Vintage Dirt Bike Q & A

After seeing several posts about hopping up MR175's, I wanted to share with the group what mods I had done. I wanted a CR125 with more power to compete in the age classes (+40/+50). With the help of a local (Seattle) vintage 2-stroke tuner we spent two seasons building a CR125\MR175 vintage racer. We used a '75 CR125 rolling chassis and a MR175 motor with the following direct swaps.'76 CR125 transmission (6speed), clutch (straight cut primary gears), rotor and electronic ignition, '76 ignition cover and exhaust manifold. The cylinder was ported adding bridged exhaust eyebrows and a rear piston fed boost port and alterations to the port timing. A DG radial head was milled and reshaped to keep compression at 160psi and a copper head gasket was used. A 36mm Mikuni and Sudco intake manifold were used and adapted to a stock '74 CR125 airbox with an aftermarket carb boot. A custom down pipe was built to match the porting. The final result was just what I wanted but it took two years of pipe, carb trials while racing part time to find what worked the best. I will be happy to answer any questions and I will post pictures of the cylinder porting. Collin

Collin,

It's been a while since I've seen anything here about an MR175, and now four posts at once - excellent! Thanks for taking the trouble to post pictures, I know it's not easy here.

The bike's really cool! I know what you mean about years of work, hopefully I'm getting close to the end of my tunnel too... The CR/MR bike is still in the shop (I couldn't get it out if I wanted to, there's still too much snow), and I've spent the last couple of months concentrating on the MR175 since riding it wasn't an option. Today I'll put the top end back together, after about a month of changing the porting to what the computer said it should be.

I'll try and add some stuff to the rest of your posts in the near future, and I'm sure I'll have some questions too. Here are some pics of the '73 CR250M head I adapted to the 175:


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

yes, but it would be nice to have all 4 at one topic together.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

I agree but website only allows 5 pictures per post

then you have to klick on "post reply" and do them in the next post.
or use an image hoster like imageshack or similar.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Coll,

While I think of it - how has your DG head worked out? Mine's had nothing but issues from the start. I had a crack welded up and severe detonation damage mostly machined away, but last night I went to put it on and found that the front right bolt landing has been machined roughly 1/8" lower than the rest, which in turn are all about 1/4" lower than the stock head. One bolt bottomed on the stud, leaving me with no compression. I could just throw on some washers and live with it, but that's just not the way I do things - looks like another round of machine work is in my future.

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

Hey endo I live about an hour out of seattle and could really use someone to tune my mt250. I dont know of anyone that will do it and would like to know if your guy would do it?