What is the purpose of the breather on the Magna?

[MagnaPHX]

Greetings to the board.

I have a 3rd gen Magna that has a breather tube that attaches to the bottom of the gas tank closed to the frame. I noticed this last night when I was breaking down the bike
to get to the airfilter box.

I read therought the service manual but I couldnt find anything that went into detail about what this part actually does?

[lragan]

There is a tank vent tube near the back of the tank on the bottom.  The vent line leads away and down to prevent fumes from igniting by the hot engine or exhaust pipes.  This vent is necessary as temperature increases can cause high pressure in the gas tank, forcing excess fuel pressure to the carbs, or on cooling, preventing fuel flow to the carbs completely.  All gasoline tanks of any size need to be vented.

There is another advantage to this tube if you ever need to work on the tank from the outside with heat.  A plastic tube placed over this vent leading well away from the work area, (like 15 feet or more) takes gasoline fumes out of the way -- if your gas cap seals properly.  I was able to "pop" a dent by first heating around it with a heat gun, then cooling the center with CO2 spray using this technique.  Only works on small dents, but it worked.

Use this technique at your own risk.  It is always risky to heat any container with gasoline in it.

[snoopy]

Use this technique at your own risk.  It is always risky to heat any container with gasoline in it.

I'm hoping that you at least emptied the tank before doing this.

[lragan]

I'm hoping that you at least emptied the tank before doing this.

Yes, I did, but it probably wouldn't matter much.  Gasoline doesn't burn -- the vapor from it burns, hence the need for carburetors.  As long as there is not both vapor and oxygen in the tank, it will not ignite. 

[magnagregcan]

This may be "old wives tales" but 2 ways to remove any oxygen is to either fill the tank with water, or purge the tank with car exhaust (CO).

[lragan]

This may be "old wives tales" but 2 ways to remove any oxygen is to either fill the tank with water, or purge the tank with car exhaust (CO).

You can purchase a can of "duster" gas for a few bucks.  The ones I buy are liquid, and can be used for rapid cooling if one inverts the can while pushing the "spray" button on the top.  The gas is non-flammable, and can be used to purge the tank.  Simply invert the can, spurt some liquid in the cap, then replace the cap quickly (with the exhaust tube in place).  The expanding gas will remove enough oxygen to increase the ignition temperature beyond what you can reach with a heat gun on the outside of the tank.

If you need to weld to the tank, that is a whole different can of worms, as the temperatures are quite adequate, even with small amounts of O2.  Filling with water probably won't completely remove the last vestige of gas, and it makes welding very difficult.  A fat tube to the exhaust of a car into the gas cap might work, but I would not try this at home...