I just ordered $220 for top end and misc parts


So looking forward to wrenching on my "new" 1991 KX250!

Bought it, and a 2004 KX65 for the boy, for $1600.

They run good and all, but the 250 seems to gurgle at low and mid throttle. I've only run it around the yard, but ran it as hard as I could.

Figured I should take the carb apart and clean it and note what jet and needle setting are in there. As long as I'm doing that, I should check out the Boysen reeds in it. As long as I'm doing that should just put a fresh top end in it.

I'm all new here, but have read a lot. There are many mentions of putting a new piston in an older "new" bike. Just a good investment to do as don't know the history. Would hate for the thing to dissinegrate and make things worse. So, I'll take the advice here and do it. I like wrenching on this stuff, I have the tools and mind set. It doesn't look that hard.

While ordering the piston kit, I also have coming:

-a spring for one of the foot pegs

-packing material for the pipe

-one plastic radiator guard (only "L" available, no mention of a "R", hope it fits the right side)

-oil seal for behind the sproket

-top end gasket kit

Just wondering if the thing has been rebored and is over size now.... Guess I'll find out when I take it apart. I'd bet BikeBandit will let me return stock diameter piston for "big bore" piston if I need it.

So, that is all.

Just making a thread and post here that I'm going to do some work on my first-in-a-long time "new" dirt bike.

I guess I'll be earning some street creds here that I'm at least smart enough and capable enough and into it enough to do the right maintnence.




Yea? Thanx.

Kinda hate to have to put $220 into it...and take it all apart when it seems to run fine.

But, guess it's something I should do.

Not like it wont run the same, or better, when I put it back together.

I'm just "out" the cash, an evening and a couple busted knuckles.

At least it wont be weeping oil from behind the sproket. The right foot peg will be sprung down instead of flopping around.

Something to do.


That is one clean 91... looks like you got a heck of a deal for the two bikes.

When you have some spare change sitting around get rid of that ugly black fender and itll be mint!


Yea...that was after I cleaned it :smirk:

Never really noticed the black fender. Now that you mention it. I was thinking it would look cool all blacked out. Rims and number pads and shrouds and seat and all. Not that I'm going to dump money into it for cosmetics.

But then I see all these threads of rebuilds to stock conditions and it has kinda grew on me. The "KX" on the shrouds is so late 80s. But, it's period and pretty stock and is in good condition. Maybe just a stock grean front fender would be worth it. Would cost a lot less than customizing it all.

And... a stock 1991, 20-year old bike has some draw above and beyond a stock 2005 bike.

When I have it apart, I'm going to try to clean the pipe. It's 100% rust now. Not pitted and through and bad....just a sheen of rust on the surface. Maybe a wire wheel and scotch brite will get it back to metal? If not I could paint it black.

I'm learning and enjoying it.


On a personal note I will not order a piston on a strange bike unless I have the size at hand,not that they wont take them back just for the waiting reason. Be sure to oil everything up after you take it apart so it dont rust(if you have to wait) Good luck on your build...Bike looks good

Or...drop the pipe off at my buddy that has a sand blaster? Would that work?

Then what, just leave it bare metal? Maybe spray some WD40 on it from time to time?


I'll see what works.


Thank RB....the piston is ordered already...

I'm just hoping it's stock anyway. The previous owner didn't know if it was bored-out or not, so he didn't buy it new 20something years ago.

I asked on a thread here if there is a way I could tell if it was bored out or not.....

Just figured if I have to take it apart to find out, might as well have the piston and gaskets here anyhow. Hope it's stock. Seems to be....other than the Boysen reeds and ProCircuit pipe.

Good point on oil everything real good if it does have to sit for a week plus to get the right piston.

Thanks guys.


If its not stock then soak your cylinder and head in oil then wrap it in oil soaked rags and it will not rust. You should have no problem with it.Oil would be your best friend for every piece if you was to have to wait on stuff. Hope this helps...

