Cracked gas tank theory

This weekend my 99 YZ400 fuel tank developed a crack at the rear of the tank. I thought it was only hitting the 2000 426's and up, guess not. But what I did notice is that I had been running without a one way vent in the gas tank vent line for about a year. I then put the vent back in and instantly started to notice quite a bit of pressure in the fuel tank hissing out when I would stop riding. My tank would actually swell up. I am wondering how much this had to do with the tank crapping out? I know a fuel tank should be able to handle it but given the reduced thickness of these tanks, I won't be putting a one way valve on my next tank and I yanked the one off of my wife's YZ125 just in case. This sucks, but atleast I didn't get barbecued.

Like I told my buddies, it's the first time this bike has ever let me down.


99 YZ400

Nah, it's from the rear of the tank bending like a hinge from rider weight. It flexes when we move forward on the tank.


Not to be an A**hole, but are sure the one-way vent is going the right direction? I've made plenty of brain fart mistakes like this myself.

Your right about where the tank cracked and the reason of rider weight making it crack, I was just thinking the pressure build up doesn't help an already weak tank.

As far as the vent being the put in the wrong way, if it was in backwards the bike would run outta gas due to the vacuum. But I looked at everything and it's in correctly. I don't think anyone is an A**hole for voicing their opinion, I welcome any input as long you leave my momma out of it...



My 99' did the same thing last summer.

swelled up and actually caused what i think was some sort of VAPOR lock??? or pressure lock????

unscrewed the cap and released about 96psi !!!lol

At one point i was on it pretty hard trail riding and the bike stumbled at half throttle as i went over about a 2ft jump in 3rd gear. nearly thru me over the bars!! I stopped and noticed a hissing which i thought was the radiator but just the tank. I rlsd the press and the symptom went away.

I also had a 1way valve on and it WAS in the correct direction. (that was the first thing i checked!) I too have put my bike back together with "anal cranal inversion" once or twice!!! :):D

The stock cap has a one way valve (a.k.a. check valve) in it. I have never had a problem with tank vacume or pressure, but I have cracked 2 tanks already.

I was told by a local racer that you shouldn't use a one way valve, and also take out the ball bearings from the cap...this will help it vet properly.

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