'03 YZ450 Overhearting After Engine Rebuild

Today I finally got around to fixing a couple of problems my bike had 6 months ago. During my last trip out with the bike a bolt fall out of my motor and the engine wouldn't start afterward. I torn the engine down and found the piston had scored the cylinder wall. So I replaced the piston and cylinder with OEM parts, and while I was at it I installed Hotcams Intake and Exhaust Cams and a Hinson Clutch Basket. So today I spend the day putting the bike back together and everything went great, however, after letting the bike run for about 20 minutes coolant will start pouring out the drain tube. The bike never did this before installing the cams or other new parts so I would assume the water pump is working like it should and from here I pretty much don't know what else to do. So if anyone can help me with this overheating issue I would appreciate it greatly. Thanks in advance.


Check your head gasket.

If the water is bubbling and foaming while running and gets worse when you rev it a bit, you have a bad head gasket. If the oil in the motor has water in it or its got a white or light tan color to it, you got a bad head gasket.

I bought the Athena complete gasket kit for my bike so all my seals and gaskets are brand new but I'll still give it a look. I'm going to change the oil again tomorrow so I'll be able to check the color of the oil too. Thanks

... after letting the bike run for about 20 minutes ....
Please tell me you don't mean you let it idle that long. What do you mean by the statement above?
Please tell me you don't mean you let it idle that long. What do you mean by the statement above?

Maybe he wanted to cook something on it :ride::):lol:

Sorry for the off, but just felt the need to do it :))

someone told me that if you let your bike idle for more than five minutes it could blow up because it will over heat because it cant cool properly when no air is being pushed through the radiator. your bikee proboly just overheated causing it to cough up radiator fluid.

sorry guys, wrong choice of words I suppose. I would start the bike, ride it up and down the street a few times and then come back to the driveway and put it on the stand and let it sit and idle for a bit. Total time being about 20 minutes. I let it sit and idle long enough to loosen the rear axle and tighten the chain and that was enough for the coolant to pour out of the line.

seems normal to me...

hapens all the time here when people drives their Yz's on the cities and has to stop on the lightsigns for two minutes

+1 on WR's compesating tanks

A warmed up YZ450 will blow coolant within 90 seconds or less of idling at a standstill just about every time. You could have your cap tested, but this all sounds normal to me.

okay thanks guys. I'm going to pull the bike apart again to check the valve lash again. I'll let you know how everything turns out.

well I just got the bike back together after checking the valve shims again. Everything is running great and the bike isn't overheating now like it was.

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