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1985 Frame Compatible with 1991 motor and components?

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Hello, I am in the process of rebuidling a wrecked 1991 XR250 and I am having a diffcult time finding information on the compatibility of other year model XR250's. I have found a titled 1985 frame and I am posting here to see if anyone may know if the 1991 XR250 motor, suspension , etc will fit on the titled 1985 frame.

 

Thank you.

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The 84/85 XR250R had a dual-carb versions of the 250rfvc engine with a slightly smaller frame than the single-carb 86-95 xr250; the 85 frame is more like the xr200 frame.  The smaller frame might be a slight advantage for woods-riding. 

 

The single-carb motors can be put in the earlier frames, but there's some effort involved.  There are threads on this exact topic.  here's a short version from my memory: 

 

The dual carbs have their intake boots straddling the centered rear shock, from what I understand, the 86-on xr's have the shock offset, so intake air routing will need consideration. 

 

Also, the clutch arm on the later motor was moved higher on the clutch cover, which means that a new bracket would need to be fabricated for the old frame.  Speaking of clutch covers, that's where the later motor's oil cooler plumbing originates - the earlier motor didn't have an oil cooler, so retaining the cooler will require a little more frame modification for mounting the cooler itself up at the steering head. 

 

The later forks (together with the triple-trees) will move without much issue, but attention must be paid to ride height and geometry.  Also, the 85 had a drum brake.  The swing-arms can be swapped, but the 91's rear disk brake will need its pedal, master cylinder, tubing, and what-not.

 

Sounds interesting for someone with motivation.

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