'91 KDX 250 Build thread, all advice, tips, criticism welcome

[PacificNWRider]

Got a new to me 1991 KDX 250 because it was too good of a deal. No such thing and I know this. My dream of building the ultimate PAC NW trail bike took control and made me bring this bike home. In my eyes the perfect bike would be a combination of a 4T & a mid 90's 2T I believe that the KDX250 is that bike

Now I got my work and learning cut out for me. Going to try and bring everyone along on the ride.

Here is the bike on the way home.


My initial findings on the bike were: missing right gas tank plastic, some groovy Blue Bigfoot stencils (maybe someone will recognize it), some dents in exhaust-though nice pro-circuit pipe/fmf silencer, terrible chain & sprockets, non stock headlight and rear fender, shock seals blown & possible more damage, terrible tires. Needs new levers, brakes completely gone through... yeah I need to go through this entire bike. Old nasty air filter, and dirt and sand in the airbox..... The carburetor looked very clean, the overall bike was pretty clean.

It had very low compression, my bet is all the dirt and SAND in the airbox, either that or the NON MIXED GASOLINE IN THE GAS TANK. and the kickstart wasn't catching, but when I looked over the bottom of the frame and there was not a dent or ding on it, I thought...this bike hasn't been ridden THAT hard?. No evidence of 2T goo all over the cylinder and silencer. The swing arm linkage seemed fairly decent with a very small amount of play, and the bearings in the stem were tight although not perfectly smooth.

All in all, I was going to rebuild any bike I bought from the ground up, So yeah I brought it home... Let it begin...

[javjacob]

You should have a nice build when finished. I dont have any experience with the KDX250 so Im interested to see how it turns out.

[PacificNWRider]

You should have a nice build when finished. I dont have any experience with the KDX250 so Im interested to see how it turns out.

Thanks, I hope so. It will be close to factory refreshed, and it will probably never be worth what I sink into it in terms of resale. But I've ridden new bikes that are alot more expensive... and for a fraction of the price, these bikes are absolute machines. It's going to be bit of a small journey, but I do think it will be worth it.

[PacificNWRider]

So I got the bike all apart... More pictures of that stuff later, I want to get to the meat and potatoes.

I took the head off, and it looks great. The head gasket was another story. It was COVERED in black RTV.. unless dirt bikes are different then other engines, this is another sign that the previous owner was not your average experienced dirtbike enthusiast or mechanic.

First looks at the cylinder and piston are better than I thought they would be. This piston is suspiciously clean, for what I was expecting. It has some interesting air-flow patterns? Looks as though the air is ramped and at the highest speed at the leading and trailing edge of the ports nearest to the Exhaust port, the direction of the air flow. It seems that a guy could use this as a map as to where porting would make the most difference?

[PacificNWRider]

The cylinder is off and I've been getting to know it.

Starting to see what the nikasil plating is, and what it looks like. The piston has alot of wear on it, I think its been rocking in the cylinder for a while. I am wondering if what I see is smeared aluminium or plating loss. There are also what appear to be bottom ring ridges, and possibly some staining from a temporary seize, riding, or a long period of sitting. Either way im sending this thing out to be repaired and re-plated at millennium. I have been reading the website and other reviews on this site and it looks good to me. I thought everyone might be curious to see the cylinder, possibly learn something with me.

In the first picture you can definitely see the nikasil coating and the aluminum cylinder. The coating looks to be somewhat thick here at the base of the cylinder.

A few pictures of the cylinder and my initial thoughts, then a couple pictures withOUT my red paint all over it so you can see it. What do you think?

[KDXGarage]

Got a new to me 1991 KDX 250 because it was too good of a deal. No such thing and I know this. My dream of building the ultimate PAC NW trail bike took control and made me bring this bike home. In my eyes the perfect bike would be a combination of a 4T & a mid 90's 2T I believe that the KDX250 is that bike

Now I got my work and learning cut out for me. Going to try and bring everyone along on the ride.

Here is the bike on the way home.


