Leaking coolant yz400f (weap hole)

I recently purchased a 1999 yz 400. The previous owner was very good about maint. and care. He told me that he recently had the water pump updated to a 426 one. He said that the 4oo's had a common problem with leaking.

So after the first ride I noticed I was leaking coolant out of the weap hole. It only drips when idleing but at speeds it pisses it out. I replaced the two oil seals and the impellar shaft taking great care while installing new seals. This dis not cure the problem. (all the old parts looked to be in good cond. except for a little wear on shaft)I have spoken with some local techs. but they could not offer much advise.

Is this a common problem with yz 400's?

I have always owned two strokes and have been away from riding dirt for about three years so I am kind of out of the loop.

Any help or suggestions would be greatlt appreciated.

Both me and my brother have 99 YZ400f's and have no problems with leaking water. It will boil over though if riding on nasty goat trails around the Tucson area.

Don't worry 'bout it TOO much.

Last winter when I let my bike set near the front of the garage where it is a lot cooler, The first time I went riding in Jan it started leaking out of the weep hole.

I watched it for the day and barely used any coolant.

When I got back I called my dad and he told me what it was. It turned out to be a $.96 part w/my disc. and about 3bucks w/o

All it was was the waterpump seal.

12min to change it and I had no clue what i was doing!

Just pull the waterpump cover and then pop out the seal with a small flat screwdriver.

I tapped the new one back in using a socket that was exactly the same size and keep it square!


Better than a leaking radiator or something worse! :)

I'd guess you may have distorted a seal lip when installing it. If you put in a new shaft, seals and the bearing, that should have 100% cured the problem. I'd go back in for another look.

Ditto what Tuner said. Perhaps something is obstructing coolant flow leading to it being forced past the seal lip and out the weep hole. I imagine a faulty radiator cap could also cause this by allowing excessive pressure build up.

But that is pure conjecture on my part.

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