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Yzf 250 conversion to a 150 thumper

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I am thinking about converting a blown up yzf 250 to a yzf 125 or 150. I would buy a new engine and make it fit the frame. I am a novice at fabricating. How much headache am I looking at. I want to use it to race a thumper class. I am a bigger guy and would like to have the extra leg room with a larger frame.

Thanks

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not sure if it would work. you would need large engine mounts and thats the easy bit. trying to get the engine to bolt onto the swing arm wouldnt work. what about the carb? use 150 carb and try to get to work with 250f air boot? dont think it would work. go to enginering forum and read the kxf exc build thread. he put's a 300 exc motor in a kxf frame. huge job with litteraly 100's of hours. it should be on the first page or two

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Not surprised it would be a huge job. I think it should shelf this idea. What about a yz 125 frame. Would a 125 engine fit in that? I am a ktm guy and know nothing about the yama dog. Seems like alot of guys are putting yz parts on the ttr.

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Best way to do this is get a 144 kit for a YZ125 2T. Cheaper in the long run. If you want to blaze trails, then what I would do is sleeve down the 250 cylinder, and contact a piston manufacturer like Wiseco and see what they might have. The engine is a short stroke design similar to an XR200 XR185, so that's the way to do it, just not sure of suppliers of parts. Good luck.

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Do a little research, check the swingarm piviot bolt on a 125 and a 250F if they are the same diameter next check the width of the two motors at the swing arm bosses on the motors. The other major diffculity may be the exhaust port location.

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Keep in mind with the engine/swingarm interface you can always slightly open up the holes in the engine and mill the engine narrower. You can also bush down the holes to a smaller diameter and add spacers to make it wider.

Is there any reason that the swing arm pivot needs to pass through the engine at all? Other than the fact it is a really good mounting point. If you were clever with your design I see no reason why you couldn't use that interface on the engine to attach to the frame in a different way. You'd need to be careful about this though depending on what you'll be doing in the way of racing and the fact you're a big guy. A 90lb girl derping around mild trails, you could probably just support the engine with the head stay, but you would need to do a better job if it's some 300 lb gorilla casing 100 foot jumps every day.

Depending on your time, skill, money, and how bad you want it, I wouldn't say it's a question of can't or can not but possibly a matter of should or should not? I would say go for it, but it's not my time money or want to do such a thing:thumbsup:

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