My initial findings on the bike were: missing right gas tank plastic, some groovy Blue Bigfoot stencils (maybe someone will recognize it), some dents in exhaust-though nice pro-circuit pipe/fmf silencer, terrible chain & sprockets, non stock headlight and rear fender, shock seals blown & possible more damage, terrible tires. Needs new levers, brakes completely gone through... yeah I need to go through this entire bike. Old nasty air filter, and dirt and sand in the airbox..... The carburetor looked very clean, the overall bike was pretty clean.

It had very low compression, my bet is all the dirt and SAND in the airbox, either that or the NON MIXED GASOLINE IN THE GAS TANK. and the kickstart wasn't catching, but when I looked over the bottom of the frame and there was not a dent or ding on it, I thought...this bike hasn't been ridden THAT hard?. No evidence of 2T goo all over the cylinder and silencer. The swing arm linkage seemed fairly decent with a very small amount of play, and the bearings in the stem were tight although not perfectly smooth.

All in all, I was going to rebuild any bike I bought from the ground up, So yeah I brought it home... Let it begin...

"missing right gas tank plastic"... usually called right radiator shroud

I would suggest you look on eBay for a used replacement. OEM new is not available, and would be EXPENSIVE if it was.

Those Bigfoot stickers are fake. If he was blue, he would be MUCH more easily spotted.

Shock rebuild and parts are similar on USA 1989+ KDX models. If you look around on here for shock info, you won't find much that says KDX250, but the 1989 - 1994 KDX200 and 1995+ KDX models are very similar. The spring rates and body shapes are a little different.

A 1990 - 1991 KX125 / KX250 rear fender fits fairly well on the KDX250 if the rear hand holds are gone, which I assume yours are.

The fork guards are missing. Keeping rocks off the chrome tubes is pretty important.

From Kawasaki, the cylinder has an Electrofusion coating. It is not Nikasil (different brand of coating / plating). Electrofusion does not come with the normal level of cross-hatch marks that are common on Nikasil and aftermarket plating.

On your cylinder, the dots do look odd. My main concern is the wear around the ports.

The piston is probably a Wiseco, as it has the laser scribing on top. OEM does not have that.

Based on the neglect from the rest of the bike, I would not read anything from the burn pattern on top of the piston.

Thankfully, no one put a damn sleeve in it.

[PacificNWRider]

"missing right gas tank plastic"... usually called right radiator shroud"

You know,... we had this talk before.... my woman was like, "well then what are they called on my bike" .. shes got a little XR, with no radiator, but has the shrouds.., so instead of "fairings" we decided on "gas tank plastic" laugh...

Those Bigfoot stickers are fake. If he was blue, he would be MUCH more easily spotted.

I would have definitely seen a blue one by now.... they are actually blue PAINT, someone really like their sasquatches!

Thank you for the parts interchange information, that will help greatly.

Ok.. I see, not Nikasil, but Kawi-fusion.... ... were they among the first companies to do this? Seems like Kawi was just way ahead of the game back then. I believe they are the ones who introduced the power valve too??

Yeah there is alot of pitting, or porosity as it is usually called. Flaws in the casting revealed during the boring process. But this looks excessive and they way it is patterned arouund the ports looks almost like an old repair.. or sand blasted through the intake? Interesting... I am curious to see what Millennium will say about it? Do you know if the ring ridge and pitting will typically fall under the "repair" category? Do they like to weld this stuff up or over bore?

As far as the piston goes, I will post some more pictures of that, but I guess its good to know it's not still stock after 30 years.

This piston is worn heavily in the shape of the exhaust port, I swear a thousandth of the piston is smeared on the cylinder. Im thinking someone slapped a piston in it, without mic'n it, (a bit too small) and then proceeded to RTV everything back together, and the piston been slapping since

[KDXGarage]

Whether air-cooled or liquid-cooled, I guess they direct air to cool.

The 1974 KX125 came with Electrofusion. They did not invent coating / plating in general.

I don't know who invented the power valve, but not Kawasaki. I think the '85 KX's were first to have it for the green bikes.

How does the head look?

