WR400F Boiling Over When turned off HELP!

My 98 wr400f seems to boil over when i turn the bike off after some slow trail riding. One of my friends says i should remove my stock silencer insert because it is causing too much back presure.

Will removing the snorkel change the jetting so bad my bike may over heat?

Taking out the snorkel or removing the baffle will not make any difference in wether or not the bike will overheat or not.

You said it all in your first post why your bike is overheating. Slow trail riding.

Bikes that are liquid cooled require air movement to keep the radiator from boiling over.

Like Mo said...u need airflow through the radiator, or it will boil over. I have had the same prob on my '03 wr450 and my 96 cr500. If ya lug it..it will do that. You may want to try water wetter, or other products like that. It may help a little.


"live well..ride free" :)

I second the water wetter usage. I still lug slow and hard but overheat less often with this stuff. I'm also in the habit of checking/topping the radiator before each ride.

Watter wetter and Silkoline pro-cool will help , when you stop to rest shut the bike off dont let it sit there and idle You haft to keep air moving through the radistor's to keep it cool , ride a bit faster.

Raceanything,I have also noticed from numerous jetting experiments,if your pilot is rich this can worsen the low speed boil over condition.If you richened your pilot,go back down.if your running the stock pilot/air jet combo make sure you have the fuel screw set correctly.As mentioned above water wetter or engine ice coolant will help also,good luck.

You could also try a cooling fan from a modern trials bike!,the cooling fans on these bikes are no bigger than the cooling fans in your computer?,as far as i know these bikes run the same kind of electrics,so fitting one on should not be a big task.If you find the over heating is due to slow trails and is not down to jetting or blocked rads(fins straight and able to flow air),then this is a mod i would go for,they should fit in quite tidy i think! :)

what's up anytime,i ride my yz in the trails all the time and never overheat,i use water wetter,but your problem sounds like, you need to jet alittle richer and check and make sure your accelerater pump in your carb is working,if it's not you can get this same problem. :):D

Use some insulation tape over your header. This will stop heat from transferring to the engine and radiator.

"Engine Ice" is another ready-to-run product that seems to work well. I often putt along helping beginner 50cc riders (lots of slow speed,idling, etc.) and never have a boil problem. Doesn't even seem to get too hot. Just don't idle too much, like they said and take her up to speed occasionally.

You might want to make sure your water pump is working right. I think a small electric fan or two would be an excellent idea for slow-trail riding. Should be (but isn't as far as I know) an available optional part for WR's IMHO. Also, I was never very impressed with the collapsing stock plastic radiator louvers. I installed Flatland guards and I think my rads get more air now.

I've got a couple of questions for you.

First, Has this bike always overheated in the tight trails or did the just begin recently?

If this is a recent thing, then the first thing you should check is the radiator cap. The 400's are notorious for overheating when the spring in the cap begins to loose it's pressure. The simple fix is to take the cap to your local auto parts dealer and purchase a 16lb cap to replace the weak 1-3 lb cap that comes on the 98-99 WR400's. a 16lb cap will raise the boiling point in the radiator by 70 degrees. additionally (with the bike cold) Start it up and check for coolant moving through the radiator. It is not uncommon for a waterpump fin to break from fatigue. I recently put Boysen high performance pumps in both my YZ and WR and noticed a dramatic cooling difference.

Lastly, If a $6.00 cap doesn't completely resolve your issue, then the worst case senario could be that the head gasket has either cracked from years of heating and cooling, and hot exhaust gasses are entering a water jacket causing the overheating. (You will NOT see water in the oil or vice versa) This can be detected again...Cool Engine... Remove cap... Start engine and let run for about a minute...Then look for foaming (air bubbles) in the coolant tank....If you see them it is not good news and the base gasket should be replaced. If this were to go on for a long time then the head would crack and become a very expensive job.

However this is not that common, and most likely you just have a tired radiator cap.....

I have been racing my 99 WR400F in all kinds of weather in all kinds of terrain since it was brand new....It should not be overheating as you describe.

The Water Wetter or Engine Ice (Which I use) are good alternatives to standard anti-freeze and have a little higher boiling point...

Good Luck...

Bonzai :)

The cap may have a weak spring, but more likely, your having trouble getting rid of heat. Slow trail riding will keep combustion temps cooler from the richer mix. I suspect the radiators are not getting rid of the extra heat from either restrictions or a coolant flow problem. There is less air flow in the radiators during slow trail riding, but this should do it, the radiators are large enough to dissapte the heat naturally without air flow. They are made no larger than they need to be, so and reductions in coolant flow etc can put it over the edge. Air flow at high speeds assist only in the extra heat generated then, but air flow across the radiators isnt neccesary during slow trail riding. Is your coolant good? With a good 50/50 mix? Weak coolant wont absorb the heat as well and lowers the boiling point as does the lack of pressure in the system.

I've been riding my 99 wr400 in tight trails, slow pace sometimes, lots of mud, even starting and stoping constantly, idleing and things like that. Never have experienced a boil-over. I would take a look at your radiator cap. I don't think that this is a naturally occuring thing for liquid cooled bikes-even though the XR guys like to make it out to be. Those damn XR babies. :)

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now