Vintage Dirt Bike Q & A

I finally got to try out my newly-modified MT125 pipe in the dirt today, so I figured I'd post the results for the half-dozen people worldwide who might be interested.

This pipe had almost the exact same powerband as the Jemco MR175 expansion chamber it replaced; a smooth surge from maybe 3,000 to 8,000 rpm. Off-idle response was better than the Jemco pipe (probably due to back pressure), and the MT pipe was way quieter, maybe 6dB or something like that. The stock MR175 pipe is nothing like this; it's got good low end, then a hard hit in the low midrange followed by a flat rev-out, like a vintage four-stroke. Drilling out the OEM MR pipe 1/2" helps top end, as I mentioned in another post. The Jemco pipe did have a bit more power, maybe 1 hp more from 3K - 8K.

I also tried this pipe a whie ago in a '74 CR125M with a rev-ported cylinder, 36mm carb and leaking crank seal . These results are obviously pretty meaningless, but at any rate... the MT pipe had better low end than the LOP pipe that was on the bike, and started to get some decent power around 7K, but the cylinder has a very narrow 8K - 10K powerband so the pipe was signing off as the motor was kicking in. On a stock motor & carb, the MT pipe would have worked much better, especially if a high-compression head was thrown into the mix. I had to make up my own center and rear mounts, and drill a small (invisible) hole in a frame gusset.

NOTE: The MT125 pipe has to be modified slightly to bolt on the MR175 (the header flange has to be bent upward slightly). I've included a picture that shows what needs to be done. The header should be heated over a large area starting at about 1'' in from the inlet flange and continuing up the pipe at least 4" or so. The modified pipe tucks in better than the stock MR pipe; my leg just barely touches the heat shield, and probably wouldn't touch the pipe body with the shield off (I'll try that tomorrow). The center pipe mount may have to be bent inwards slightly, but the MT rear pipe mount as shown in the parts diagrams should work fine as is (unfortunately, I don't have this mount and am using a tie-wrap now).

If you're looking for a quiet up pipe to use your CR in the woods - as I was - your options are pretty limited. The MT125 pipes are always showing up on eBay and may be a good alternative to get you up and running quickly.

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