please help??


my 98 yz400 was over heating in slow stuff

so i added a overflow tank off a wr.

the cap isnt an air tight seal and it seemd to empty but never fill

still the bike over heated

so i tried a 1.6 rad cap off a kx500

ok so as far as i can tell its not bubbled over since,

but twice now there has been water leaking from the front of the barrel, from the o-ring on the metal tube that connects to the water hoses.

i assume the higher pressure from the rad cap is highlighting other weak links in the cooling chain.

ive replaced the o-ring once and am about to do it again.

ive been filling the radiator to the top with pure distilled water, i guess the higher pressure cap isnt allowing the excess water to vent.

ideally id like some WR / YZ426 radiators which are slightly wider.

any ideas how i can cure this so i dont keep blowing orings?

Take the cap off completly and put a big ass ballon around it, that way the pressure stays even as it heats up.

I got the idea watching me Grandpa make wine, Dang Polocks :)


If your bike is running lean it will run hotter

I run WATER WETER with 50% antifreeze & 50% distilled water

I also run race gas ( Sonco Purple 110 oct. )

that seems to make it run cooler too,FYI you must re-jet for race gas

I all so use HEADER WRAP on my head pipe, keeps heat away from rad., hose & water pump

PS -00 YZ426 with stock cap & no over flow tank

I ride mostly tight trails with no problem



a) Try a 1.2, 1.3, or 1.4 bar cap.

:) Only fill the radiator enough to cover the fins.

Hope this helps.

If you have a lighting coil maybe you can get a small [thermistat controlled] fan from a road bike and attach it to the back of one of your radiators. If you ride in tight stuff often then IMHO that would be a worth while investment.

will the bike run lean if the gasget between the header pipe and muffler is leaking?


Yes, if it is jetted lean. Just being a smartass. No, it will pop and backfire which may be confused with too lean symptoms.

UncleRich has a good point. My bike loves the lean side on the track, but when I follow my daughter on woods trails, I go one up on my pilot. It will run considerably cooler. IMO if you are in really tight stuff, you are probably mostly on the pilot (less than 1/4 throttle) most of the time anyway. Worth a shot.

Good luck,


Uncle Rich

That Header Wrap looks like a good thing (specially where im ridin) , but its not all that pretty, what color is yours, can u use any kind of heat paint coating or does that come with it ??

I found some Header Wrap at

25 feet for $28, good deal or rip off ???

Thanx :)

I just recently got rid of my '98 YZF. The '98 models had a tendency to crack that metal fitting you are referring to. I assume you meant in front of the cylinder (barrel in the UK). I would look into replacing that entire fitting if I were you. Also, I had trouble overheating in the twisty stuff. The lack of airflow along with the excess heat from the clutch will cause the bike to boil over. I never changed the rad. cap but did as a previous post. I ran 50/50 mix of distilled water and antifreeze and also added redline water wetter. The antifreeze mixture actually removes heat better than straight water. The wetter allows the heat transfer rate to go up by decreasing surface tension between the aluminum the radiator and the water. I didn't have any real trouble after that.

PS. Maybe you just aren't going fast enough. haha

My header wrap is white (was white)and I got it from Summit Racing ( 1" wide )

I realy dont care what it looks like so I didn't check for colors ( I don't even have stickers on my bike )

I don't remember how long the roll was, but it's enought to do 2 - 3 head pipes

I don't remember what it cost, check Summit Racing and Jeg's and compare with the price you got.


i recently switched coolants and noticed motor runs running 50/50 water/coolant friend got me coolant from his work they use it in forklifts and heavy machinary.its a synthetic antifreeze[its designed for engines with limited airflow] its looks slightly thicher than normal coolant.i would give you brand name except he brought to me in 2 liter pepsi bottle.ill ask him who makes it and let you know.


Please let us (or me) know what the results are from any upgrades or changes you make, I can see myself running into this problem (I hope not though) as I take tighter trails every ride.


One thing to be careful about header wrap.

My son and I raced 1/4 midgets race cars and we wrapped the header pipes. It does tend to make the steel pipes rust.

If you want a good tight fit, wet it first then wrap it. As soon as you start the engine the wrap will usually smoke like crazy, but then dry out quickly.

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