Vintage Dirt Bike Q & A

I just got back from testing the modified MT125 pipe on my MR175. Two thumbs up.

This bike normally doesn't like to wheelie, but today it did. This modded MT pipe has less low end than any of my MR175 pipes, but it has more top end than all except the hybrid MT/MR pipe I put together a couple of weeks ago. I think the top end on both these modified pipes is about the same, but since the MT pipe has less down low it seems like it has more up top, and that top end comes on fairly hard (hence the wheelies). It also will easily break the rear wheel loose and slide in 3rd gear now, which it never really did before. It pulls OK until maybe 5K, then really kicks in and zings right up to 8-9K, maybe higher.

This pipe should also be a good match to a stock or modified CR125M or MT125; on these bikes no bending is required, as the 125 exhaust manifold is at the proper height and angle. It will obviously bolt right onto an MT125 frame, but center and rear pipe mount mods will be needed on a CR125M (I did it last year, it wasn't too bad).

The mods are a three-stage deal:

1) Heat and bend inlet flange up about 1/32" (see pic for bending marks, and rusty area where heat was applied)
2) Remove access plate and Swiss-cheese spark arrestor w/drill
3) Modify center and rear pipe mounts

This is much less work than the hybrid pipe, which also requires a bunch of cutting and welding; I think the results are worth it, but you might want to try just the MT125 pipe mods first. If you find you need the same power all the way down to nearly idle, then the hybrid's for you.

If anyone's interested in trying it out, let me know and I'll make up detailed instructions. The MR175 motor - like any 2-stroke - really needs a decent pipe to perform well, and these modified Honda pipes seem to work just fine. I wish there were lots of good pipe options available and all this work wouldn't be necessary, but that's just not the case with these bikes.

Composite picture below - from top to bottom: bent header section w/dimensions and marks, rear mount area, modified center mount w/spacers, and pipe on bike (ignore the tie wrap, the welder's shop was closed today and I wanted to try the pipe out). Underneath that is the hybrid MT/MR pipe, after filling in the heat-shield mounts w/JB Weld and painting.

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