Vintage Dirt Bike Q & A

74 cr125m only runs on high revs,no low end power
AS long as I am giving it full stick it won't bog down,if I have it in too high a gear I will not get any power unless I change down, so it seems to be reving quite high all the time.
Is there some way I can get the power band to come in at a lower rev?

My CR never had really good low end, but when stock it pulled hard from the midrange on up. After I ported it and threw on a giant carb on it, it ran like yours - either bogging or screaming, with me shifting all the time. Later the LH main bearing wore out & took out the LH crank seal and it got much worse, unrideable really.

If yours is stock, I'd check all the basics (compression, timing, leakage, etc.) and see what you find. More compression, possibly a different piston (some had shorter rear skirts and weaker low end) and pipe mods could help.

Ray

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

Hi and welcome to the club of CR125 tuning difficulty! I have been riding/racing '75 CR125's for a few years. These are the most finicky engines I have ever tried to tune for racing.

I tweaked, tested, re-carbed, re-piped, and re-jetted so many times I can't even remember what combinations I've tried.

I talked with a premier 'old school' porting guru (in his 70's) a few years ago about my plight. He told me the '75 5 port cylinder is VERY tricky to properly upgrade for racing purposes. He also said the '74 4 port cylinder is the easiest to tune. I happened to have one of the cylinders and threw it on. He was correct, they are much easier to tune.

Two of my friends ride '75 CR125's. One is stock and one is not. The stock setup when fresh is as quick as any other 125 from the period. The other re-tuned one (ported and piped) seems to be just as fast but pulls very well from low RPM's, which is unusual for these CR's.

My best-running race CR125, has a '74 cylinder, very long custom pipe, and a 32mm mikuni. It also has a WEBCO head which raises the compression while cooling better. I bought a wide variety of needle jets, needles and main jets. I was trying to compromise between better low to mid power without losing too much top end.

It takes several hours of testing to get the best combination, because the 2 stroke is a system that starts from the air filter and ends at the pipe stinger. The reed valve Yamahas will almost always pull better from the bottom, but the Elsie with it's 6 speed and lighter weight should allow equality overall on most tracks, and pull ahead on longer straighter tracks. Of course the rider makes all the difference ;o)