine everything else on the bike is. Original tires, chain, and sprockets and all the plastic. It really is a time capsule. I told her my initial assessment revealed low, low, hours and what a shame that her friend stuck a slug. She said she had ridden it a total of 3 times at maybe a couple of hours each - it shows. She then married...started snow skiing and the bike just sat.
I've stripped the bike down to the last bolt and am cleaning and regreasing everything. My question to some of you folks, many of whom I am sure have owned an '03 CR125 is not how to make it rip. I know, I know... I've read countless threads on what a great-handling bike with a gutless motor this is and how the YZ's and KTM's humiliated it.... at will. We don't want an Eric Gorr spec or a stroker or a big bore, so please don't go there.
I have rebuilt the KYB forks and shock, respringing them for her weight at 125 lbs.
As far as the motor goes. What makes sense is the 36mm airstriker in place of what is supposedly (I have no idea as I have never ridden this bike) a blubbery 38mm TMX? Perhaps a set of Vforce 3 reeds as well as a 5mm pipe spacer?
For those who have performed the airstriker swap, I understand the in/out carb necks are identical to the TMX but how about the throttle cable. Are they a straight swap or?
Thank you all for your collective wisdom, experience, comments and suggestions.
ODG
A flywheel weight and a big long silencer will tame it down.
Other options include a G2 throttle tamer, smaller rear sprocket etc and retarding back the timing a bit.
If the TMX38 is in good condition and is jetted well, then it will save a lot of time and faff to leave it, as opposed to fitting a PWK and having to get the jetting right. I'm not sure that 2mm smaller on the carb bore is as significant a change as people think
Cogent response, RussB. Being ignorant to how this thing runs, I've fallen victim to the social media propaganda. What I need to do is get it together, break it in, then assess the requirements for its intended useage. In reference to the carb, as well as the condition of everything else on the bike. I redundantly reiterate: It shows minimal wear everywhere. Had it been mine. It would show more useage in an hour than it does now.
The G2, big can, sprocket, timing, and a steahly weight are sensible suggestions. If anyone else has more to add, I'm all eyes.
ODG
They are different motors, but we have an '07 CR125. The kid we bought it from (thankfully low hours) had put a lawn mower spark plug in it to get it to run because the jetting was so far off. We did a top-end overhaul and bought a JD Jetting kit and lived happily ever after (bike is used for MX). I'd work with the Mikuni before junking it. Our bike runs really well with a Pro Circuit pipe, JD Jetting and is otherwise stock in the motor department.
I currently still own an 03 CR125 and trail ride it on occasion.. Unfortunately the motor went south years ago....but I do still remember how I had it set up. Drop a tooth on the front sprocket to a 12t....or find a 53t or 54t rear sprocket. Stock reeds (or a set that is the softest) offer the best "roll on power". Pipe spacers work really well to lower the rpm "hit", but the spacers that make any noticeable difference start at 10mm...the studs in the cylinder for the flange aren't long enough to fit a 10mm spacer...so it's not an easy modification.
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I just remembered that I also used a pro x piston for the 04 and up RM125 part # 01.3224. This piston requires the head to be machined to matched as the wrist pin to Crown Heights are different
Thank you, godog. Being fully aware of pistons being instrumental to port timing and the availability of cast/forged through several sources. What was the intended purpose?
ODG
The Suzuki piston mod and subsequent lowering the port timing is the same as "decking" or removing 1mm from the base of the cylinder. I chose this mod with the head machining so I wouldn't have to worry about any modifications to the power valve actuator arm or pre load on the governor itself.
Got it, godog. Admittedly, being the neophyte I am in this particular case, my naiveté glows like a beacon. The chiming in from RussB, JMCR250, and yourself provides some 'cud to chew'. You folks can objectively analyze any changes you have made on your machines, leaving those like me to subjectively parse through your much appreciated empirical advice which is very helpful indeed.
Coming up on 67 years old, I come from an era where we'd use the kill button to scrub velocity while keeping the top end lubed up on pre-powervalved 125's when keeping them pinned WOT. Being a decent intermediate on the cusp from the mid 70's to '82 I was 'given' a then-new 1981 YZ125 by a dealer who sponsored me, which was the advent of liquid cooling on MX'ers. A true contraption it was, what with the single radiator on the steering head—which heated the forks up to the point of fade making you pogo from one berm to the next. I gumby'd too many times and blew out my knees (go figure), broke my right radius and gave it up. I still ride though, albeit at a much saner, more age-appropriate pace with my brother on our late 90's xr250's or CT's, or our BMW boxers and Aprilia road bikes. I still cough dirt and bleed mud, and at this age it's fun to share and reminisce and learn about what's new and interact with like-minded folks such as yourselves.
In speaking with my brother on this, we've decided we should get her out in the dirt on one of our dirt bikes in order to deduce her off-road aptitude. Heck, I haven't slung a leg over a tiddler in decades. When I get this thing together along with observations gleaned from riding with her, we'll be better poised for any changes beyond a crisp and clean running as per OEM bike.
Thank you so very much everyone.
ODG
Good luck with the bike ODG. I've got a couple of years on you, but still enjoy thrashing my MX two strokes at age-appropriate speeds and risk levels.
Once you get it up and running, I think you'll enjoy playing with the 125. It's a good platform, just in need of some carb/motor fiddling to make it really enjoyable.
If the cylinder is still out getting replated you could have them skim .5 mm from the base.
Then use base gaskets to dial in squish to 1.0-1.1 mm
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