KXF Valve Seats?

[KXF329]

Well, as has almost everyone's, my KXF took a dump on me. A friend from another forum pointed me in this direction, so I have come to you guys for help.

I'll start off with what happened, and how it was acting before we tore it apart, to what we are looking at right now.

In my last race, and my last moto of the day (4 total) my bike died on me at the end of the first lap. When I got it re-started it sounded unusually deep, like a blown out 450. It was also backfiring ALOT and would not rev out at all.

We have since taken it apart, and the left intake valve was really loose, the other 3 were all really close or within spec. We checked the cam timing, and everything there was ok. We took the cams off to get to the valve caps, and once those were off none of the valve springs are visibly broken.

We then sprayed some carb cleaner into the intake tracts and both intake valve seats are leaking pretty good. Not dripping, but you can easily watch a wet spot grow on the valve. The exhaust valves did not appear to be leaking.

My first question is would a really loose valve and leaking intake seats cause the running condition I described? Or what do you guys think it is?

My second question is can the valve seats on these bikes be machined, or do I need to have them replaced.

Thanks,
Jason W.

 

[Rich Rohrich]

The valve seats can be recut using carbide cutting tools by any competent cylinder head shop.

Generally what happens is valve springs begin to wear which leads to the hard coating wearing off the face of the titanium intake valves and causing the softer titanium face underneath to wear out. That's the leakage you are seeing. The fix consists of, recutting the valve seats and replacing the springs and valves. Sometime in December Kibblewhite will be making available a kit that includes stainless steel valves, new high quality springs,lightweight titanium valve retainers and a stronger valve cup/tappet. Until that kit comes out the stock parts are still probably your best bet .

 

[bclapham]

Thanks Rich- Jason is a good kid, although he is a bit of a whiner when it comes to track prep! :)

Luckily, he tells me he has a KX125 to ride for a while, so that will keep him out of trouble. Just out of interest, what will be a ball park cost on the full KibbleWhite setup and your installation costs? I have a feeling you are gonna get asked this question quite a bit and that will grow old very quickly!!!!

 

[bclapham]

one last thing- where does this leave kids that want to ride the Stock classes? its gonna suck if you have to drop $1000 on inferior OEM parts before you drive out to Lorettas!

 

[Rich Rohrich]

one last thing- where does this leave kids that want to ride the Stock classes? its gonna suck if you have to drop $1000 on inferior OEM parts before you drive out to Lorettas!

Right now the YZF/CRF Kibblewhite setup is $475 parts and labor. That includes stainless steel valves, springs, titanium retainers, valve seals and the shims to set the valve clearances.

The RMZ/KXF engines will require tappet replacement as well so those will be more expensive. The RMX/KXF tappets have a nasty habit of breaking if you increase the valve spring pressure so you can't use them reliably with the better quality springs .

That's a good question on the Lorettas thing. Unless the AMA changes the rules I'm afraid a lot of guys are going to be forced to race during the season with the good Kibblewhite parts and then have to keep a totally stock head on hand to race Lorettas.

 

[KXF329]

Thanks for the speedy answer Rich. Do you know of any other companies that make reliable valves and springs?

Also, my dad is wondering of the loose valve and leaking seats would cause the sudden stall, and poor running conditions I experianced (wouldn't rev, excessive backfiring, deep sound note), or if we should continue disassembly and tear down the bottom end to look for other problems.

Once again, thanks.

 

[Rich Rohrich]

Thanks for the speedy answer Rich. Do you know of any other companies that make reliable valves and springs?

Not for this engine. Using OEM parts or waiting for the Kibblewhite parts to be released are the only real options at this point.,

Also, my dad is wondering of the loose valve and leaking seats would cause the sudden stall, and poor running conditions I experianced (wouldn't rev, excessive backfiring, deep sound note), or if we should continue disassembly and tear down the bottom end to look for other problems.

Once again, thanks.

When the intake valves leak combustion pressure backs up into the intake and causes some pretty erratic running.

While you are at it you might want to take a look at this thread too :
http://dirtrider.net/forums3/showthread.php?t=105314

 

[KXF329]

Ok thanks Rich. Bruce originally gave me the link to that thread, and that's what led me here.