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81cr450 04-07-2013 06:33 PM

RMX250 halfway build
I'm pretty good with motors , but the shiney stuff is not my thing. So this is only halfway. I may powdercoat eventually but so far it's just to get it running & riding
What I bought was a 93 rmx250. I like the wide ratio trans of the RMX & Guy Coopers' whip in the early 90's Dirtrider always had me wanting an RM. I'm more of a big bore 2-stroke guy so the motor really needed to be breathed on a bit. I ended up just using the 93 trans & clutch. The jug is from a 91 RM which has the biggest exhaust port of the series. The case & crank are 1995 because they started using crank stuffing in the later motors. The head is a 95 rm because it actually has a squish band. I'm using VF3 reeds because they reduce crankcase volume too. I would have done some stuffing but I am lazy.

Here I gotta say thanks to Bix & Mike Wier. Mike runs OEM-CYCLE.COM Bix', place is called , these 2 gave me a smoking deal on the crank & jug. Both parts were to me in 3 days.

I had others supply parts but these guys stood out.

I'm also using a new PSI pipe I found on ebay for $125 shipped, talk about SCORE.

I intend to dual-sport this for a bit , but as with anything I build its always for sale. As part of the dual sporting I went with a KTM LC4 tank & grafted it to the bike. I figure I need the extra range of more fuel. I really wanted a 6.6 gallon 2003 yz250/450f tank but couldnt find one so I took what was cheap.

My photos will start with the motor & I'll kinda go from there as I complete the build. The handle bars on these are way far back & I have a trick for that. The plastic & seat cover colors that are available now dont match the old suzuki yellow & I have a few tips on that.

I do have a question though , have any of you used permatex anti-sieze on your powervalves. I've been using it on my last 3 powervalve motors & it takes a few rides to really loosen up but once it does it works quite nice. It kinda helps keep you from beating on a fresh motor because the powervalves dont react as fast as normal so you kinda ride a DOG till the lube melts in .

Sorry for the long post but I'm kind of a Blatherer

Pics to come

81cr450 04-07-2013 06:42 PM

On the piston I windowed it & dropped the boysen port before the replate. On a kx500 it equals 2.5 hp in the mid range. The reason I think it works like that is if you think of swinging a bucket to keep the water in the bottom the highest point of pressure is in the very bottom so a window relieves this hgih pressure point.

The piston is also skirt coated with an antifriction coating & ceramic on top
Here's the boysen port drop

81cr450 04-07-2013 06:51 PM

Powerseal does beautiful work as well.

Heres where the bike is at

Yes it's in my kitchen & yes its a work in progress. You can see the tank on it at this point but the shrouds still need to be attached.

The 91 jug needed some port work to smooth out the powervalve area & the powervalves had to be relieved as I went 2mm overbore

You can see the rechambered head above as well & below here you can see how the everthing is flush when the valves are open, they were 1/4 inch deep before the work.

The RMX is know for having excellent fuel economy due to such a small exhaust window, you can kind of see it here when the powervalves arent open

81cr450 04-07-2013 07:00 PM

The crankcase stuffing I was refering to I lost the pic to but you can kind of see it in these 2 photos here. The aluminum insert on the second pic is where it's at.

Veshrah Japanese rods are the good stuff, no wiseco China junk

81cr450 04-07-2013 07:23 PM

This may be a bunch of crap no one cares about but it might also help someone understand what theyre paying for so...

Heres an illustration of the head with no squish band off the RMX vs the RM head with squish. The clay is there to illustrate what I'm talking about.

The bar mounts are off of a honda ascot 500 & they put the bars forward a 1/2 inch at least. I had to turn a spacer to on the lathe to use them though & the retaining nut is small enough I worry a little that it wont be strong enough

81cr450 04-07-2013 07:41 PM

The little details are what pay the dividends. Intake matching helps a bit too.

third eye 04-08-2013 06:46 PM

very cool, looks like part of the return of two strokes

Foot dragger 04-11-2013 02:56 PM

That should be a runner,I sold an RMX PSI pipe a while ago on ebay,,could it be?
I found 8 RM/RMX's all in parts in a pile for so cheap I couldnt pass it up.
Also one with a current CA plate in complete bike form but it's gone now.
Ive got a few odds and ends left but most has sold off.

