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Overheating?? 426 Help needed

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Ok, quick background. 2002 YZ426F which I use for both MX riding, trail riding, and supermoto. Never had any issues with it once I learned the secret handshake for starting it. Other than a Pro moto billet end cap the bike is stock.

Long story short...I was in a race a few weeks ago, and it took forever to set the grid bottom line the bike started to overheat from too much idling...finally a green flag. I finish the race with no cooling issues. Since then I have changed the oil, topped up the coolant(only lost a few oz) and went trail/MX riding this weekend. Now the issue is after/during every ride I am losing coolant through the overflow. Just a few oz lost this weekend, and the bike never failed me when riding. Any thoughts on what might be causing this issue...even with a 3oz catch can it overflows Could i have a bad radiator cap? Oil looks good, and so does the coolant. No white smoke...:thumbsup: I'm at a loss. thanks for reading this book, let me know if you need more info, or have a suggestion

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Assuming you have not changed any carburettor settings I suggest two actions. First drain the coolant and fill with new fresh coolant. Second change the radiator cap.

You say you are using a catch bottle, does it have the capability to allow any overflow to be drawn back into the radiator ?

I use a catch bottle which is set up to allow the overflow to be drawn back into the radiator. While the level in the catch bottle does vary depending upon riding conditions (I ride hot desert) the radiator itself is always full.

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Assuming you have not changed any carburettor settings I suggest two actions. First drain the coolant and fill with new fresh coolant. Second change the radiator cap.
Very sound advice. :thumbsup:

A shop can test the cap for you, BTW.

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I don't mean to sound cheap, but $41 for an OEM radiator cap ***!!!!

I have read about replacing the 1.1 cap with a 1.6 cap. Any thoughts on going this route?? Sounds like the OEM Kawasaki part will "bolt on" and is only $27

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I have read about replacing the 1.1 cap with a 1.6 cap. Any thoughts on going this route??

It should not be necessary. When things are working as they should, 1.1 bar (16 psi) should be enough. But as long as the cooling system hoses, etc. are in good shape, I don't see a problem with it.

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you usually can find those 1.6 bar caps on ebay for under 20.

when coolant gets hot it expands, and you are usually supposed to give the radiator a bit of air on the top, like fill above the fins, to give the coolant room to expand.

so you might have just topped it off, and it just purges what your topping off

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you usually can find those 1.6 bar caps on ebay for under 20.

when coolant gets hot it expands, and you are usually supposed to give the radiator a bit of air on the top, like fill above the fins, to give the coolant room to expand.

so you might have just topped it off, and it just purges what your topping off

I found a 1.1 bar cap that fits my 99 Yamaha yz400f at Kragen Auto for around $12, there may be a 1.6 as well but I didn't look for one...

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Are you saying that when you take the cap off after filling it to the top then running it...it went down a little ? because If you fill it to the very top thats going to happen.. when the fluid expands and exceeds the caps pressure rating your going to lose a little..how much are you losing and how full are you filling it?

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