426 overheating Badly

2 replies to this topic
  • racerx573

Posted September 24, 2008 - 01:37 PM


Here's my scenario, I have the engine in a quad frame for MX.

I have a 42 pilot
172 Main
Needle is on the 3rd clip (stock needle)
It's ported, K&N filter, 13:1 piston, 450 cam mod.
Wet sump kit

It has no response off the bottom end, but screams once it is wound it, but takes a while to get there. I just checked out all the jetting guides and printed them out to try some different combos.

My plug is burning kind of a grayish brown, and my exhaust tip is a little white, so she was running lean.

When I come off the track the coolant is down to the fins in the top of the rad, and she is blowing steam furiously.

I installed a catch bottle, and a high pressure radiator cap.

I figured since it was running like that I may have a leaky head gasket, so a new one is on the way, and there was a minute amount of drip from my weep hole, so I ordered a new mechanical seal.

I am going to try all of this, and run the jetting I was running before.

But if anyone else has any insight, I would greatly appreciate it.

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  • grayracer513

Posted September 24, 2008 - 02:06 PM


First, confirm that the flywheel key is intact by checking actual TDC through the spark plug hole against the timing marks. Then check the cam timing.

  • racerx573

Posted September 24, 2008 - 02:26 PM


I've got the whole head off right now still, I am gonna check all the timing and whatnot in reference to the YZ450 cam mod thread. I bought the engine used, so something could be out of line. What should I check for when checking the flywheel? Thanks for the info.

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