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KX500/250 Hybrid questions?

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This is a post I have been wanting to make for a while but I have just gotten registered by an admin (Thank you!). I wasn't sure which category this should be posted in since it covers both KX500SF swap and modification of the 500. Either or if it is in the wrong location feel free to move it.

To start:

I am building this KX500SF with a 99 KX250. However I will be using the KX500 swing-arm instead of the conventional 250s. The collars to make up for the space will be pressed into the case. The cylinder I am contemplating using is the 87 jug(any input old vs. modern?), I also have the original cylinder. I will have this bored .080 over. Reason for the 523 is because I have a brand new crank and piston, the piston is .080 from the tear down of the engine. We are using the hollow cases to get the mock up for the welds.Makes handling the 60 pound motor much easier.

There will be extensive port/head work done on this by a professional machinist with access to BiMotion. Now for the fun stuff. We're wanting to use the 99 KX250s stator/flywheel/CDI/etc, I will add a flywheel weight. What we're hoping to achieve is a cleaner power curve and utilization of the 38mm Keihin carburetor with the throttle position sensor. We haven't done the measurements yet but we're aiming to take about 4mm off making it a 42mm carb. What I'm asking is has this system been used? I have read there may be engine damage caused by using the CDI of a 250, wondering if it is true or not?

It's not so much modification as it is repair but the water pump gear on my crank shaft has 4 shattered teeth. It's been decided it was from the previous idiot who thought he was a master mechanic, trying to force the clutch side case on with a hammer... I see on the schematic for parts this gear is pressed into the crank. How do I go about removing this pressed gear? I have tried heat with no success.

How do you fellow big bore riders like the reed spacers?

Also a repair, it seems this motor grenaded or something and scuffed up the inside where the crank sits also it cracked the cases in a couple spots. The Previous owner(he told me he was the last one in the engine) instead of doing it correctly just molded some JB Weld and used that as the patch work. I'm having this tig welded to fix it but is there any suggestions to removing the JB Weld? I read 600 degrees for 10 minutes renders the JB Weld useless but am uncertain the cases can withstand those temperatures?

I know you guys have probably seen these questions a million times but I searched for days before I made the post and could not find solid information regarding my issues. I may be missing something so please feel free to inquire and if you can view my album of this build here is my link: http://www.facebook....59972637&type=1

Thank you all in advance.

Edited by Bodyrot
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I had copied my other post I made at KXRiders. Sorry about the mess. Yeah dude I give it a good 4 months of doing shit RIGHT, get my powder-coating and graphics made. I'm hoping to hit 80 RWHP usable in the trails. I should also mention I'll have a custom pipe made for this using the BiMotion software. I'm hoping some electrical veterans can chime in.

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Hey bodyrot

Nice project dude - good luck

quick question.......

I am building a KX500AF at the moment in a KX250F frame and am having trouble with the gas tank clearance on the spark plug.

I have been told that there are shorty spark plugs on the market that could possibly do the trick, only thing is I;m batteling to find a compatible shorty for my recomended plug NGK BR8ES. Do you know of a forum or where to look for a compatible shorty plug to prevent me from having to heat gun my gas tank?

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grasshopper you ask many valid questions... lol i find great satisfaction building bikes. first off if your going through all the trouble to modify i feal that newer is better (better handeling) (better resale value) (better parts availibility) having said that i wish you secsess. i,m no whare neer being a excelent rider but their is a word of difference between my 97 made into a 500 and my 04 made into a 500 . it sounds like your about to dump lots of cash into this project.neer my home their is a 01 kx 250/ 500 for sale for 2200 the guy put a 38 mm carb to improve botom end, pwk 39.5 was stock. a friend of myne built one using a kxf frame on that bike he ended up notching the head to clear the frame witch really is no big deal. dont be shy about puting a heat gun to the tank heat a bigger area than you need to change take your time as the heat takes time to penetrate the thicknes of the tank have some sahping tools like a old spoon maybe a broom handle get the area so shine from the heat then start crafting . on our cr500 motors we put in 250 ignitions because it reduces the gyroscopic efect then the bikes handle better in tight trails not to mention start easyer and rev higher. but i have no experiance with a kx250 ignition swap. service honda does the famous cr500 af and they also do a kx500 implant into a new frame maybe you could check their website and steal their best ideas? lol good luck on your build

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Some decent info out there. Personally when reshaping a plastic gas tank I would use a baseball to reform it with, anything less I personally feel would be easier to punch a hole in the tank. You can use the plastic getting shiny as an indication you're getting too hot. I suggest just reshaping your tank man, It's not too complicated and you're only going to lose a FRACTION of the fuel capacity the tank is capable of.

Any of you got experience with both 1987 and 88-02 cylinders? It's this cylinder with a 9.5-10mm read spacer that has caught most of my attention.

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89-04 cyliders are pretty much the same. They have a 3 KIPS valve layout. The 86-88 only had 2. Im using the NGK BR8ECM in my 04 250/500 conversion. I had to slightly reshape the stock 04 tank for clearance. I also spaced it up at the front with a 1/4" aluminum spacer for added clearance. With 2.2 gallons I can make 28 miles if on the gas or just under 40 just taking it easy.

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I'm pretty certain 88-02 are the same jugs and 85-87 was the 2 part powervalve? I could be wrong but from what I've researched those are the number I had come up with.

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89 was the first year for 3 KIPS valves, I bought a 88 brand new and it was the last year for the 2 valve cylinder. The 85 didnt have any exhaust valves (1st year watercooled) and the 86 was the first year for KIPS.

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Heh wow, that's weird... Wonder why they classify parts as 88-02? Are they referring to the bottom ends I take it? Thanks for the info dude, appreciate it.

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