Plumbing motorbike carburettors for deep water crossings

If you have ever pointed your motorbike at a section of deep water only to find your engine coughs and splutters or even dies, it’s highly likely your carburettor breathers are not plumbed to handle this.  The carburettors on some motorbikes are setup from factory to tackle deep water crossings however most are not, so if your bike falls into the latter category a bit of re-plumbing will most likely be required. 

Why is it necessary to re-plumb the carburettor for deep water crossings?

Without going into technical specifics a basic overview of how a motorbike carburettor works will make this clear...  Your throttle does not directly control the flow of fuel, instead it actuates a mechanism in the carburettor that metres the flow of air being drawn into the engine. Fuel that is held in the carburettor reservoir (fuel bowl) is then drawn into this flow of air through the jets to create the ideal ratio of fuel and air for combustion. This works on the basis that air flowing past the venturi in the carburettor has a lower static pressure (the faster air moves the lower its static pressure) while the fuel supply remains at atmospheric pressure (14.7 psi at sea level) allowing the process of the fuel to be successfully drawn into the flow of air and mix. The job of the carburettor breathers is to allow air movement and pressure compensation within the carburettor reservoir area to maintain the level of atmospheric pressure so the carburettor can function correctly. So, when you ride your bike into a deep section of water the ends of the carburettor breathers will be immersed in water blocking the flow of air required to maintain atmospheric pressure within the carburettor and your bike will most likely run like a sack of crap.

Photo 1 of Plumbing a motorbike carburettor for deep water crossings
Basic diagram demonstrating how air in the fuel reservoir needs to remain at atmospheric pressure

Why not just run the carburettor breathers up under the seat then?

To firstly address why the breathers are as long as they are, this is a safety precaution.  If you have a not so glamorous moment and your bike goes down the carburettor can become flooded and excess fuel can pour out of the breathers.  The long length of the breather hoses are to ensure this draining fuel is kept away from the hot exhaust, electrics etc. So, with that in mind if you just run the breather hoses upwards (under the seat for example), the next time your bike goes over and fuel runs into the breather lines, you will have created a U bend in the breather line for the fuel to puddle in and no way for it to escape inadvertently blocking the flow of air required to balance the atmospheric pressure for correct carburettor operation.

What about rerouting the breathers and plumbing them into the airbox?

In most cases this is not a good idea. The aim is to achieve atmospheric pressure for the carburettor breathers and this is most often not achieved in the airbox as this subject to continual pressure changes from either/both the crankcase breather and carburettor air induction. There have been many reports (notably the DR650) that when the carburettor breathers are plumbed into the airbox it causes very irregular acceleration and pulsing at idle.

What about running no breathers at all?

Although you will increase the height in which you can cross water, foreign debris can easily make their way into the carburettor, plus if you do take your bike into water deeper than the carburettor breather stalk you will most likely draw water into your carburettor.  Not to mention every time the bike goes over you will have fuel spilling everywhere.

Plumbing in T’s and Y’s into your carburettor breather hoses

So what's the best way then?  Well, we need to keep the functionality of safely draining fuel away plus add the functionality of maintaining an open air way at all times.  The best way to do this is by plumbing in a T or Y joiner into your breathers. Below are photograph examples of a Mikuni TM28 carburettor

Photo 2 of Plumbing a motorbike carburettor for deep water crossings
Mikuni TM28 carburettor right hand side: The factory breather setup allows adequate excess fuel drainage however will not perform well in water at a depth that blocks the ends of the breathers
Photo 3 of Plumbing a motorbike carburettor for deep water crossings
Mikuni TM28 carburettor right hand side: Having the breather routed upwards allows the carburettor to breath when going through deep water but will not allow excess fuel drainage as it will pool up in the U section of hose and cannot escape
Photo 4 of Plumbing a motorbike carburettor for deep water crossings
Mikuni TM28 carburettor right hand side: Plumbing in a T allowing both excess fuel drainage and breathability from the hose that is routed and secured under the seat (detailed below)
Photo 5 of Plumbing a motorbike carburettor for deep water crossings
Mikuni TM28 carburettor right hand side: A variation of plumbing in a T
Photo 6 of Plumbing a motorbike carburettor for deep water crossings
Mikuni TM28 carburettor left hand side: A T plumbed into the left hand side of the carb
Photo 7 of Plumbing a motorbike carburettor for deep water crossings
On my DRZ250 I have the breather hoses from both the left and right side routed up under the seat and affixed to the frame with cable ties. After numerous deep crossings I have never had any problems with them located here
Photo 8 of Plumbing a motorbike carburettor for deep water crossings
On my DRZ400 I have the breather hoses from both the left and right side routed up under the tank looped around the steering head and zip tied in place
Photo 9 of Plumbing a motorbike carburettor for deep water crossings
Crossing a High Country river - DRZ400
Photo 10 of Plumbing a motorbike carburettor for deep water crossings
Crossing a deep section on a West Coast 4X4 track - DRZ400

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