Vintage Dirt Bike Q & A

I am building a 1975 MR 175 for AMA Vintage Cross Country and have a few questions. What is a paint code for Glare Green? I really don't want to buy the $70.00 per 8 ounces I see online. We will probably redo a couple of tanks so the bike looks good at the start of the race. Also I believe the bottom triple clamp is not aluminum, will a CR125 triple tree swap and if so which models will work? Thanks for your help.

I believe the Glare Green paint code is G-44. The '74 (definitely) and '75 (possibly) CR125M steering stem/triple clamps will fit the frame. I assume the bike must remain fairly stock?

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

Thanks Mr Ivers, it has been impossible to find out what G-44 equates to in today's paint codes but I will keep digging. I have read articles about your MR's and they have been very helpful. My plan for AMA Vintage is to follow the "Pops" Yoshimura and Captain Jack Sparrow interpretation for rules, i.e. that rules are only guidelines. My goals are to build a bike that can win the Cross Country race. I will also at some point field a motocross version of an MR175 and the idea of a MR 175 Trials contender seems to be an easy build. I am fortunate that I have a friend with a boneyard that I can get into for various parts. I will be asking the AMA about what forks I can use, what type and model of front brake is allowable, and I have an AMP link here that I think I can replicate and mod to fit the MR. We have found out in the field, an MR with a 1976 CR 125 swingarm on it, it is the same length as the stock MR arm and should be made of Chrome Moly so we may move the mounts back and use that. I do not know if you are aware but National Enduro Champion Terry Cunningham made his step up to the elite on an 1975 MR 175. He rode it in District 11 races and such and that led to his factory Husky ride. TC and I are the same age and from the same area, as are Jeff Russell and Kevin Brown, 2 outstanding riders who may have had even more success if they had raced MR's earlier in their careers. I have seen some photos of the 1979 XL 125S and some of those seem to have a bigger fork that looks similar to the MR fork but with bigger tubes. The travel is too long for the rules but that can be corrected when the forks are fixed. I have Boge shocks for the Cross Country bike but will fit something better for the MX bike when I start on that bike. I have emailed Eric Gorr about reed valves, big bores, etc and I think I saw where you had done some of the engine mods. I appreciate any knowledge you have on the bikes, I have an article from the Sept 1976 Motorcyclist mag about Jay Tullis' bike and the mods he made. If it has not been scanned and uploaded I will try to get that done. I have been told that Popular Cycling had a good mod article as well. Thanks again for the reply and thank you Nathan for this great site. David L. Patton, Jr.

David,

If you can use these, I'd suggest:

'79 XR185 forks (the MR175 or early CR125M damper rods should work for travel restriction)
CR125M ignition (w/balanced crank or flywheel weight)
MT125 transmission
gutted MT125 or MR175 pipe w/silencer

The porting diagram / info in my 'MC175...' thread should NOT be used - I thought I had deleted this from everywhere I'd posted it, but apparently not. The porting below works extremely well:



I'll try to update that thread when I get the chance - a lot of info in it has been superseded, and there's a ton of new stuff as well.

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

Thank you very much for the info. We will be building a big bore reed valve motor, will the porting specs work for it as well? We will use a reed mount from a 86-99 Cr 125 that Eric Gorr recommended with either a VForce or Boyesen Reed assembly. He mentioned that there is a 2mm oversize piton out there and they can lengthen the stroke but that may be beyond our budget right now. Since I need the crank rebuilt just to be safe I plan on the longer rod option, hopefully before winter so I can get everything together by Spring. I appreciate the fork info, now I will wait for the AMA to tell what the rules will allow. Thanks again.

My reed motor is still in the initial planning stages, but I've cut out the intake section from an '86 Kawasaki KDX200 air-cooled cylinder for use with VForce3 reeds and a Keihin PWK35 Air Stryker carb, and will also use the KDX piston, rod, and bearings (Crank Works rebuilt an MR175 crank with these parts recently, for a friend of mine). I'll be using my engmod2t simulation program over the coming months to fine tune the piston-port cylinder and develop the reed one from scratch, I'll pass that info on as soon as I have it. Nothing fancy; single channel-type boost port & semi-trapezoid exhaust, L/R ports from reed chamber to transfer passages. I don't expect the port timings to be very different from what I'm using now - perhaps a bit more transfer duration - but the pipe will be. It will be great to finally have a decent amount of crankcase volume to work with.

Re the big-bore setup - I expect the 171cc reed motor to make about 35 or so rwhp, which is probably more than most of the drivetrain can reliably handle. Even version 1 of the piston-port motor (I'm guessing maybe 26 rwhp - the simulator indicates @ 32 at the crank) managed to crack the OEM piston nearly in half.

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

Awesome. This is going to be fun.