Carbon build up on two outside intake valves only

[yamaha491]

I have an 06 YZ450F with 90 hrs of use. I decided to rebuild the top end mostly just to check on the condition of the head and cylinder. I replaced the timing chain and piston along with tearing down the head cleaning things up and lapping in the valves. In doing so I noticed a few things about the valves I was hoping I could get some answers and or comments on. The first thing I noticed was that the two outside intake valves had quite a bit of carbon build up on the top (intake side) while the center intake valve was perfectly clean. The second thing I noticed after lapping in the center intake valve was that it had a slightly larger valve seat width. The third thing I noticed was that my center intake valve clearence was only .002 after a temporary assembly ( the spec being .004 - .006) . I had checked the valve clearence at 70 hrs and all the valves were in spec but were on the tight side. Based on what I discovered I had a couple of questions.

Is it normal to see more carbon build up on the outside intake valves?

Is it normal to see a little variation in valve seat width due to normal wear or just a little difference in machining?

Is having to re-shim .002 - .003 normal for a motor with 90 hrs?

I would appreciate any input on this matter.

Thanks

[grayracer513]

The clean center intake is normal. You will notice the center port is also cleaner than the outer two. The fuel droplets in the intake air are relatively heavy, and are much more prone to run down the center port than to swing outward and run down the outer two. This washes more of the fuel, which is loaded with cleaning additives, over the center intake. Interestingly, it also has the effect of concentrating the bulk of the fuel in a relatively rich mixture near the center of the combustion chamber, making it easier to ignite, something like the Honda CVCC car engines.

The wider seat and the .05mm shortfall in clearance are wear at the valve seat. As long as this is wear at the seat only, you could reassemble the head and use it, but of course you can not know how much longer the valves will last.

Notwithstanding what is said anywhere else, hard coated titanium valves such as those used in the YZF should never be lapped. The valves absolutely depend on the nitride coating applied to the faces for their ability to wear well enough to last for even a few hours use. The coating is unbelievably hard and durable, but it is also exceptionally thin, often less than .0005". Lapping the valves runs the risk of compromising the coating and promoting its early failure.

When the valve clearance will remain stable over time, the valves are OK. When they continue to close up within 3-6 hours of run time after shimming, they need to be replaced before they snap a valve head off.

[yamaha491]

I appreciate the quick response. Thank you.

About the Quote on lapping the valves.

Notwithstanding what is said anywhere else, hard coated titanium valves such as those used in the YZF should never be lapped. The valves absolutely depend on the nitride coating applied to the faces for their ability to wear well enough to last for even a few hours use. The coating is unbelievably hard and durable, but it is also exceptionally thin, often less than .0005". Lapping the valves runs the risk of compromising the coating and promoting its early failure.

You make a good point about not comprimising the Nitride coating but why does Yamaha suggest lapping the valves in their service manual?

[grayracer513]

I believe that is a carry-over from the steel valve days that has not been removed from the manual. You see that in the trade all too often. It may be that somebody thinks it's a good idea to lap Ti valves, but as I said, I don't, and I don't know of a good engine builder off the top of my head who disagrees with me on that particular point.

[yamaha491]

I definitely see your point. Based on our conversation I am strongly considering finding a reputable shop that specialize in modern 4-strokes to re-cut my valve seats and install new valves and guides. I am located in Indiana is there anyone you would recommend on this side of the country who could turn this around pretty quickly.

[Birdy426]

Eric Gorr does good work up in Wisconson...http://www.eric-gorr.com/

[grayracer513]

I definitely see your point. Based on our conversation I am strongly considering finding a reputable shop that specialize in modern 4-strokes to re-cut my valve seats and install new valves and guides. I am located in Indiana is there anyone you would recommend on this side of the country who could turn this around pretty quickly.

In my opinion, seats need to be ground, not cut, at least on the finishing pass over the actual seat itself. There's no problem cutting top and bottom angles with a cutter to put the seat where it belongs, but no cutter can ever put the finish on a seat that a good set of stones does, and this is a key element in how long the refinished valves/seats will last.

Contact Ron Hamp Cycle in Michigan and see if he can help.

[yamaha491]

Heard any good or bad about Millennium Technologies. They are in the add section of Dirt Rider magazine.

[grayracer513]

They are a TT sponsor, but I don't know much about them one way or other.