Tank

Hrvatski

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Fuel tank

The main function of fuel tank is, of course, to store the fuel for motorcycle engine operation. The tank must enable entrance of air into it (breathing) in order to avoid creation of vacuum when fuel goes down in consumption. Also the tank must have a security mechanism to prevent leakage in case when motorcycle falls down. Additionally the tank should have the "reserve" functionality which warns the driver that most of the fuel will be exceeded soon and that (s)he has to go to the petrol station to refill the tank.

Vacuum petcock

manual valve
(vacuum petcock)

The first xv750 Virago motorcycles manufactured in period 1981-1983 worked on the principle of free fall of fuel from the tank into the carburetors. Some Virago models had only one fuel valve and some two. The vacuum petcock has a small diaphragm as valve that allows or prevents the flow of fuel, and this diaphragm is controlled by a vacuum (sub-pressure) supplied from the engine cylinder. While the engine is running, a vacuum is created (and it is brought by a small tube to the vacuum petcock) letting a fuel to flow. When the engine is not running, there is no vacuum and the vacuum petcock diaphragm blocks the flow of fuel, so it is also a safety mechanism preventing the fuel not to leak when the engine is not running.
 
These valves usually have three positions in which they can be set manually:

• "ON" (normal) - the valve operates in the usual way controlled by a vacuum diaphragm and allows flow from the main tank when the engine is running,
• "RES" (reserve) - the valve allows supply from a reserve tank (it can be either a physical separate tank or only the lower part of the same tank) and it also is controlled by a vacuum membrane,
• "PRIME" (or "FLOW") - a fuel flow from the tank to the carburetor is allowed without the influence of diaphragm and vacuum (i.e. fuel goes even when the engine is not running). This is necessary in cases when the carburetor should be saturated with fuel for starting when it is very cold outside.

The fuel sender

In period 1984-1987. on models xv700 and in period 1987-1988. on models xv535 and xv750 the fuel sender and corresponding indicator (bulb) was built on motorcycle alerting the driver that fuel is exceeding and that tank should be switched on reserve. This fuel sender is immersed into fuel inside the tank and when the fuel level falls below some threshold it sends the information and fuel indicator lamp is activated. It's up to the driver then to manually switch fuel supply to reserve.

fuel sender (2GV-85752-00)

indicator panel with red fuel indicator (2GV-83530-01)

Electromagnetic fuel valve

Afterwards the fuel sender was abandoned and new electromagnetic fuel valve was implemented. This valve is mounted under the main tank, and its two tubes are immersed into the tank fuel. The valve passes the fuel either from one or from other tube. This device had several revisions during ther xv535 production:

3JB-24500-00	1989-1990
3JB-24500-01	1991-1992
3JB-24500-02	1993-1997
3JB-24500-03	1997-2001

electromagnetic fuel valve
(3JB-2450-03)

Fuel system on xv535 Virago

Yamaha xv535 Virago - 1987-1988
only one (lower) tank

Yamaha xv535 Virago - 1989-2003
two tanks

First versions (from 1987. to 1989.) of xv535 had only one lower (main) tank with volume of only 8,6 L (includes 2 L reserve) and gasoline was tanked under the seat. Upper tank did not exist then! Actually, there was an imitation of tank so called dummy tank positioned here for esthetic purpose. This fuel tank allowed the driving autonomy of only 100-150 km and very soon it appeared to be insufficient and that was one of main complaint of motorcycle owners. Yamaha promptly responded to this and in 1989. upper dummy tank was transformed into real tank where both tanks had capacity of 13,5 L (2,5 L reserve). The reason for moving tank down under the seat is lowering the motorcycle's center of mass which achieves better driving stability is more according to chopper form.

refueling the lower tank under the seat

The fuel from upper tank goes through rubber tube and fuel filter into main tank residing under the seat. On xv535 manual fuel valve (petcock) is substituted with electromagnetic valve (with corresponding  ON/RES switch on right hand handle) and electric fuel pump was added pumping the fuel from lower tank into carburetors.

Yamaha xv535 Virago - tank under the seat with electromagnetic valve under it

The electromagnetic valve has two input tubes immersed into tank towards up, one higher and one lower. Usually, when tank is full, then fuel is taken from higher tube (the valve status "ON"). When the fuel level drops under the input hole of higher tube, the higher tube cannot pass fuel any more and electromagnetic vale should be switched to take fuel reserve from the lower immersed tube (the valve status "RES"). The fuel then goes into fuel pump which pumps it into carburetors.

The additional rubber tube (breather) allows the exchange of air (pressure) between upper and lower tank in order not to create vacuum in lower tank because that would prevent the normal fuel flow.

As precaution measure, in order to prevent leak of fuel from tank when motorcycle falls down, the fuel pump stops to pump fuel 5 seconds after the engine stops (regardless of ON valve status).

Fuel tank on xv750

xv750 Virago models have only one (upper) tank from which the fuel flows freely into the engine carburetor. In the first versions from 1981, there were two valves on the tank, one on each side. Later, only one valve remained.

Yamaha xv750 Virago - 1981-1983

Yamaha xv750 Virago - 1984

Fuel tank onxv1100

On model xv1100 (like on xv535) there are two tanks, but here the upper one is bigger and main tank, while the lower one is smaller tank. Also here there's a fule pump.

Yamaha xv1100 Virago - 1986-1999