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Discussion Starter · #1 ·
Hi, I have a desire to build a Honda MT125r replica. I have most of the parts and Airtech makes the fairing and I like their parts. What I really need help with
is the expansion chamber. I have a 1975 Mt125 motor I hope to use, it's complete and in nice shape and I don't want to spend the time or money to chase down a CR motor, most are beat down and parts are expensive. I know the MT125r had a 6 speed, and the MT is only 5 but I'm ok with it.
I had a MT250 in 1974 and after it was broken in, I bought a Bassani pipe for it and that pipe really made it go. I also understand that back then the hot trick for CR250 guys was to take some engine parts from the MT250 to tone it down and make it a better desert racer? I believe the Mt125 shares the same design as the CR125m, and I was hoping that I could boost the MT engine with porting and a pipe and a bigger carb. My question is? does anyone on this forum have any old magazine articles or books on tuning for theMT125 or an expansion chamber plan they might share with me please? any leads on a fuel tank would help too,
 

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Discussion Starter · #4 ·
Thank you, I actually found a copy of that, and it has a lot of useful pictures for reference. I really am trying to do something a little different than that. From what I've read and research, the MT125r was pretty much the cr125m motor in the MT frame? They say it was so "pippy" that it needed the six speed trans to be any good. The MT motor has less power, and the CR pipe is made for the CR motors porting and higher compression. I think that there were people back in the 70's who had the MT125 and tuned on the MT motor and made pipes for them to hop them up and that's more what I'm looking for. I could buy a new DG performance pipe for the CR125m, but I would have to pretty much put a CR head and Cylinder on my MT cases and port match the cases to do that, and I don't know if I want to get that deep into it. I didn't want to buy the two-stroke pipe building software and do the math on it and build my own, last time I did I got a nasty computer virus in the deal. I'd much rather find a pipe plan and porting measurements that were designed for the MT motor. The old bike magazines used to print articles about this type of modifications pretty often. I have the "Vintage Collection series of books and the two-stroke book has a lot of reprints of this type of thing, but it doesn't show
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the MT125.
 
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Discussion Starter · #6 ·
Wow, I just saw the Cycle World article you posted, I was able to print it and add it to my stash of info, thanks! A lot of neat info, I doubt I will be able to 12500rpm out of my MT motor but I'm only building a replica bike that I might try out on an open track day, so I want a little tamer motor. This project is just for fun? I will also have to find a fuel tank and that looks to be a real problem. I haven't found anything like it yet.
 
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Discussion Starter · #7 ·
Thanks for the link to 13x 2-stroke. I signed on there and hope they can help me out also.
So, what I actually have, is a 1975 MT125 frame and motor and a CB200 front end with a hydraulic disk brake, 18" wheel. I will probably get the Airtech fairing and tail; I liked their parts for my CB450 replica build and might paint it to match since I have a lot of paint left over. I don't know that I'll go all out with this build, I will probably just clean up the frame and build up the motor the best I can. The other parts, I will just try to copy the photos as best as I can. The exhaust and fuel tank will probably be the toughest parts to replicate. The photos in the Cycle World article help allot. If I can get the specs on the pipe, I can build one using the MT head pipe to start off of.
 
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Sounds awesome fun, I would gear up to build my own pipes from scratch until I made one that worked for what I was riding. I'd be tempted to build an intake manifold to install a reed valve on it to get rid of the narrow power band too. Was terrific fun riding today we blazed 2km of new riding area :cool: I crashed hard a couple, going to go attempt to straighten my favourite brake lever now, hopefully it doesn't snap.
 

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Discussion Starter · #10 ·
Well, I thought about it more and decided to look into a cr125m lower motor. Everyone who responded about road racing, said I really should have a 6 speed trans so I'm going to try that Then I will just get the DG performance pipe and use the MT motor for parts, lots of the parts interchange. I figure after hearing all the stories, here, and my own mistakes through the years, maybe since I asked for advice, I should take it!
 
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