Recently built 450 but still no compression.

4 replies to this topic
  • poswr450

Posted October 23, 2010 - 08:19 AM


i blew a head gasket out on the trails.

has not run since.

the intake valves were looking pretty shot so they were replaced. The center intake valve will not make clearances even with the smallest shim. I had .02 taken off the top of the valve but it still wont clear with the smallest shim. Could this be my compression problem?

Also i put a weisco 13.5:1 piston in. The stock piston was really wobbly in the cylinder. The new piston fits well and im still left with no compression.

New timing chain was also put on.

Any help would be great.

  • CanadianWR450

Posted October 23, 2010 - 09:25 AM


The 450's have a auto decompresser. You can not do a "normal" compression test, you have to do a leak down test. Is this how your determining you have no compression?

As far as I know, if you replace valves, you also have to have the seats redone in the head as well (usually at a shop). Someone can correct me on that if I'm wrong.

Is your cylinder in spec? If the original piston was "wobbly", I'd diffidently be taking a hard look at the cylinder as well.

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  • Birdy426

Posted October 23, 2010 - 06:45 PM


Yes, you should have the seats cut when you replace the valves, but even with the seats uncut, you should still be able to shim the valves. Are you sure you have the right valves in the right spots? The center intake is a different part number than the outer intakes. Also, are you sure you have the collets installed properlu?

  • djrevz

Posted October 25, 2010 - 11:17 AM


Theres obviously a sure chance it will be the valves but have you tryed feeding petrol through the carb and given it a few twists of the throttle? these thumpers work as a type of hydralic with the fuel system, were if you have no petrol in the system it will feel like there is hardly any compression but the second it has full feed of fuel the engine has perfect compression.

  • poswr450

Posted October 26, 2010 - 06:09 AM


Im going to have a new seat put in the valve thats giving me problems.

As for the fuel, its getting fuel, i gave the throttle a few twists while trying to kick over.

And for the auto decompressor, thats kind of what we were figuring.

Not going to have a chance to work on it for a few weeks because im away at school, but will keep this updated and let you know whats up with it as soon as i know.

thanks for the replies.


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