1992 xr250r
 

In my search for a used 2 stroke dirt bike, (which was really hard to find a 2 stroke at a decent price) I can across this bike. It needs quite a bit of work cosmetically and engine wise. I test rode it and it ran and shifted great. The problem is it smokes and uses oil and needs a top end rebuild and it has a tick I need to investigate. These bikes were ahead of there time with a 4 valve head 6 speed tranny, aluminum swingarm and aluminum wheels. The guy I bought it from owned it for 8 years and decided to sell it. I bought it with a extra set of knobbies for $450 so I really couldn't pass it up. I look forward to tearing into this thing. If anybody knows of somebody with parts for this bike please let me know. I don't know what I need yet but I will be looking for parts some new some old. Thanks

Nice! :tup: looks to be a complete bike. Hope you can find parts for a decent price. The reason i sold my 200x was because even used parts were ridiculously priced. You should still be able to find some after market parts for a good price. Ebay is usually a good source for them.

Nice find...I'd have bought it too.

Nice find herbie and rebuild kits don't look too bad!

https://www.ebay.com/b/Motorcycle-Bi...59/bn_94120309

Thanks guys!
I know everyone loves a good tear down, so here's what I found so far. One rocker face is worn out bad with a divot in it. The camshaft has locking pins in both ends of the camshaft to stop it from moving. Well they were both missing and you can see in the one pic where the camshaft was moving and the sprocket bolts where wearing a groove in the housing. That's the noise I was hearing. The piston and cylinder bore looks good so a good hone job and a new set of rings should be sufficient. I think the oil usage and smoke either came from bad valve seals or valve guides. I'll dig a little deeper when I get more time.

The XR engine design on the top end is their one real weak point. Well known to have rocker and valve issues over the years.

That seat cover is epic, btw.

Yeah, that double rocker design is kind of silly, Seems like it could of been done with a single (less moving parts) rocker.

Epic is right!:tup: I'm not sure it's something I would have done and I will be putting a new seat cover on it but it was kind of ingenious.

The valves looked pretty good other than the build up. I gave the guides the good ole redneck pop test and they sounded good.:tup:
The valve seals were super hard as expected. I started cleaning the carbon out of the exhaust ports on the head and in the good old chinariders fashion, I shaved off a little extra :lmao:

Looks like I will be able to get everything I need for under $200 which I'm stoked about. I'm really surprised some of these parts are still available.

Honda used the twin rocker arm arrangement because of the RFVC design, with the valves not being planar to the camshaft. There's not really another way to do it, except putting valves in normal places.

True, but a twin camshaft would have been more reliable and less wear imho, not to mention easier valve adjustments.

The problem is that the valves move towards the piston, and if you had like a shim over bucket setup with the cams acting directly on the valves, you still run into either the cams not running planar to the crank (and the lobes would have to rotate independently of each other), or you'd still have to have rocker arms that pivot in plane with the valve travel.

They probably looked at a bunch of ways of doing things before they settled on this, and they probably decided that this was the lightest and worked the best. An extra cam, sprocket, and longer chain would probably have similar if not heavier rotating mass, with more machining required. As we know now, a conventional 4-valve head would have worked better, but they were pushing the envelope at the time, probably without computers to help them design it.

I'm reminded of Yamaha's decision to stubbornly stuck with the 5 valve engine, even when it was no better than 4 valves. I think sometimes manufacturers stick with unconventional solutions for marketing purposes.

However, as far as I know, these RFVC motors have some of the highest specific output for a mass produced, air-cooled single cylinder motor, so they must have been on to something.

Good reply Franque, I agree with everything you said. You seem to know your stuff! :tup:

Before and after hone

Looks like it will do the job!

Why is it when you decide to pick up a project that you love to do, things just pop up that take your time away from it? :hmm:

Anyway, I decided to open the right side crankcase cover and have a look at the clutch. I found a little carnage inside, seems the oil pump drive gear took a beating. Fortunately I found one on Ebay that looks good for $7.50