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Lost my compression!

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It finally happened, but I was expecting it since I went through it last year on a 2002 CRF-450. I pulled the valves out and the intake valves both definately look cupped.

One thing that surprised me and kinda threw me was I was expecting the exhaust valves to be steel like the CRF!!! Are they titanium? A magnet didnt stick to the exhaust valves so I think they are.

Now.... it made sense on the CRF that the intake valves were cupped and the exhaust were ok because they are a different material, but... if the KX has 4 titanium valves why would my exhaust valves appear to be fine while the intake valves are cupped??????

Anyone shed some light on this, I would think the exhaust valves are being subject to worse conditions such as higher temperatures however the intake valves could be exposed to dust/dirt which may cause erosion on the faces?

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I think the intakes go first because they have way more thermal cycling. They get cold air with fuel mist dumped on them, and then fire. Also on the blue bikes, there has been a correlation between the valve the accelerator pump squirts the direct stream of fuel on, to have this intake go first. That valve would get even more thermal cycling. Some people say its heat that causes problems, but as you point out the hotter valves do better. The fuel mixture also flows across the portion of the valve that is cupping, so that surface is getting the full effect.

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Its a 2004...

So you're saying the exhaust valves are titanium as well right? Makes sense because the magnet wouldnt stick to them.

I'm still confused as to why they dont show any wear when the intakes do. What are the differences...

exhaust - more heat, consistent though

intake - more range of temperatures, cool intake air then hot.

# of cycles of the valves are the same.

how far they open should be the same.

seats are the same.

Perhaps there is some truth to the dust eroding the valve faces???

It was almost predictable when they would fail though since I've seen it before on the CRF... I rode the KXF all year quite a bit with no problems, I never revved it too high though (ie rev limiter) because I felt that the high revs contributed to my CRF failure.

But then last weekend we were riding in the sand and its not usual for me and I was revving it higher and shifting less, then by buddy was on it and a couple times he bounced it off the rev limiter. Then sure enough the next time I tried to start it it had no compression.

So that sort of points towards the valves floating and slamming against the faces causing this BUT you'd expect to see the same thing on the exhaust side too.

So...... I have no answer

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same was on my bike: intake valves almost destroyed and exhaust valves like new (except from the carbon on it)!

I´d say that this is caused by the "mysterious microdust". Cause I had that problem when I took my backfire screen out. And now with the screen inside the bike again, no problems at all for about half a year now. 😢

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So you think your screen filters out smaller particles than your air filter???

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