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XL250s 1979 more power

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Before I start I may as well mention I have an XR500 I use in MX. I also have an XL250 motor kicking around. I occasionally find the XR a bit of a handful, so was wondering what can be done to the lowly XL motor, for MX racing. I have seen they can be taken out to 305cc, but can find no info on how to do this?

Can a jug and piston from a later XR350 be fitted, as I think 350-400cc would be ideal? Any advice and thoughts greatly appreciated

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No there is a lot you can do to that motor  . To start with what you got is pull the base gasket out ,carefully seal barrel and case . Pull both side cases break the balance chain take the balance shaft out don,t worry about the mane shaft one . It will run fine . Powroll mite be able to help with a bore kit . What you need to do is take to me . There was a lot of things we did for the race motors .

 

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1982 XR/XL bottom end with 6speed and a gear driven balancer.

XT350 piston (12mm Oversize)in a custom sleeve, cases machined gives 338cc!!

Ported inlet  of head to match an XR500 inlet manifold and carb.

     Still revs like a 250 but total power to match the 500, and soooo much lighter and manageable.....

 

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Yes that is the better combo , but he  has a 79 . The gear box will not take the big bore  for long . I still have factory race  cranks and race parts in my garage . We built big power but the weak link was the gear box . We did not break a 6 speed box ,but they would not shift well with the toque and HP we were making . 

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What is the weak part of the gearboxes? I know you can weld steel rings on to the earlier motorsport XL's Are gearbox parts interchangeable with the 500s?

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1 hour ago, chamber66 said:

What is the weak part of the gearboxes? I know you can weld steel rings on to the earlier motorsport XL's Are gearbox parts interchangeable with the 500s?

Yes, 5speed clusters interchange. Have found 250 gears in 500's before. Not sure how long they'd last......

 250 may be different, but 500's are better off with balancers intact. Smoother, free-er revving (comparatively...)

Edited by Muzz67

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I have taken the balancers out of the 500, also have the smaller flywheel. I remember your posts on this before

 

Looks like I will try the 338cc set up  probably

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raced VMX on '79XR250 and 500 for years.  bikes really similar on paper, yet 250 able to be ridden flat out all day. 500 much more tiring to ride hard.

  mates all had heavily modded 500's (540's and 590 cc) but didn't have much advantage over my standard one, except on longer  straights. HP not everything in slippery conditions.

  338 was pretty good, could ride hard and long, not much slower than standard 500.

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In 500 and the 250 5 speed it was 3rd gear. there was after market gears for the 500 years a go .

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52 minutes ago, Muzz67 said:

raced VMX on '79XR250 and 500 for years.  bikes really similar on paper, yet 250 able to be ridden flat out all day. 500 much more tiring to ride hard.

  mates all had heavily modded 500's (540's and 590 cc) but didn't have much advantage over my standard one, except on longer  straights. HP not everything in slippery conditions.

  338 was pretty good, could ride hard and long, not much slower than standard 500.

This is the problem with my 540 Curtis. Tracks over here are fairly tight and slippery. I am going to drop the motor in to a 76 Husky frame, if I go ahead with it. we have a pre78 class over here and the 250 motor is class legal

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