Blowing water out of exhaust

99 YZ400

The bike started up first kick tonight. Road it about 3 miles and ran like a top. Shut it off for about one hour and checked and filled the radiator, which was pretty low. After filling the radiator the bike would not start. Never fired once after dozens of kicks.

While kicking it over another guy noticed some serious amounts of water started blowing out of the exhaust pipe. Checked the radiator and sure enough it's very low again and possibly empty. I did leave it out in the rain for a night accidently last night but the air box is bone dry.

A friend is telling me it's probably a head gasket or cracked head. Thought I would ask for another opinion and a recommendation on what to do next.


Check for water in the cylinder by removing the plug and cranking it. If it's there, it's most likely a bad head gasket. Cracks in Yamaha heads are rare.

Is it possible to remove the head without removing the cams? Guess the question is can you get the cam chain off the bottom gear and slip the whole assembly off?

I'm getting ready to pull the counter balancer and I'm hoping it's somehow possible to slip the cam chain off the gear and pull everything out at once to replace the head gasket. Maybe it will be obvious once I get in there.

Nope. The only way to get the cam chain off is to pull the cams, and the cams cove up the two head bolts on the left side, anyway.

Got it all taken apart and the cylinder is full of water. The gasket looks pretty good.

You can see on the front throttle side there is a dark spot between the combustion chamber and one of the water channels. In my unprofessional opinion it looks like there was a leak.

The external head bolt on the same side of the leak wasn't as tight as the other. The front bolt was also more loose than the rear. It seems like maybe the headbolts where not torqued down correctly or maybe just came loose?

Check the cylinder head and the crank case sealing surfaces for cracks/scratches, if everything looks ok I would buy a new head gasket and give that a try.

Turns out there is a low spot and a chip out of the head on the edge of the water channel. Had a bike shop in town take a look and he suggested I have it welded and have the machine shop plane it down a couple thousands. One of the valves is so close to the head surface that any more than a 1-2 thousanths off may cause valve issues. He thought it would take 20 to get rid of the chip.

Anyone know if there is a minimum head thickness I can check for?

I think this was a known issue when I bought the bike :thumbsup:

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