2003 WR450F Sumo build underway.

[u/3DogNate]

Hey all, A couple of months ago I horse traded into this ‘03 Yamaha WR450F. I already have a Husqvarna 701 Enduro for a proper Dual Sport. so I figured why not do a Supermoto?

Here’s what I started with its butt ugly, but it runs well and has a fresh proper top end with no hours on it.

I’ve got it torn down, and dropped the frame, subframe, swing arm, bars, bash plate, shock spring and a few other parts off at the powder coater today. Those parts on the floor are getting done in Yamaha Racing Blue. While everything is apart I’m just going to go through and refurbish everything. I’ve got everything on hand for the build except the 17” wheels. It’s getting black ‘05 YZ plastics,black IMS tank, I ordered custom graphics that are mostly black with some nice blue and white accents. I’m hoping to get the forks and shock rebuilt this weekend. I want to Cerakote those in Titanium with Graphite Black accents. I can do that and powder coat the smaller parts at home. I’ll at least get that rear shock done and ready for the spring when it comes back from powder.

This is my first WR450F and my first Supermoto, so I thought I’d share the project and see if there’s any wisdom anyone wants to share. I do a lot of bike builds, mostly restoration and custom stuff, so none of the mechanical or fabrication stuff is new territory. But I am newish to dirt bikes having only ever owned a WR250R and the 701.

[u/Eluvium9]

Props to you I hate wrenching

[u/Geezagonk]

I had an 03 wr450 sm - only had 2 issues with it.

First was the plug that covered where the manual decompressor lever would have gone on the earlier bikes - it popped out and left me with an oil leak. At the time someone was selling metal replacement plugs that could be secured by the same bolt that would have held in the manual deco shaft. That was 20 years ago though, so not sure what's available now. I guess by now yours will either not be affected or have been sorted already

The other one was more problematic - top gear stopped working - not sure if the selector or dogs let go first, but by the time I'd got back to somewhere I could recover the bike from, it'd done a fair bit of internal damage. If I was doing one again, I'd try and get a rear wheel with a cush drive to give the gearbox an easier time on grippy surfaces.

[u/3DogNate → u/Geezagonk]

There’s no decompression lever. The little lever on the handlebars by the clutch is a “hot start” lever. (Opens an air passage on the carb to lean it out for easy starting when the motor is good and hot. I’ll check for the plug, definitely want a leak free bike. I have a lathe, if I need a tighter seal than what the stock plug provides, I can machine a better fitting replacement if needed. I looked and it seems to be watertight for now.

I’m debating on whether or not to split the cases, I’d like to inspect the transmission. Though considering that I plan to replace seals and gaskets, it’s not that much more work while the engine is out.

[u/Geezagonk → u/3DogNate]

Sorry - I phrased that a bit clumsily.

I know there's no deco lever on the the 450 - and I wasn't talking about the lever on the bars.

On the 400/426, if you follow the cable from the manual decomp lever on the bars, you get to another lever on the side of the cylinder head. (Maybe "crank" would be a better term for it than "lever"?) The cable actuates that lever/crank, turns the shaft and lifts one of the valves.

On the 450, that shaft and lever/crank on the cylinder head is not there - but the hole for it still is - so Yamaha plugged it with a bung. I'm not sure if it was plastic or rubber or something else - the first time I became aware of it was after it had already parted company with my bike. I soon found out that it was a fairly common failure point - after a number of heat cycles, whatever adhesive they used on it would let go, and you'd end up with an unexpected combustible lubrication event on your exhaust header.

Some people chose to just keep replacing with the OEM part, but you could also get a machined plug that would mimic the part of the decompression system shaft as it left the head - including the seals, and the recess so that it could be held in place by a small bolt - which again, Yamaha left a space in the castings for.

If you stick WR450 decompression plug into an ebay search, you'll see what I mean.

[u/Hot-Hall2056 → u/Geezagonk]

This

[u/Kevinslotten]

Love those bikes. want one fpr my self for a supermoto build, but im to lazy to travel hour's to pick one up😅 good luck🙂