91 KDX250 Rebuild

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91kdx250
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91 KDX250 Rebuild

Post by 91kdx250 »

Hi all, just pulled the barrel off to discover a fare bit of movement in the bottom end bearing for the conrod. I can tip it left and right like a pendulum.

I'm pulling it down due to a lack of compression. I was told of the KX head gasket to get more compression, but then advised it might shorten the life of the bottom end. Well that might need redoing as well.

Anyone tackled a bottom end rebuild and advise me of what I'm up for $ wise and difficulty. Definately something I would take on, unless there is something in the job I need specialist tools for.

The clock shows 12000kms and the speedo has never worked since I bought it so wear is an unknown factor.

Any help....Cheers

Any
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Re: 91 KDX250 Rebuild

Post by KDXGarage »

Hello.

Yes, there are several threads on here about bottom end work. You will need special tools to split the cases. Replacing the connecting rod will require either taking it somewhere or shipping it off
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Thanks Jason, top end wasn't as bad as I thought. Just rings and a hone for now. There is no up and down movement in the big end so for now a freshen up will do.
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Re: 91 KDX250 Rebuild

Post by KDXGarage »

Did the cylinder have a sleeve installed at some point?

Normally, if it had the Electrofusion coating, honing is a no no.
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91 KDX250 Rebuild

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Yes its been sleeved and running a 1mm oversize wiseco piston. $40-$50 to get it done, so I bought a hone for $25 and I'll do it myself. Any tips?
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Re: 91 KDX250 Rebuild

Post by KDXGarage »

Do you have tools to measure the bore so that the correct piston is installed?? A certain clearance is required.
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

I took the piston and barrel to the local bike shop, owner and mechanic went over it with me and said it only needs a hone and rings. I just don't want to over do it with the hone. I'm planning on you tubing it before I start. I have verniers for the piston but no bore micrometre.
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91 KDX250 Rebuild

Post by pumpguy »

A vernier caliper is not really the best tool for this measuring job. You really need to be able to measure to 1/2 a thousandth (.0005").

Besides, as you say, you can't measure the bore which needs to be measured in several places since the wear is always uneven.

See if you can borrow or find someone that has a micrometer and set of telescope gauges or inside micrometer. Micrometer and dial bore gauge would be even better.
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Re: 91 KDX250 Rebuild

Post by KDXGarage »

I THINK he knows a guy. It's the guy he didn't want to pay the extra $20 to. :oops:
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Haha, its not the coin, its the learning experience. When you compare the job getting done to JB cans, its a cheap service. I have a car engine to re-ring also, the honr will come in handy.
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Re: 91 KDX250 Rebuild

Post by KDXGarage »

LOL :grin:

The normal micrometer works well on the piston measurement. Verniers are not normally used. An inside micrometer as several places down inside the bore is the way to go, as mentioned.
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91 KDX250 Rebuild

Post by 91kdx250 »

Well I got it all back together, but no spark. Any common spots I should look at. I had the brown and white wires unplugged and the grey double plug for some time. Put inox in the connections and they feel firm, otherwise I'm lost. Ah and I disconnected the kill switch.

Any clues?
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Update, next morning the bike had spark. Kicked it over choke on and it fired up and ran on a couple of kicks. It ran for nearly 1 min, got the choke off and hit the kill switch. Kicked it over and it fired up again, ran for about 30 seconds and conked out. Still showed spark but fired once and again then zilch. Had the plug out and no spark again. Is there a common spot or a way to check with a mulit meter without stuffing the meter from the magneto?
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Ok, igniter plug under seat in front of air box was dirty as ****. Cleaned it and sparks away.

Back on the first day.
When it stopped after second start up I pulled off the carby and cleaned it out. still didnt start and and I fiddled with the mixture screws. and still wouldn't start. after i put it back on it was leaking out of the overflow so I must of bumped the float adjustment. We gave up at that stage and went to a car rally. So now its a carby problem.

Today.
With the carby still overflowing I got it to fire a few times but not run. I dropped the float level and stopped it overflowing but no fire. I have turned the mixture screw everywhere from 1/2 a turn 180 degrees to 3 turns out 1080 degrees. Everytime the plug was wet. I adjusted the float by the book at 16.5mm and still no fire. Tried adjusting the needle from second clip to first clip and 3rd clip. Still no fire. Plug still wet, spark still there. Compression still there. Apart from finding a really large hill to roll start it, I'm not sure what to do.

It is running the standard 158 and 45 jets. Do I need to drop the jet sizes now it has more compression. (it started and ran nicely the first time before conking out) All i have done since is play with the carby. It was very smoky on first start, but we oiled everything on assembly.

Any help would be great. The book says 1.5 turns on the mixture dose this mean 540 degrees from closed? all needles look fine and no damage.

Could I of broken the top ring or ring grove with my hone and new rings rebuild? Compression good....

Standard carby. Any help?
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Re: 91 KDX250 Rebuild

Post by KDXGarage »

It is 540 degrees. Sorry it won't go. Keep checking one thing at a time.
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Wasn't fuel, it was the 90 second rebuild, because that's how long it ran fore period. Checked the reeds and no obstruction but I could see scores, took off the exhaust and I could see scores, pulled the head and barrel and it was score city. Has anyone tried an eBay chineese special piston before? I can get a 68.15mm piston but are they a waist of time? My bore miced at 67.4mm.
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Bit the bullet and went a rebore and wiseco piston. Full resto is now the go... I'll need to do both front and rear shocks, modify the frame for long range tank and wider seat. Powder coat the frame blue, new engine seals, expansion chamber, and some of the plastics. $$$$$$$$$$$$$ :roll:
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Re: 91 KDX250 Rebuild

Post by 91kdx250 »

Picked up a KX 250 seat today to match the KX Long Range tank. The KX seat is too narrow for my but so will have to strip down both seats and join the front of the KX seat base and foam to the KDX base. That will give me the widest seat with safety seat to match tank. Also picked up the re bored barrel and Wossner piston. Should end up 258cc. Next I'll have to get the bike to the workshop where I work and modify the frame to take the tank. When i first sat the tank on the bike it fouled with the throttle cable on top of the carby. Once I get it all aligned I can start stripping it down for powder coat.
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