hey guys, i've recently got 01 wr 426 and cant find any reference of thermostat in my manual. Does this bike have one? It's getting really cold here where I live, so I think it's a good idea to have one. what do you think? thanks.

Nope. Doesn't have one.

which hose would you mount it on?

OK, so am i going to have problems running it on flats in 30-ish temperatures? what about inline thermostat housings? like this one:

Mask one radiator with strips of duct tape and add or substract strips until you find the right operating temp. This is assuming you have a Vapor computer (or silimilar) as a means of measuring your coolant temperature.

Max, well you could put it right in between the engine head outlet and radiator, so it gets the hottest water the cooling system will see.

sxp, I dont have any computer with water temp indicator. And even if I had, it would be pain in the ass to duct my radator in the middle of my ride, when I find some new long and straight section. 100$ for thermostat housing seems a bit pricey. I'm gonna find out what size thermostats tiny cars are using and try to make housing for them. offcourse if I find time between my rides :smirk:

We run our bikes in the winter in the mountains here when the temps are in the teens and 20's with no issues. Usually a bit of cardboard on the radiator air deflectors is all I ever see anyone using, just to stop the air and snow from flowing straight onto the radiator fins. Just make sure the coolant is mixed properly for the temps that you will be riding in. WR Dave

Ok, got ktm thermostat from ebay, will be installing it this weekend.

I can take photos if anyone is interested.


Here's a pic of it, main flow direction is from bottom to top of the image. The main section is straight so it will fit inline in the outlet tube behind left radiator, and return flow will go into T fitting that I'm going to install just after left radiator outlet hose.

Definitely keep a build thread for this going. I'm curious to see how this works out!

Ok, then some theory before practice :p

So first I want to say that the problem of overcooling really exists on these bikes. I had my fuel screw set up spot on on a hot bike, it was really responsive then and I was quite satisfied with jetting. After that, I and my mates had to do 2-3 mile relatively high-speed driving to another trail. When we finally got to the trail, I found that my bike would not idle without playing with choke, I was like o.O but after a bit of playing with choke the problem got fixed itself. The only explanation I could find for this is that the bike overcooled during the high-speed/low-load cruising and thus threw my jetting out of the window. We need some hot engines here! :thumbsup:

Now a little bit about thermostats. There are 2 kind of setups with thermostats:

1. Just an inline setup with no coolant bypass route, so when the thermostat is closed, it just blocks the flow of coolant. This setup is easiest to do as you just insert thermostat into engine outlet hose and put radiator inlet hose on it, no any other plumbing at all.

2. Traditional setup with coolant bypass route. in this case, the thermostat regulates how much of the flow will go through the radiator and how much just back to the engine, without any cooling. The flow of the coolant is almost constant and doesn't depend on position of thermostat. In this setup, you need to do everything you need for the first setup plus to route bypass route back to the engine.

The choice might look simple, the first setup is the one. This is what I thought at first, but take a look at the bold texts. The main issue here is the speed of coolant flow. In the inline setup, when you hit the thermostat regulating temperature(i.e. thermostat blocks some of the flow), the coolant in radiators is cooled even more, because it stays in radiator for more time. This would create a big difference between engine outlet and inlet temperatures. The slower the coolant flow - the bigger the difference would be. This could create even more problems then just over cooling of engine: bottom of cylinder being cooler then top, could cause a taper in the bore and probably some other really nasty stuff you dont want to happen with your engine, use your imagination :mad:

In the second setup however, when thermostat would hit its regulating temperature, it would just reroute outlet flow to the inlet of the engine and mix with it as much cool coolant from radiator as needed to maintain the regulating temperature. The temperature difference between inlet and outlet points of engine would be minimal of which is achievable with given coolant pump capacity while the temperature itself would not drop below certain value (70 deg C with this KTM thermostat).

whew, think thats all :bonk:

So what do you consider cold in Georgia? For every 10-15 degrees change in ambient temperature, there is a slight change in "optimum" jetting for a bike. However, most people jet for their most of the time temp and run it. Here in Minnesota (and Wisconsin where I am originally from) it get COLD, like well below zero cold. Then a jetting change is needed and you really need to be concerned with coolant temp.

I eould think anything above 40 degrees ambient should be just fine without a thermostat. All we do here is like was suggested earlier either cardboard or ducttape the radiator a bit to cut down on some air flow. It's not rocket science, just slightly impede the air.

More power to you, but after reading you reasons I think you had some debris or something with your fuel system, not an engine temp issue.

No thermostat on this bike.


Well first I have to say I live in country Georgia, near Russia ye know. And about running thermostat vs. not running it: you could probably get away with slightly rich jetting without thermostat, but the main issue that you get without running thermostat is engine reliability. You could run any automotive engine at 50 deg. C but it would fail much more early then the one that runs at 100 deg. C and also perform much worse the hotter one.

I don't think that it was debris in my carb that caused that kind of situation. I had cleaned my carb a week before and in case of jet blocking, the problem would have not cured by just idling for 2 minutes...

beezer, nice speedway bike out there. Do you have any onboard computer installed on it to measure the coolant temp?

Russia like across from Alaska?

I use the bike for dirt track in summer and ice in the winter.

I go up 1 size on the pilot and main jet in the winter.

No temp gauge just warm it up good and then ride it like it ain't mine.

A thermostat makes no sense.... I have never ran one in any of my WR's, and we ride into the 20's around here in the winter. Furthermore all the KTM's I ride with ALWAYS overheat in the tight technical stuff and they often point to the t-stat as the reason why :p

Hi all.

What happened with this thread?

Hitman, did you install the thermostat? This is exactly what I want to do for my -12 WR450!

What bike it that ktm thermostat from? Sorry to revive an old threat but I want to retrofit the same one onto my wr426

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