Ran 04 WR450 out of water!!

Okay. My kid ran his 04 WR450 into a tree at an enduro this past weekend, smashed the radiator on the left side and cracked the top tank. We gambled and tried to get it back to the next reset/gas before the water ran out.

We lost the gamble and the bike died 2 miles before the gas stop.

It rolls over, so it didn't sieze.

It seems to have good compression, but with the auto decomp I'm not sure I'm getting a good feel for it.And I haven't done a true comp test yet.

It has spark and seems to have fuel too, so I ruled out any kind of overheat sensor that it might have.

There is a plastic plug in the head that faces forward just above the header that is now leaking oil onto the header. I imagine it got hot enough to melt that enough to leak. What is that plug for?

My guess is a blown head gasket or a cracked head.

I'm looking for suggestions from the hardcore yamaha 4stroke mechanics what to look for. I am very familiar with 4st bike motors, but not the 450 yamaha.

The is a decomp plug, the bikes are famous for blowing it out. There are 'fixes' for it that are prermenant.

I'd do an oil change, looking at the oil and filter for any particulate, Assuming the oil looks normal, I'd simply refill and test the bike. Run with the rqadiator cap off, looking for bubbles and at the exhaust, looking for 'steam'. After this, we can chat more.

I know this is no help to you, but may save someone elce some trouble. I grew up on a farm, around lots of equipment. Dad always told me that if there is any question what so ever about oil or coolant, SHUT IT DOWN NOW.

It's just a dirtbike and you'll get it fixed relativly cheaply. Could have been your truck....


I'll keep you posted. It may be a few days before I can dig into it. I have to clean and do maintenence on the borrowed bike my kid rode for the 2nd day, and get the radiator fixed first. Stay tuned.


If it was a trail ride, shut it down works, but racing is a different story.

And I think trucks are cheaper to fix than dirtbikes, seeing that my bikes are worth more than the vehicles I drive.

I told my kid that his bike is broke, but your coming home in one piece and that is alot to be thankful for.

I cleaned the bike tonight, hooked up the batt charger, hit the estart and it runs.


I'll get the radiator fixed, change the oil, get a good look at the filter for shavings, and take the kid for a trail ride to test it out before the next event.

What is the fix for the plastic plug above the header that is leaking a bit of oil?

Oh yeah. I filled the radiator with water ran it at idle for a minute or two, and I see no bubbles in the water. Good sign!

For the decomp plug. the TT store sells a kit to resolve it, so do a few other manufacturers.

Keep a real close eye on the coolant level and exhaust. If the coolant drops more than normal or you see steam out the muffler then there is a head gasket issue, at a minimum. Also listen very carefully for any sounds emanating out of the engine that are not normal. May not be a bad idea to pop the exhaust cam out in inspect the journals as they are often one of the first to suffer from oil/overheating.

What is the fix for the plastic plug above the header that is leaking a bit of oil?

An 18mm freeze or oil plug from the auto parts store works great.:)

I cleaned the bike tonight, hooked up the batt charger, hit the estart and it runs.


I've overheated my 400 enough a couple of times that the engine quit. I let it cool and restarted with no apparent damage. The same thing happened to me in an old camaro too, leak in the rad, overheated and shut down. Cooled and started with no problems.

I think that the engine gets so hot that the heat ignites the fuel instead of the spark, throwing off the ignition timing. The valve clearances may also tighten up enough to allow some blow by, losing compression.

It seems to be more of a 4 stroke thing. 2 strokes tend to grenade, 4 strokes just want to take a breather. I'm sure that if you ran hard steadily it would wreck something, but usually you hve to back off for a turn once in a while, and that's when the engine will shut down and refuse to refire.

how would you know frostbite? you gotta use a space heater to keep the antifreeze from freezing :)

i think mine stalled at 350 degreese cooled it back down added water from a my last water bottle and took it into the creek to clean the rad

you probably are ok, i just get worried i cant fix a 4 stroke

how would you know frostbite? you gotta use a space heater to keep the antifreeze from freezing :ride:

:D busted! :worthy: Good one. :)

After checking the flywheek key and that being okay, I was baffled. It still missed on idle, would just die sometimes and was hard to start.

I cleaned the carb 3 times and still had no luck. It was just like the idle circuit was plugged or not working.

Then it accurred to me that a friend of mine had the same bike and I could swap carbs and other parts to eliminate or pin point the problem. We swapped out the carb with no difference at all. Next I was going to swap the CDI unit, then I looked at the plug cap/coil unit and thought it was easy to try. Swapped it and it ran like it was brand new! I swapped it back and forth a few times to make sure I didn't just disrupt a wire.

It's the coil for sure! It makes sense that the overheat killed the coil, seeing that it is part of the plug cap. It's too bad I took the whole top end off.

The good news is my son gets to ride Stumpjumper now. :thumbsup:

Thanks for all of your help and suggestions.

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