Glad I read this, this works perfect ont he pipe. I picked up a 99 kx 125 in horrible shape as a basket case to rebuild and it had a PC pipe on it like that. Use a wire wheel on a drill, and wd-40. After you get it all off, use some scotch brite pads and some more wd-40 to finish it off. Will look GOOD. 124.jpg this is what it looked like before. I could of sworn I had a picture of it after, but I can't find it. It will restore it to that bare factory looking finish. It is a lot of work but well worth it. Once you get it all done up, after you wash the bike everytime you'll need to hit it with a bit of wd-40 to keep it from getting back to being a rusty mess. Maybe touch it with a scotch brite pad again quickly. The small upkeep is worth it not rusting out looking like crap requiring loads of time to fix it up to look nice again.

Aren't the head and cylinder aluminum?

Anyway, I'm going through the same thing right now with the '98 KDX220 that I bought a few weeks ago. I'm thinking the same thing that you are. I didn't know anyting about the bike so I took it apart and started ordering parts for it. So far I've fixed the kick starter, kick stand, ordered a new gas tank, Wiseco top end kit, seals, jets and some other stuff. I bored the carb to 35mm and I'm fixing things as I find them. I'm out a little more money than you are at this point, but the KDX should be a lot of fun when it's done.

I am more fond of Honda's and Yammys but I hear GR8t things about the kdx's....I have heard so many people say they are one of the best trail bikes out there for the buck...I wouldn't mind finding one myself but the finance situation for me is tight at the moment

cool cool cool

I'll post again when the stuff gets here and I'll take a couple pics....everybody likes pics.

Of coarse, I'll probably be posting some questions too. Like: How/where do I drain the coolant out before I take it apart so it all doesn't flood into the lower end?

That site didn't have the service manual for this bike, so I still don't have that. Going to go look for it somewhere else...or stop by the local Kawa shop and order one from them.

Will need torque specs for the fasnters.

Somebody asked; "Is the head and cylinder aluminium?" Yes, they are.

My KDX has a drain plug in the bottom of the water pump and a one in the bottom of the left radiator. I had to open all of the vents to let the coolant out.

coolant drain plug should be right on the water pump cover.

You dont need to make a mess with oiling everything. spray or rub on any crap you have laying around on the inside of the cylinder. The nikasil will rust but everything else is aluminum and will be fine.

just a heads up so you know std. bore sizes are as fallows for the 91 kx250

67.40 std.

67.41 std.

68.00 .025 over

68.50 .045 over

another thing is that if it doesnt have a coolant drain bolt is just to pull the hole water pump cover

Great, thanks.

As mentioned earlier....I bought the standard piston and hope it's a standard jug size.

I'll see when it gets here and I take it apart. I'd hate to take it apart just to check and then have it all sitting there in peices for a week(s) waiting for the parts. I'm hopeing and planning to take it all apart, swap everything and put it back together in one eve. Some rainey Friday night.

It's not like I'm some pro and can't live with out it. But, we have five acreas and I have a nice big garage and it is a nice escape most evenings to put the gear on, fire it up and rip some laps around the house track we have. A litttle get away from the daily grind.


It's been bored.

My piston came in and it's to small.

But at least I got to go all the way in there and see it.

Didn't take four hours.

Wont be but two nect time.

The piston in there looks fine. Just a slight vertical rub. Cross hatch on the walls is mostly all still there. Rings are blemish free. No gunk anywhere. Reeds are straight and true. So....I put it all back together and ripped it across the yard.

First bump start and the thing is screaming?! Had to drag the brake and clutch it to not go flying. If it was totally clutched it would go crazy revs, so had to keep a bit of a load on it, yet engaged was fast?!?!?


Repacking the silencer couldn't have done that? Spraying out the carb? WTH< ! :smirk: Must be some sort of air leak? But so constant...and the twist grip feels wierd?


I routed the throttle cable wrong and it was 1/2 throttle. Thumb screwed the slack setter all the way in and got a normal idle out of it. Now that I know that's the problem, I look for the reason. The cable is pinched under the tank. All's good.

Silencer wasn't that bad. only a couple dozen holes at the end were carboned over.

Out the new spring on the foot peg.

I have the oild seal for behind the sproket, but didn't get to that yet. I'll just keep topping it off until next rainey day.

I'll let that piston stay in there for a little while more too.


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