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
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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 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.