I have one broken sub-valve (RHS) and one stripped sub-valve (LHS) from my 1990 KDX 200 and from what I understand I'm not the first to have this exact same scenario. (e.g. Buckhat) Does anyone know why these parts fail so readily? Do you think steel ones would be more reliable? Are there any steel ones made out there? The bike's only got 150 easy hours on it since the last top-end was done.

I think they fail because they get all gummed up with spooge (not entirely possitive though). If there were steel gears in there..somthing more expensive/harder to replace might go out, there's got to be a weak link somewhere in the chain.

Quote:
I think they fail because they get all gummed up with spooge.

That may be true but my valves were not all gummed up with spooge. They had a slick oil covering but were not gummed up at all. I also have a '98 220 which I've recently purchased ... If I have to replace these valves (~=$38/valve) this often, I won't be happy.

There are a couple reasons 89 to 94 KDX200 sub port valves break and all have to do with the valve being jammed/stuck in their bores. One reason, already mentioned, is that the valves become coated with carbon over time and seize. To help avoid this be sure to clean the valves with each top end service. Some people have polished or hard anodized their valves, which may make it more difficult for carbon to stick and make them wear longer. The second reason is that over time the hole at the bottom for the sub port valve’s bore, where the shaft is inserted, becomes enlarged and the sub port valve’s shaft wears and becomes smaller. I’m sure the added stress of carboned up valves only accelerates this. Once the hole is enlarged the sub port valve has a poor/sloppy fit and becomes wedged in their bore and snap. If you’ve replaces your sub prot valves only to have them break again shortly after this is almost for sure your problem. There are a couple things you can do to fix this. Obviously if you replace a damaged sub port valve the shaft will be larger than the worn original, but the diameter of the hole where the shaft is inserted will still be over sized. I’ve had good luck in the past using this method to resize the hole: Using a pointed punch and hammer make four punch marks around the hole no more than about 3 or 4mm away from the hole’s edge. Evenly space the punch marks so they end up making a square pattern around the edge. The idea is to deform the aluminum and reduce the size of the bore. Test fit your new valve. You should be able deform the material around the hole enough that the valve’s stem no longer fits into it. Measures the valve’s shaft and using a drill bit the same size carefully redrill the hole ensuring you’re hold the drill vertically and going no further than the original depth. If the hole is badly worn I expect you may not be able to deform the material around the hole enough to enable you to redrill it. If that were the case I guess you’d just have to be happy knowing the fit is tighter than it was and go from there.

If you have access to a machine shop you may be able to resize the hole and install a brass bushing?

You should have any problem with your 220's valves, they were redesigned in 95.

I hope that helps,

David

Has anyone thought of having the valves coated by HPC or similar company?
I had the piston done on my old 83 KDX250 and found it eliminated carbon buildup on the crown.

1. I've got some additional thoughts on causes of this problem -- and a partial solution -- posted on [this page]. I've also added a link there back to this thread so that future readers will also have access to C. Dave's method.

2. jpm200: What does HPC stand for mate?

High Performance Coatings they are in Leongatha Vic.

www.hpcoatings.com.au

or for those in the US www.hpcoatings.com

Aha! Thanks mate. Looks very interesting. I'd love to have the top of my piston coated to keep the carbon off. I've polished mine to a mirror finish to try to minimise it, but this is a pretty futile step that doesn't do much for long.

The radiant heat absorbed by the black carbon would have to be worth a few percent of the horsepower potential...

I like the look of their aluminium exhaust coating too.

Cheers

Thanks for the great replys guys.

C-Dave,
How much bigger should the hole in the bottom be compared to the shaft of a "new" sub-valve? Service-albe limit?? If the hole is too enlarged, I think I'll go the brash bushing route. Is brass preferred because of the thermal expansion properties?

Boot,
How many hours do you have on your bike now with the modded bearring collars? That looks like a great idea!! Have you had a chance to check things out yet?

You guys are the best!!

I'd say I've only got somewhere around 30 hours on the modified collar now. It's been winter down here, so most of the year's riding is yet to come. I've only had it apart once (after seizing the new Wiseco piston...) and the powervalves were still working fine. However, this was very soon after doing the mod. My gut feeling is that it's still working because I can feel and hear the powervalves open at 6000 rpm, and the top end rush and bottom end grunt seems as strong as ever.

Sometime in the future with some more hours on the engine I'll pull it all down again and post a report on my site.

If the left valve does go again on me, I'll be trying Dave's neat trick of distorting and re-drilling the lower bushing.

I think Dave suggested a brass bushing not for thermal reasons but for wear characteristics? A phosphor-bronze bushing would be the ultimate material to use, but brass is a good approximation and would wear much better than the aluminium cylinder alloy. I don't think clearance would be a religious issue either. You'd just want it close enough so it's not sloppy, but not so tight as to bind things up.