yz 450 center intake stuck closed?

7 replies to this topic
  • derment

Posted February 05, 2009 - 10:57 PM


Im new to this site, I took my bike in to have the valves adjusted and the shop said the valves were good, but there was a pinging noise as if the piston and cylinder need to be replaced. I dont burn smoke out of my pipe and the bike only has about 25 hours on it (2008 yz 450). So I went home and took it apart to check the cylinder walls to find that they are in great shape. I looked all other parts over and found the center of the 3 intake pistons to be brand new. Meaning there was no carbon build up like there was on the outer two pistons. It looks like it has not been opening? I dont know if this is normal or not.

  • rufusz

Posted February 06, 2009 - 03:55 AM


What are you driving? A locomotive? :))
Just a joke, you are confusing pistons with valves. You have one cylinder, ergo you have one piston that moves in the cylinder, and you have 5 valves (3 intake, 2 exhaust). It's normal that exhaust valves have carbon deposits because after the gas ignites in the cylinder, it is evacuated on the exhaust port. Exhaust valves open, gas, and other left-overs of the burning process, are "pushed" out along the valve, and that's why it has much more deposit than the intake valve. One of the valves to get stucked closed is nearly impossible without breaking several other parts. The camlobes would push it down, and if the stem would broke, all the other assembly parts of the valve would migrate somewhere to make trouble (cylinder, engine, crank).
MX engines are known for being loud, if they shimmed the valves to spec, you don't have to worry about them. Go and listen to some other 450 engine to see if your makes more noise or not.

  • DC5039

Posted February 06, 2009 - 05:16 AM


Pinging is often caused by bad fuel, try chucking in some fuel with a higher octane level and see how that goes.

  • matt4x4

Posted February 06, 2009 - 05:27 AM


Well, there's only 2 things that would stop one valve on a cam operating 2 or more valves from opening, either the lobe is no longer there (impossible) or the top half of the valve stem is no longer there and the valve miraculously stayed in it's guide - also nearly impossible.

I believe the center valve gets the most direct flow of intake gasses passing through, as well, the cam lobe is timed slightly different than the outside two, essentially washing the valve clean better than they do on the other two, so it looks newer.

If you want to see whether or not it opens, once you put it all back together, keep the carb off the bike, move your kickstarter down slowly and look through the intake port at the valves - you WILL see it opening and closing - you can view the same on the exhaust valves from the exhaust port side.

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  • derment

Posted February 06, 2009 - 07:29 AM


thank you guy's for your help. sorry it was late when I wrote that it was a intake piston, I meant intake valve

  • grayracer513

Posted February 06, 2009 - 07:32 AM


Think about it: If the valve were stuck closed, what would happen when the cam lobe came around on the intake stroke? Didn't go quite that far with it, eh?

What you see is normal. The intake tract carries fuel droplets atomized in air, moving at high velocities. When this mix arrives at the intersection of the 3 individual ports, the air tends to go down the center due to its inertia. Some swings left and right to the outer ports in response to the pressure created by the larger main port meeting the smaller center port, and also due to the low pressure created along each side by the widening of the main port at the branch. The fuel does the same thing, except that it's much heavier than air, and so has a much higher tendency to go straight down the center, rather than out to the sides. The result is that the center port flows a much higher concentration of fuel in the air than the outers do, and with that fuel comes the fuel system cleaning additives, which then keep the center intake cleaner than the other two.

  • grayracer513

Posted February 06, 2009 - 07:37 AM


the (center) cam lobe is timed slightly different than the outside two, ....

Not really. It looks that way, but that's due to the fact that the valve enters the head at a different angle that the other two. The timing is the same.

  • derment

Posted February 06, 2009 - 10:20 AM


thanks again, Im new to motors. Im a contractor and can build almost anything but this is the first time I have ever tore into a motor. Im very cautious and through. But now Im understanding how it all works.

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