Ive still got a FMF 91/92 RMX pipe on there for sale but nobody wants it even for cheap.

81cr450 04-13-2013 06:11 PM

The PSI pipe I got is new old stock. The guy had a few when I got mine.

On to a place to put my :kat:

The seat has been some work. Mating it to a non stock tank & the sukiest seat cover ever. The seat cover has been the biggest problem on multiple levels. Its the worst fitting one I've ever replaced, even with using new undamaged foam.

The other problem was the color didnt match the 90s rmx color at all. Its like neon lime yellow & the older plastics are a orangish yellow & it just clashed :puke2. I could have gotten a black one instead which I would recommend , but the easy way isnt my way.

Plastic for early 90's rm's is hard to find let alone multiple colors & I wanted the newer yellow to match the seat. I could have gotten the the factory fit tank in black but no black shrouds. So I tried to get a natural colored 6.6 yamaha, the only color they still make is blue. Which just left me randomly hunting for a large tank with shrouds. I lucked out & picked up a new lc4 tank for $80 on ebay.

Mounting it was pretty slick. I had to cut off the original seat catch, represented by the cardboard in the pic, then weld in a new tank hold down there.

The next step was the inner front mount, it's kind of an aluminum handle, with a rubber hose over to keep from wearing on the tank. Bolts right into the factory tank mounts

These tank ears just slip right on

For the rear mount I turned down a piece of derilin on the lathe. Figured it would wear decent against the tank.

Feels snug no slop when mounted. I still nee to attach to the radiators, but they have some nice loops that will make it easy enough.

To hook the seat to the frame / tank area I used an old cutting board of my Wife's cause it seemed like it wouldnt hurt the tank at all. It worked out to where the airbox now draws air from the front of the seat. You can see where the derelin bushing goes under the silver washer.

The middle layer hooks the seat to prevent side to side movement but also alows for an air chanel

I also used an old milk jug to create a dust shield from the rear. I'd of liked it to seal up closer to the vents but you gotta be able to take the seat on & off

81cr450 04-13-2013 06:23 PM

Real easy to attach just used my trusty stapler

The seat pan needed the old attaching points removed & a new one riveted in. I had a old cr250 seat to rob the front catch out of.

You can see the gap here under the front of the seat for the air channel . It's decent sized under there I just couldnt get a better pic. & no my fat :kat doesnt smoosh it down to where it cant breathe.

81cr450 04-13-2013 06:27 PM

From a couple feet back the gap doesnt look too horrible.

Yea my picture skills need help.

Foot dragger 04-16-2013 04:16 PM

Holey Moley,youve got some time on your hands,and some skills as well.
I tend to buy what works and ride it till it dont.

Looking good though. Ive got some 90's RMX/RM wheels around if your needing spares

81cr450 04-16-2013 05:32 PM

I have some sort of mental disease that wont let me just buy something & leave it alone. Theres only 3 bike's I could even consider buying new right now, ktm/Husaberg 300, Hayabusa or Ninja 1400. Those would even need some tweaks from the factory. I dont have buckets of cash though so I kinda gotta build what I want. The time on this hasnt been to bad though , not compared to an aluminum framed bike.
I already have a spare rear wheel though , thanks. They are factory excell's on the rmx you should strip the rims & sell them as such & sell the hubs separate.

bobfab 04-18-2013 09:12 AM

Nice build, its the litttle things. Is that a DRZ400 IMS tank on there?

Im looking at buying a RMX this summer for next winters rebuild needless to say im following along :lurk

PSchrauber 04-18-2013 12:52 PM


Originally Posted by 81cr450 (Post 21132625)
Powerseal does beautiful work as well.

Heres where the bike is at

Yes it's in my kitchen & yes its a work in progress. You can see the tank on it at this point but the shrouds still need to be attached.

Kitchen is this your kitchen holy cow what a mess :eek1

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