Maintenance & Technical > KX500 Original

the 250/500 project

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supermotokx:
Hi All

This is it!  :shock: The first pictures of my '98 kx 250 with the '96 500 engine in it check out the gallery. I know I'm late but I've been very busy the last weeks. The pics are kinda crap, but I'll have a lot more to come with more closeups, the finished pipework, brakes, cooling system etc. Stay tuned.

Martin  :twisted:

John:
Congratulations!

That's a mean looking bike you have now!

Have you noticed any change in the way it handles? (Obviously you have  :oops: ) I mean any positive and unexpected news?

/John

supermotokx:
Well there are plenty of changes:

1) The frame fells alot stiffer and so does the swing arm which means it steers better going out of turns on the throttle. The stiffer frame probably allso accounts for the better steering going into turns, but that may be the 250 forks as well (way better for supermoto than the 500 forks).
 One of the first things you notice is how you sit on the bike. You're able to get much more weight on the front because of the tank and seat of the 250. That really helps going into turns.
 Because I had to modify my FMF pipe, I've lost a little top end power but feels like I've gained more low end. But it's not really a problem, i still haven't done anything else (like v force, big bore ect) to the engine so I think we can get that power band back. I did look for a complete made-to-fit pipe and found that Jolly Moto (Italian company) and Van Hasselt (Belgian company) makes race exhausts for the 250/500 combination, price: 5-600 $. That was not really within my budget.

Martin

John:
Nice!

Now that you mentioned it, how much have you spent on the conversion? I have a KX125 and a KX500 motard. I am planning to do the same thing, depending on the amount of spare time (= none).

I think I read somewhere that you had to cut in the 250 frame to fit the engine? Is there something else apart from the expansion chamber that caused you problems? Can you change the sparkplug and jets easily?

Sorry, loads of questions... Maybe you should write a Haynes manual  :wink: . I'll definitely buy it!

/John

supermotokx:
Well here's my haynes manual:

1) 250 Swingarm (alot stiffer than the 500) is too wide for the engine
Solution:
-Grind approx. 2 mm of each side.
-Do the same on the 500 engine mounting bracket (quite difficult to do as it has to be precisely alligned with the swingarm bearings)
-Cut the swing arm bearings down to size.

2) The 500 engine has a 15 mm bolt hole. The 250 swing arm has 17mm  :roll:
Solution:
- Drill the engine hole to 17 mm.

3) Engine is big compared to the 250, even when lowered as much as the mounting brackets lets you do (actually the brackets are not the problem, but you have to get the bolts through as well...). This means no room for the tank.
Solution:
- Cut off the top engine bracket and throw it away.
- Modify the tank with plastic padding to fit the sparkplug (use chemical metal - no big problem) or raise the front of the tank apx. 10 mm.

4) Exhaustpipe doesn't fit the 250 frame.
Solution:
- By a new exhaust (600 $)  :(
- Cut up the old one, turn it and bend the last part (where it's not expanding). Get some weld-god to weld it back together.

5) 250 engine fittings doesn't fit the 500 engine.
Solution:
- Cut off the 250 fittings.
- Weld on new ones to fit the 500 engine (piece of cake  8) . You will not have to cut in the frame at any time).

And you're done  :D

Parts used from kx 500 :
- Engine
- Ignition
- Cooling system (you could use the 250 I guess - I just didn't have it)
- Karb (but not airfilter and airbox)

Parts used from kx 250 :
- Everything else

Upsides:
- Handling  :twisted:  :twisted:  :twisted:
- Looks  :twisted:  :twisted:
- Coolness  :twisted:  :twisted:  :twisted:  :twisted:

Downsides
- Changed powerband (from the exhaust fiddling)  :evil:  
- Tank must come off to change sparkplug  :evil:
- Work hours  :evil:  :evil:  :evil:

Cost:

Hours in the garage: 75 (most was thinking about: How the hell am I'm going to do that??)
$ paid: 75 for the exhaust rebuild.

You will need to have a well sorted toolbox the fix all the precise drilling and grinding.

I would think that thework needed to be done is the same for a 125 frame. But you need to check if the frame is too small for the big 500 engine.

Let me know if you have any more questions. And I'm sorry about my crap English, but I'm from Europe so I can't help it.

Martin

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