Having tried out a new Indian restaurant here in my home town, and not having had a curry since last time I visited the UK, I made an erroneous choice from the menu: A fiery Prawn Vindaloo. My digestive system is now indicating this was unwise and I have been sitting in the bog all morning...I think Johnny Cash wrote a song about it: Ring of Fire...
I thought I use my sojourn in the bathroom to write something to bore you all with, but mainly to help some newbies that have trouble with the gas tank breather and, judging by the frequent posts, it's a lot.
In the good old days the gas tank breather system consisted of nothing more complicated than a tiny hole in the gas cap that allowed air to flow into the tank as the gas was used up. If no air can get in no gas can flow out. These holes could sometimes get blocked up and the cure was simply to poke something sharp into it or blow them out.
Today things are a little more complicated due to certain safety and emission laws. For example gas caps are not allow to vent raw gas fumes into the atmosphere. To avoid this they have no breather hole at all. There's a spring loaded valve in their centre but this is a blow-off valve that opens only under certain pressure if the gas tank becomes pressurised by expanding gas and fumes. In some States of the US these caps are tested (on cars at least) as part of regular inspections and, I believe, even replaced free of charge if they fail the blow-off test.
The part that lets air in nowadays is a separate system that fulfills three roles: To vent the tank, to allow excess fuel to overflow to the ground rather than spill over the tank and on to a hot engine, and to close off the flow of gas out of the tank in the event of an accident causing the bike to tip-over.
In certain States, (California for example), this system is further complicated by the requirement that all raw gas fumes expelled during engine off conditions are captured in a charcoal canister and then released back into the engine at pre-set conditions and intervals.
Testing the breather
To test the operation of this breather system disconnect the hose from the gas tank (it's towards the rear right hand side). This hose has no clips but it fits very tightly on a long metal stub welded to the tank. You can feel for it by running your hand around the right hand edge of the tank, towards the rear just ahead of the carb. On EFI models extra care is required to ensure you don't disturb the right hand MAP sensor hose that lives near by.
![Image]()
It can be difficult to disconnect without lifting the tank. Before replacing it smear some silicon-based grease on the stub to ease fitting and future removal. I say silicon grease because ordinary petroleum-based lubricants tend to rot rubber components.
Tools required: Simply your mouth and lungs.
First test:
You should be able to blow down it as much as you like and should feel clear if there's no obstruction.
Second test:
You should also be able to suck on it normally without feeling any
obstructions, but if you increase suction you should feel it block up. This is the Roll-over valve operating.
This valve has a little ball that blocks the outflow of fuel if the bike is laid over to prevent fuel spillage and risk of fire. The strong suction you can create even with your mouth brings the ball into operation blocking the suction.
The valve looks like a small black plastic cylinder about 2" long with an "UP" marked on it to indicate the correct position, it must be positioned upright. It is sited just in front of the airbox, close to the top of the crankcase.
![Image]()
If there's an obstruction in the hose and you can't blow or suck through it then there are three possible reasons:
1/ The hose is trapped or pinched somewhere under the tank or along its path around the frame.
2/ The roll-over valve is faulty. Perhaps the ball is stuck in the closed position. This can happen if you regularly ignore owners manual instructions not to over-fill the tank beyond a certain level. The breather system then acts as an overflow when the fuel expands due to heat and some fuel remains inside the valve. The evaporation of this fuel leaves behind a varnish or gum-like residue that can cause the ball-valve to stick in the closed position.
3/ The hose is blocked somewhere along its length by road debris, water or even gas that has drained down from an over-filled tank and got stuck in there.
Disconnect the first section of hose from the top of the tip-over valve and blow through it. It should be clear. If so then the blockage is either on the valve or the short length of hose that goes from the bottom of the valve to a point under the bike by the swing arm pivot tube.
A good tip for avoiding future problems with this short hose is to locate where it exits by the swing arm pivot area and practice a long slash cut on the end with a craft knife. As standard it comes cut flat right across. Do the cut so it faces towards the front of the bike, this way the airstream will suck the hose clear.
This is an oversight by the Factory (slash-cut drain and breather hoses are common on Jap bike) as even a drop of water can block the exit and cause fuel starvation symptoms. A slash cut prevents this for some obscure scientific reason I dont understand...something to do with venturi effects or meniscus or
something with a silly word in it...
.
If you go on to post number 11, Verde has kindly supplied a couple of photos illustrating how this is done.
Quite a few posters have experienced fuel starvation problems following a ride in the rain due to water blocking this hose so it's worth doing that slash-cut.
On California models this hose terminates in the EVAP charcoal canister and there's very little I can tell you about that as I don't know the system.
Here's a last drawing ilustrating the breather components common on the whole range, except California models with the EVAP system.
