Tips on reviving this big bore K250...

Hey guys,

Picked up a KX250 for $550 from some druggies out here in eastern KY. They claimed it only needed a coil and it would run perfectly... I gave this statement absolutely no credability but the bike looked solid for the money. Its missing chain and spockets, has the wrong shifter, no spark plug, etc but the bearing seems tight with the exception of rear wheel bearings, fork seals are good, suspension seems fine, engine was not siezed had decent compression (though not great) and transmission shifted though all gears.

Plan is to get the right parts on it, give it a top and, new coolant, trans oil, etc and run the wheels off it. Started taking it apart as soon as I got it home... First thing I noticed was the carb was foul... I'm talking so gummed up I had trouble getting to need valve OUT of its seat. Got that all cleaned up and then onto the "bad coil"... Both resistance and bench top power supply test showed the coil as ok. Produces a nice strong blue spark and resistance is .5 ohm on the primary and 8K on the secondary. So much for THAT. Pulled the top and off and was pleased to find no major damage... Cylinder was sleeved but no big deal. Piston was a bit worn (not nice and tight in the cylinder) but I was replacing it anyway... Started measuring to make sure it was stock size. Piston measured 68.2mm at the thrust surface... &%$#@!? Stock size it supposed to be 66.something... Even .060 overbore was like 67.something... Entering the part number showed it was a piston for a 265 big bore kit... Sweet! Only issue is the vin checks out to a 92 and the part number is showing 93-00 year compatibility. I am going to assume they've swapped engines or parts of engines at some point. Now for the bad news:

The big end rod bearing does not move freely... Its not out of spec for clearance... In fact it has no play at all. I THINK its bad... The condition of the carb suggests it may have sat for a few years (that or than fueled it with gello instead of premix for awhile) and its a bit dry... I am HOPING that its just a little stuck or something so I put a bit of 2 stroke oil on it and work it in a bit and am letting it sit... If it moves freely after this should I run it or not chance it and fix it? If the answer is fix it I am thinking I will buy a new crank assembly (found a used one on fleabay which looks nice and claims no up down play for $138 BIN) and do the work myself... This the best route? How hard is splitting the case on the (probably) 93 KX250? What tools will I need?

Finally where is the best place to find parts for the 265 kit? I need a piston and rings obviously but I assume the gasket set will be different as well?


Call Eric Gorr.

Call Eric Gorr.

Thanks... Checked out his web site and called but no answer... Sounds like hes pretty busy so I will call back... In the mean time I checked on the bike during lunch... Bearing is much better but still some resistance and now the resistance is uneven... Definitely gonna replace it. I called a nearby dealer who I trust and they quoted me 2 hours labor max to swap the crank if I brought them the engine with everything already off it. If I wanted they'd open it, let me clean it up and give it back to them to put back together if I wanted to save more (said that'd probably get it under an hour). That said a Tusk case splitter is $60 so its pretty much a wash (dealer charges $65/hour) and then I'd have the tool and the expirience to do it again so I'm thinking I'll do that.

Tips on actually doing it? I'm gonna wait to buy any parts until I get it part on the off chance I find something seriously messed up inside that makes just sourcing a good used engine a better option... From videos and stuff it looks like the general idea is to take pretty much everything off the engine (including the 2 bolts under the magneto... LOL), pull the fly wheel with some sort of special tool I havn't researched yet and then split the case using a case spliter? From there I gotta new bearings on the new crank... Do they just go on or do they have to be pressed on?

Get a case splitter. It's a valuable tool, eyven if you don't know how to use it, you can figure it out with time. I had m case split and crank changed by a retired Yamaha mechanic in his garage, he said it wasn't near as big of a job as the dealers make it sound like. At least if you split a case you can get a good look at the tranny and stuff too, I'd inspect everything, the clutch, the transmission, the crank, the case itself and what not before putting it back together. Your better off fixing up your engine most likely because then you'll know what's been done to it, what shape critical internal parts are in and you'll have the experience from doing it as well.


Just ordered a Tusk case splitter on Amazon for $60. I will keep update the thread with my findings from inside the bike.

Just FYI thought I'd share a few pics as I go though this process...


Just after removing the cylinder... Nasty looking.


Piston: Wesco p/n visible on top... Got a few scuffs on the side but nothing really big enough to hang fingernail on. Thrust surface (just below wrist pin) measures 68.2mm... Meaning its .3mm down from what its supposed to be.


Cylinder head... You can see the edge of the sleeve.


Close-up... Can see a little of the machining done inside to make the powervalve work and maybe improve the flow. Incidentally the PV was much cleaner than my KDX was when I did the top end on it... Had the same oily nasty gunk that was on everything but not much in the way of carbon and everything operated freely without any binding.


Carbon fiber! Nice! Still need replacing though :(


Hmm... Not sure this press will be big enough to press my crank out... I'll just have to give it a try.

Oh yeah I forgot: If anyone wonders why it looks like I took the top end off in the back of a car... Um... That'd be because I did.

I live in an apartment during the week (my house is 65 miles+ commute) so the bike stays in the hauling vehicle and pieces come into the kitchen or go to work to be worked on... LOL. This is hauling vehicle:


And for the heck of it: The girlfriend on the KDX that this KX replaces (she bought the KDX after getting hooked on riding)


Edited by duncanstives

Nice. Good luck, doesn't seem like you're in too much trouble though just looks like it could use a freshening up. Post some pictures of the bike.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now