Vintage Dirt Bike Q & A

To all who suggested switching to a CR125 ignition, thank you. Now I just have a couple of performance questions.

What should the timing be when I check it?

Now that it is converted, I am experiencing what might be called sputtering/motor-boating. I experience this with other 2 strokes, but with them it usually seems to go away after warming up. It sounds like a raspberry or sputters and has little power when you give it gas, then may seem to catch once it hits the power range. Could this be from timing or is it more likely running rich or running lean?

Also, how much power should I feel from this bike? Is it more like my kids XR100, my MT 250, or my RM250; and where in this range should it fall?

If the motor's basically stock, you could try 2.1mm / 21.5 degrees BTDC. Right now I'm at 1.9mm / 20.4 degrees, and hope to try 1.8mm next week.

The sputtering / motorboating behavior is most likely an over-rich condition caused by carb jetting.

A stock MR175 has 14 rear-wheel hp, an MT250 about 18 along with a bunch more low end (and weight). The MR should seem noticeably more powerful than the XR but less so than the MT, especially down low.

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

I am sorry to hear about it being less that the MT, since the MT is already pretty weak; but it does tell me that I probably don't have a lot more to have to work at to get it to its expected output.

Regarding the over-rich condition: the change to the cdi ignition shouldn't have any affect on the jetting should it? Also, before I go and purchase jets, could it be something else? I strongly believe everthing is stock on the bike, so could it just be an adjustment: would the needle setting, float, or air needle give me this condition? If so, any suggestions on an approach to deconstruct the cause?

BTW, thanks for the input on the ignition question, I will check the timing tomorrow. Could the timing be a cause? Also, when you are measuring the distance btdc, is that before or after when turning the crank counterclockwise?

I think the MR175 was basically designed as a trail plonker. With a 125-sized clutch and gearbox they surely weren't building a racer; probably just trying to sell a few more 2-stroke trail and enduro-type bikes before the 4-stroke lines replaced them.


The CDI swap and/or timing won't affect the jetting, but either or both could certainly affect the way the motor runs. Did the motor run fine with the stock ignition? How was the ignition swap accomplished? Are you using the MR coil, was the wiring straightforward, how was the CDI unit mounted, etc.?

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

The original ignition failed. The rotor cracked over the w-key and broke off the brush from the point. I only had it running for maybe 20 minutes before the failure. It ran OK as I remember; that is once it was wound out a bit. I was trying to run it at 3/4 to full throttle when it backfired and died (rotor cracked). The reason I say it ran OK is that when I would gun it, I could feel it slip. That is the pitch would raise incomensurate to my acceleration. I had that with the CDI as well; though I cannot confirm in either case whether it was the rear wheel or the clutch that was slipping. (I will need to check that tomorrow.) In short, I don't remember whether it ran better or not.

I completely replaced the ignition. The only part I kept was an aftermarket kill button. The rotor, stator, coil and cdi are all new. I believe they came from a 76 or 77 cr125, but they look almost brand new. I mounted the coil by one bolt to the coil flange on the frame. The CDI I mounted infront of the airbox on the frame behind the carb. I plan to move it up under the gas tank next to the coil using the other coil flange. It was all plug and play, and after I figured out the kill button wiring, it started on the second kick.

I should note the muffler is present only as a figure. I believe it was an MT or a CR muffler. It is attached to a metal bracket in the rear and is barely linked by a weak looking spring at the front. I think I have one or two spare mufflers I got with the bikes, so I plan to replace this one. I don't know what the lack of back pressure would do to the performance.

It sounds like your clutch is slipping. A quick trip on pavement will confirm this. Barnett 501-29-06072 clutch springs are highly recommended.

You might want to try disconnecting the kill switch, just to eliminate it as a source of trouble.

A lot of power can be lost at the pipe/manifold joint. I recently fixed a slight leak there and was surprised how much better the bike ran afterwards.

The MR175 pipe has a short large-diameter tailpipe, the MT125 pipe a longer small-diameter one. The MT pipe is very restrictive and doesn't work well on the MR unless modified.

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

Them mt250 had a cloth like casket the fit I the pipe at the manifold to reduce leaks. What Did you use to reduce your leak?

Right now I'm using a Honda 91301-MG9-003 O-ring in the manifold's piston-ring groove instead of the ring, along with the circular asbestos gasket at the end of the manifold and hi-temp sealant. A Kawasaki 671E2535 O-ring should work really well once it's on, but's it's not that easy to install.

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