Item 21 is the long hose from tank to roll-over valve, 22 is the roll-over valve and 25 the short hose to atmosphere:
![Image]()
I thought I use my sojourn in the bathroom to write something to bore you all with, but mainly to help some newbies that have trouble with the gas tank breather and, judging by the frequent posts, it's a lot.
In the good old days the gas tank breather system consisted of nothing more complicated than a tiny hole in the gas cap that allowed air to flow into the tank as the gas was used up. If no air can get in no gas can flow out. These holes could sometimes get blocked up and the cure was simply to poke something sharp into it or blow them out.
Today things are a little more complicated due to certain safety and emission laws. For example gas caps are not allow to vent raw gas fumes into the atmosphere. To avoid this they have no breather hole at all. There's a spring loaded valve in their centre but this is a blow-off valve that opens only under certain pressure if the gas tank becomes pressurised by expanding gas and fumes. In some States of the US these caps are tested (on cars at least) as part of regular inspections and, I believe, even replaced free of charge if they fail the blow-off test.
The part that lets air in nowadays is a separate system that fulfills three roles: To vent the tank, to allow excess fuel to overflow to the ground rather than spill over the tank and on to a hot engine, and to close off the flow of gas out of the tank in the event of an accident causing the bike to tip-over.
In certain States, (California for example), this system is further complicated by the requirement that all raw gas fumes expelled during engine off conditions are captured in a charcoal canister and then released back into the engine at pre-set conditions and intervals.
Testing the breather
To test the operation of this breather system disconnect the hose from the gas tank (it's towards the rear right hand side). This hose has no clips but it fits very tightly on a long metal stub welded to the tank. You can feel for it by running your hand around the right hand edge of the tank, towards the rear just ahead of the carb. On EFI models extra care is required to ensure you don't disturb the right hand MAP sensor hose that lives near by.
It can be difficult to disconnect without lifting the tank. Before replacing it smear some silicon-based grease on the stub to ease fitting and future removal. I say silicon grease because ordinary petroleum-based lubricants tend to rot rubber components.
Tools required: Simply your mouth and lungs.
First test:
You should be able to blow down it as much as you like and should feel clear if there's no obstruction.
Second test:
You should also be able to suck on it normally without feeling any
obstructions, but if you increase suction you should feel it block up. This is the Roll-over valve operating.
This valve has a little ball that blocks the outflow of fuel if the bike is laid over to prevent fuel spillage and risk of fire. The strong suction you can create even with your mouth brings the ball into operation blocking the suction.
The valve looks like a small black plastic cylinder about 2" long with an "UP" marked on it to indicate the correct position, it must be positioned upright. It is sited just in front of the airbox, close to the top of the crankcase.
If there's an obstruction in the hose and you can't blow or suck through it then there are three possible reasons:
1/ The hose is trapped or pinched somewhere under the tank or along its path around the frame.
2/ The roll-over valve is faulty. Perhaps the ball is stuck in the closed position. This can happen if you regularly ignore owners manual instructions not to over-fill the tank beyond a certain level. The breather system then acts as an overflow when the fuel expands due to heat and some fuel remains inside the valve. The evaporation of this fuel leaves behind a varnish or gum-like residue that can cause the ball-valve to stick in the closed position.
3/ The hose is blocked somewhere along its length by road debris, water or even gas that has drained down from an over-filled tank and got stuck in there.
Disconnect the first section of hose from the top of the tip-over valve and blow through it. It should be clear. If so then the blockage is either on the valve or the short length of hose that goes from the bottom of the valve to a point under the bike by the swing arm pivot tube.
A good tip for avoiding future problems with this short hose is to locate where it exits by the swing arm pivot area and practice a long slash cut on the end with a craft knife. As standard it comes cut flat right across. Do the cut so it faces towards the front of the bike, this way the airstream will suck the hose clear.
This is an oversight by the Factory (slash-cut drain and breather hoses are common on Jap bike) as even a drop of water can block the exit and cause fuel starvation symptoms. A slash cut prevents this for some obscure scientific reason I dont understand...something to do with venturi effects or meniscus or
something with a silly word in it...
.If you go on to post number 11, Verde has kindly supplied a couple of photos illustrating how this is done.
Quite a few posters have experienced fuel starvation problems following a ride in the rain due to water blocking this hose so it's worth doing that slash-cut.
On California models this hose terminates in the EVAP charcoal canister and there's very little I can tell you about that as I don't know the system.
Here's a last drawing ilustrating the breather components common on the whole range, except California models with the EVAP system.
Item 21 is the long hose from tank to roll-over valve, 22 is the roll-over valve and 25 the short hose to atmosphere:
