Hey Scott,

Check out a topic titled: Fuel Too Hot (Bubbling). Read Pyro's reply about the fuel cap problems.

Last Friday at Rampart it was 90 degrees and my bike started to run terrible, also. While sitting on a log, waiting to gain the strength to contiue kicking it, I remembered this topic...

My fuel line hadn't been venting causing the fuel not to flow to the carb - bad thing. I unscrewed the cap, the tank gasped for air, then it started on the second kick.

It's never done this before, so I blamed it on the heat. Best of luck.



I actually took the cap off to release the pressure one time and blew into the hose. Air seemed to be going through ok considering gas spewed out.

I went back and looked at the "Fuel Too Hot" post, but didn't really find any solutions, just confirmations that others have the same problem.

I actually responded to that post a while back stating that my Pro Taper square PAD was causing the hose to pinch. After I removed the pad, the bike seemed to run ok. BUT, on this past ride, I didn't have the pad on.

I can't imagine that a hot day would cause the bike to do this..?

I just had the same problem. I was riding very slow speed, untracked 4-7 foot grass and neddles, and I noticed my radiator and overflow was working over time so I stopped to give it a rest. While sitting there amongst the worlds largest population of 6 foot neddles I noticed a hissing sound, I first thought it was my radiator, until I noticed that the sound was coming from my vent tube in my stem. I unscrewed the cap and wow massive pressure, I actually unscrewed it at least another 3-4 times in 10 minutes to bleed off all the extra pressure. When I looking inside the tank I could see and hear the gas bubbling! **** !

The only performance problem I seemed to be having was a hesitation and stalling when I would jump on the throttle from an idle or low rpm. I did notice that if I feathered it instead of romp on it it wouldnt die, but that seems kind of obvious.


1. Gas bubbling (to much slow speed riding?)

2. Hesitation and stalling (to lean-open up the air screw?)

JJ in WA

OK, here's the situation...

I was riding yesterday in the woods of Colorado. (Rampart, for you CO guys)

After about an hour of riding, my WR started really bogging. I was cruising the trails when all of a sudden the bike started acting like it was out of gas - chugging violently. I would pull in the clutch and feather it to keep the bike running. It seemed to happen the harder I hit the gas. If I stayed on the gas the bike would die.

One thing I noticed was that there was a ton of pressure coming out of the gas cap exhaust tube. I loosened the gas cap and there was some serious pressure goin on.

Any ideas? Jetting? Blocked fuel line?

I would think that if it was jetting, the bike would be doing this all day, not just after an hour or so into the ride....



99WR,WR timing,throttle stop removed,Twin Air filter,E-Series,carbon air box,Pro Tapers,front and rear lights removed

It may have something to do with riding at low rpms for a long time. I was mostly in first and second gear. I rarely hit third gear and when I did it was only for a few seconds...


99WR,WR timing,throttle stop removed,Twin Air filter,E-Series,carbon air box,Pro Tapers,front and rear lights removed


It sounds like you have an aftermarket pipe? From what I have read, aftermarket pipes put out a lot more heat up high due to the headpipe routing. I still use the stock pipe, but I have a BigGun on order. My plan is to use a heat insulating wrap on the pipe to prevent rider, fuel and machine overheating problems. Besides, Thermotec claims increased HP. Take a look at

Eric in WA


’99 WR400, YZ timing, IMS Tank, YZ Seat, Acerbis Handguards, Scotts Steering Damper, Renthal Bars, Devol Rad Guards, Dunlop 755F & 756R Tires

’99 Polaris 400 Scrambler –“Trail Couch, Toolbox and Lunchwagon”, ’99 Honda Z50R, ’97 Honda XR70R, Kubota L275DT and an assortment of farm equipment.


I'm pretty sure I have the stock pipe with an E-Series silencer. (I bought it used) I assume it's the stock pipe considering it has the holes where the pipe gaurd was once mounted.

I've never seen a pipe with this kind of insulating tape.. You think it really works? I'm willing to try it.

