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'75 MX 125 VS. '75 YZ 125

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I bought a Yamaha MX 125 a while ago for $100. It runs, came with about $300 worth of new parts too! As i have heard, the YZ came with some fancier cosmetic parts, different cylinder porting, and i close-ratio gearbox. As a later project, I was wondering how different the porting jobs on the two different cylinders were so I could port the cylinder myself eventually to give this bike the 18 horsepower over the normal 16. I also want to give it the peaky and narrow powerband of the YZ. Any specifications on the ports would be great.

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I bought a Yamaha MX 125 a while ago for $100. It runs, came with about $300 worth of new parts too! As i have heard, the YZ came with some fancier cosmetic parts, different cylinder porting, and i close-ratio gearbox. As a later project, I was wondering how different the porting jobs on the two different cylinders were so I could port the cylinder myself eventually to give this bike the 18 horsepower over the normal 16. I also want to give it the peaky and narrow powerband of the YZ. Any specifications on the ports would be great.

just buy a 75 yz jug and head it will be easier/cheaper

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I also believe your 75 MX has the same porting as a 74 YZ but a smaller carb. The 76 is a completley different engine.

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Oh man. I read an article on the 1976 models. In 1976 they switched the porting up.

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If you can find a used CDI unit from the YZ to lighten up the fly wheel, do some minor port work and add a bigger carb that little MX will scream.

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I'll try to do that. Should the porting job go all the way up into the cylinder? And what size carb should I use? I have a few extra ones here. Two are from modern 85's and maybe 3 from a '77 250. I'll measure the sizes off the 250 later. The ones from the 85's are 28mm. Is the current one on the MX a 26? I don't have it at my house to measure it.

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Yeah 36 would be way to much. It would be fun to try those 85 carbs with stock porting just to see what happens, just make sure they are jetted rich enough so you don't melt down. I wouldn't go any bigger then 32.

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How much richer should I run it on them? I might try running a PWK 28 on it. the other ones were a bit older and were round slides.

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