I have seen a few people with Kawasaki cylinders that, after aftermarket replating, showed the porosity. If that one had been replated, I would expect to see some cross hatching marks.

I don't know on the ring ridge.

If you think it has aluminum on the cylinder wall, you can use muriatic acid to clean it off. It may very well be that someone wore it out, then slapped a new piston in there.

[kdxsully]

Can he try to clean it with scotch brite or emery cloth?

[KDXGarage]

If one is worried about aluminum smeared on there, then muriatic is the way to go. I guess one could clean it well before getting to the acid part.

[PacificNWRider]

Well its been a long few month. With all the crazy crap going on... (what a joke ) I have still managed to continue this build.

Upon further inspection of the cylinder, and suggestions from you guys, I cleaned it up best I could. Most of the shiny in those pictures was definitely smeared aluminum from the piston. I recommend if you are doing a rebuild or refresh, definitely get that cylinder cleaned up good with scotch so you can really see what is going on. (be very careful with acid, it can get in the tiny pores and can be very hard to neutralize).

Back it up a little bit before tear down- out of curiosity, I started it when I got home, just for the heck of it (bad idea in hind site, I hadn't even checked the tank yet, just smelled it, smelt fresh enough, but did not know it was PLAIN GAS). To my surprise, It started and it started climbing in RPM, so I just killed it after a second or two. I was shocked it started. I compression tested the engine after it started - It read 40-50 PSI. The kick seemed light, but not THAT light. I thought for sure my tester was broken. Time to tear into it.

Back to inspecting the cylinder - I got so caught up in looking at the cylinder I forgot to look at the piston, figured I was just going to toss it anyway, but you can learn a lot from the old parts.....

I started laughing as soon as I looked at the piston a little closer, the piston rings were UNREAL. worst egg-shaped rings I have ever seen by far. if someone managed to run this engine a little longer, they would have been in for a real treat.

It is very hard to believe this bike started like this, and my tester is not broken, some days the weirdest things happen in the shop...

After seeing this madness, needless to say, I sent the cylinder / new piston / new rod / crank / related bearings to Millennium.

More to come..

[PacificNWRider]

Parts parts parts

[PacificNWRider]

Airbox... Notice the awesome High flow upgrade, simply stab your biggest red hot drill bit into the side of the airbox and wiggle it around until you have the desired high flow hole size.

[PacificNWRider]

Inside the boot, AFTER the filter.

Healthy sand and dirt, so your bike can get the minerals and micro-nutrients that it needs!

[PacificNWRider]

Some other worn parts, stuff to look for if you guys are looking at bikes.

obviously worn sprockets & chain, i could bend the chain into a rainbow sideways.....

broken swing arm rubber - probably means there is some bite on the swing-arm from the chain

fuel petcock - hard to turn, gritty - probably means had bad fuel or dirty fuel in it, looks best to replace the whole unit they are cheap

[PacificNWRider]

You can also look at the chain guides to see how worn they are, can give you an idea of how well it was maintained, if they payed attention to chain tension.

Looks like it has a somewhat original PC pipe, it is stamped with "KDX 250 '94" on it, so not original to bike, but from that era, pretty cool.
I read pipe dents can hinder performance, and if it has a dull sound when you tap it, its probably full of carbon and needs to be burned out.

[PacificNWRider]

You get the idea, just about everything on the bike needs refreshing or replaced.

Alot of stuff happened during the Cerveza virus and I didn't get very many pictures of refreshing and rebuilding, I will post up what I have.

[PacificNWRider]

Bottom end time...

Actually looked a lot better then I anticipated...... nice and clean, no rust on the bearings

[PacificNWRider]

not bad...

[PacificNWRider]

You can clearly see what looks like to be that leftover RTV that I mentioned earlier that was gooped everywhere. Shredded around seal?

must have been smeared around the seal because of the awkward way you press these in backwards??

Look at that MESS. wonder what the other side looked look INSIDE... How does this even happen? Seal spinning? so much rtv it gooed out of the seal on the shaft and then got spun out? wow...