So you think the problem is simply caused by too much heat? I don't why my bike would be running any hotter than anyone elses though..?


99WR,WR timing,throttle stop removed,Twin Air filter,E-Series,carbon air box,Pro Tapers,front and rear lights removed

I'm using the Stealy breather cap. They've got a really cool little aluminum 1-way tank breather valve that eliminates the "kinked breather hose" problem. Everyone who sees this thing wants one...

Steahly - 1-800-800-2363

Stock pipe and silencer here...

JJ in WA

Stock pipe and silencer here...

JJ in WA

So Steve,

Were you experiencing the same problem and the breather cap solved it?


99WR,WR timing,throttle stop removed,Twin Air filter,E-Series,carbon air box,Pro Tapers,front and rear lights removed

Fortunately, I haven't experienced the problem. Since I've read several other postings about kinked breathers, I figured the addition of a "Pin Head" could solve a ton of problems, since it completely eliminates the breather hose. But if you've actually got pressure building up in the tank, the Pin Head won't help. If that's what's happening, I'd be tempted to take out the 1-way breather valve altogether.

For all you folks that have experienced this problem, what gas tank do you have and how have you routed the fuel line? If you have anything other than stock take a look at the hose routing and where the heat sources are. The fuel bubbling and high vapor pressure has to be the result of excess heat and lack of venting. If you don’t believe me, put 2 unvented fuel cans, one in the sun and the other in a cool place for a few hours and check out the difference. Heat is the ONLY way to increase fuel vapor pressure. The other phenomena to consider is also vapor lock in the fuel line. Vapor lock often occurs with high temperatures at a high point in the hose routing where a bubble of vapor accumulates preventing fuel from flowing.

On a perhaps unrelated note, I have observed a very bad bogging problem, but under a very different set of conditions. I experience it when I restart the bike after it has been hot and then sits for about 20 to 40 minutes. I suspect it is because the radiator fluid has cooled more quickly than the engine. The warm engine gets a slug of cool fluid causing it to run very poorly for about 5 minutes or so. It is so bad sometimes it will hardly pull its own weight on level ground. Once it the temperatures are evened out then it runs fine. However, no fuel bubbling problems.

Eric in WA

I've got a stock tank. This just baffles me I dont get it...

I have a feeling my bogging and stalling problem is completely separate from my fuel bubbling problem.


The subject of heat shielding came up before and I suggested a do it yourself with aluminum foil. Well, I was at Boulder Cycles and they had some shielding that you just cut to fit and stick on. I believe it was from Answer.

Can you say Vapor Lock?

I had the same problem with mine ('98 WR). On the fuel cap there is a strange valve system. Which on some bikes doesn't work. Just undo the screws underneath and take it all off, leave the oneway valve thats fitted to hose in place.

I was able to solve my pressure problems (coming from the tank).By simple removing the stock 1-way valve. Gives ya incentive to stay vertical :).2ndly I tried routing the fuel line with thin wire(to pull the fuel line feed further away from the pipe)and so far so good. But the removal of the 1-way valve helped the most.

No More Pressure....JUST RIDE

WR400 Y2K


The check valve in the gas cap vent works great if there is no increase in pressure at all inside your tank. But if the temp rises due to anything, even just sitting there in the hot sun, the internal pressure will increase in your tank. The vent allows only air flow into the tank as the fuel is consumed and not out. You figure the tank is pressed up which is gravity fed to your float bowl. This means your float bowl is now pressurized. I am not sure if these carbs were meant to operate like this. Do the old fasioned thing and replace your fuel vent. Yeah, you may get some seapage out of the vent, but at least you won't have some sort of ram fuel effect when (if?) the float drops and the PRESSURIZED gas shoots into your carb.

Hey Kevin!

You said replace it, did you mean remove it.

If not replace it with what?

I was thinking of getting the hoseless valve by Stealth mentioned earlier in this post. Once I thought about it though, its not going to be any different than what I have now except for the fact there wont be a hose to crimp...


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