Vintage Dirt Bike Q & A

I have a 1976 CR125 & would be glad to sacrifice some top end for a little more mid range power. Is a MT125 cylinder & piston a bolt on swap with a CR?

I think the MT125 cylinder has the thinner liner and would be a better fit to the '74-'75 CR cases, but it may also fit the '76-'78 cases, I'm not sure.

Unless you plan to do a fair amount of work to the MT cylinder, I think you'll be disappointed with this swap. The MT intake port is higher and the exhaust port top is lower, which (in general) are both better for low and midrange, but the exhaust port, total transfer port area, and intake manifold opening are all much smaller which will likely reduce power everywhere.

Easy things to try instead with the CR top end would be increasing compression (you may need octane booster afterwards), advancing the igntion timing slightly, and using a smaller carb (28mm, OEM '76 carb is 30mm). If you're using a vintage pipe you'll still be locked into the top-end powerband thing, but you should still see some improvement in the lows and mid.

On my CR, I'll be using the '75 cylinder (port layout similar to '76) with increased compression, porting (stock timings, flow increased), a much longer pipe, and probably a 29mm carb. The intake port will still be lower than I'd like, but it should run well everywhere but the very bottom. If you really want to dump some money into it, apparently Rich Gagnon can mate a modern reed setup to the cylinder, which should make a huge difference everywhere:

http://www.richstaylordporting.com/intake.html

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thank's Ray! My past experience was with a '74 & '75 CR125.......like 35 years ago. This past march I bought the 1976 CR125. I assumed they would have similar engines, but now I'm wondering if the 1976 powerband was enhanced at the expense of the lower end. I just have many fond memories of trail riding with the 74 & 75 cr125's, but this 76 seems a bit more wild. Are they that different, or did I just get old! I'm trying to chase some turtles with this CR but the contest is over before it gets started! I did put a 59T sprocket on the back, I used to do that back in the day too.

The '76 CR125 motor's porting is quite similar to the '75, I believe (I don't have port timing specs for any of the CR125M's as apparently Honda never put out shop manuals for them!). The '76 also has the larger carb, different pipe, and also advanced ignition timing (22 degrees BTDC vc 18 degrees on the '74).

I also went the big-sprocket route briefly (58T), but it was a big mistake for me; I was shifting all the time and going nowhere fast. The smaller sprockets let the motor pull longer in each gear, and put more power into the ground instead of wheelspin. You might want to try a 16T front sprocket with your 59T, which gives a bit lower gearing (@ 3.7:1) than the stock 3.5:1. The JT JTF264.16 is what I use, available on Amazon, eBay, etc. JT also makes steel rear sprockets (JTR269.56 is the largest).

My '74 gradually lost all of its low end - turned out to be a leaky LH crank seal, behind the ignition. By the time I found it the LH main bearing died, so now that motor is awaiting rebuild. I did everything wrong with that bike (36mm carb, stupid porting, low compression, never checked the timing, etc.), time to make amends.

If your motor has been well maintained and is in good shape, you might be able to find a bunch more mids just through compression, jetting, timing, and gearing. The next step I'd make after that would be to lengthen the center pipe section 75 to 100mm. Getting more mids through porting isn't really an option with the CR cylinder, as it would involve putting metal back rather than removing it. If you're doing a top-end rebuild, be sure not to use the optional racing piston with the -820 suffix, as this will reduce low end even more.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks again Ray for your knowledge. LH crank seal, timing & jetting all need looked at. I bought this bike because it looked to be in really good original shape, not messed with. But that probably means original crank seals too. When I got my 74 & 75 cr125's, 35 years ago they were beat, broken & in pieces (and cheap!). So, 'till I got to ride them, everything like seals etc was replaced. I'll never forget.....standing at the Honda parts counter on a busy Saturday, ordering a big long list of parts. The big mean counterman said "why don't you just go and buy a whole new bike, kid". And to further mix up my memories of what to expect out of this bike, I made a new down pipe because the old one was to smashed up to use. By accident I may have made a really good pipe? You gave me some really good input on troubleshooting & year comparison, many thanks Ray!!

No problem at all - glad to help.

The LH crank seal is easy to replace once the ignition is out, and setting the timing to stock specs can be done by determining TDC exactly, then lining up the 'T' mark on the rotor with the stator mark (it probably won't be spot on, but should be close enough to start with). As far as jetting goes, I'd first inspect your needle and needle jet for wear (out of nine eBay Keihin carbs I've bought, all but one showed significant wear), otherwise you'll be wondering why the bike's so rich and sluggish below 3/4 throttle and pilot jet changes make little difference.

I remember well standing at the counter with parts list in hand, while the guy went through the microfiche pages one by one. He wasn't grumpy, he was more like, "you want anything else, kid? How about some fenders to go with that tank?"

It could well be that your pipe worked better than the stocker. A pipe with a decent front section and a smashed-up middle would be an ideal candidate for lengthening; Jon or Dan at jemcoexhaust.com should be able to make up whatever length and diameter of extension section you need. There's also a bunch of companies making down pipes, eBay, etc.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I can attest to Rich Gagnons reed valve setup. I had him port my cr250 cylinder and add a reed valve, I was very impressed with the work. Now I just got to work out the head shake problem and the bike will be really fun.