99 yz400f

I have a 99 yz400f and it overheats fast. The antifreeze boils and pours out of the overflow. I tore it down and the pump is fine what else could it be? Any info helps thanks!

Overheats fast under what circumstances?


It is normal for this model to expel coolant after only about 120 seconds or less from a cold start, or perhaps 30 seconds when up to normal temperature if they are allowed to stand still and idle.  They need air flowing over the radiator, and they don't get that standing still.  Doing a lot of tight trail work at very low speeds will also put them over the top.


Other abnormal conditions that can contribute are a weak pressure cap or a blown head gasket. 

Over heats riding it hard or slow. No matter the day it'll over heat

Then it's most likely:


  • Weak cap
  • Scale deposits or clogging in the radiators
  • Head gasket leaking into the cooling system

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