1991 KDX250 - melted it down

[woodsy]

Hi All: Went riding with a few buds this past Monday and siezed the motor on my KDX250. I admit running at high R's for long distance on the roads (it was getting late and some of our party had to get home) probably wasnt smart. After about 45 minutes of road running it siezed up/slid the back wheel on gravel. I pulled in the clutch and broke it free. Shut her off and it actually restarted!! It didnt run real bad after words (a buddy told me he thought I had a rock caught in my sprocket :) but I could tell she wasnt right. Teardown revealed piston scuffing on the intake and exhaust side with stuck rings on the intake side of the piston.
My bike has about 3k miles on it of which I have put 1800 on. It has never been down before. Airbox lid has been cut and I just replaced (before I went on this ride) the stock filter with a UNI (anyone ever heard of having to rejet due to this? It has a FMF gold series fatty pipe on it with an FMF silencer, Boyesen reed cage with a light blue connector between the carb and cage (this is one of my questions - is this a RAD VALVE?) which I would like to replace - where do I find one?? My jetting is 165 main/42 pilot and jet needle at the second from top. Always seemed to run a little rich on the pilot but ran real good on mid/top. Jetting sound right?
Kipps was carboned pretty bad but still operating correctly - does anyone have a good way to decarbon without total removal? Know of any good chems for this? Piston top and head were almost spotless with NO carbon buildup. Bore still looked very good (thankfully) with just a small amount of aluminum from the piston scuff - where do I find a ball hone? Do they come in exact sizes of the bore you are honing or do you buy one smaller/larger then the bore? Do I hone to a cross hatch like an iron cylinder? Do I use cutting oil? Must I clean up the ports when finished honing and if so how without scratching the nicalzil/aluminum junction?
I have parts ordered and am goiing with wiseco piston and Versa KX250 head gasket/top kit. Am I on the right track with these?
I thought I would touch base here with my cyber friends and see if anyone has any suggestions about all this. If so, please fell free to HELP!
As always - THANKS!
Woodsy

 

[dirt bike dave]

What fuel, oil and ratio were you running? What spark plug?

Is your cooling system functioning like it should?

Sounds like either improper lubrication or a poor cooling system caused the seizure.

 

[Matt90GT]

wow! that was a tough read there.

First the jetting is way off on the bike. That is why you have a lot of carbon in the cylinder and exhaust. You will need to clean up the pipe and everything while you have it down. Here is the jetting specs to clean the bike up:

Slide = #6
Needle = Kehin CCH, 2nd or 3rd notch from top
Main = 158-162
Pilot = 40

The bike is jetted way off to start with. That will lean out the car and bring it close to the KX specs of the year. It is what I am running and ran out in Milican (open desert, LOTS of sand) last weekend flat out for 70 miles without a hickup.

You will want to get a complete gasket kit for the bike. Cost will be about $35. You can get one off ebay from K&S performance for the 88-92 KX250 with all the seals you need. It will also come with a KX headgasket that is thinner than the stock increasing the compression and performance.

For the cylinder, there are different sized pistons out there. Something like A-D. I am going to have to tackle the same thing this winter. Take the cylinder down to a shop and have them measure the unit and go from there.

For the Boysen reeds, you will need to contact them directly for replacement parts. You may want to consider selling that unit like on Ebay and getting the V ForceII cage. Seems to be a much better setup.