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Petcock Operation

Gasoline essential to the proper operation of your motorcycle is controlled by a “Fuel Control Valve” otherwise known as a “Petcock”.  This valve is characteristically located at the bottom of the fuel tank and can have a number of different configurations.  Fuel-Injected motorcycles rarely have a petcock.

Your carbureted bike will have one of the following:

  1. Manual Operation  

     These valves are generally found on Pre-1982 machines and dirt bikes.  This is the original type of valve used since the beginning of time.  It operates on the same principal as your bathroom faucet you need to open and close manually.

There are three positions on these ON  - OFF - RESERVE.

ON is used during normal operation of the bike.  It is the position you always return the valve to when refueling.

RESERVE is used when you run out of gas in the ON position.  When you switch to this position it may take a bit of time to refill the float bowl(s) so wait 10 seconds and try to refire the bike if you are stopped.  If you are going down the road at the time of the switch from ON to RESERVE, the time will be less, so make the switch and get your hands back on the bars as the jolt of the motor restarting could be sudden and upsetting to the bike (and you). It would be a good idea to practice this switching over before you need to do it while you are stopped

so there is less fumbling and confusion when it is needed.

OFF is used when the bike is parked. If you do not turn the fuel OFF the same result as the bathroom faucet left on can occur…..Flooding!  The gas will then run out on the ground or into the crankcase, diluting the oil and possibly ruining the engine.  The tendency for people to overlook this needed shutting off of the fuel is what  necessitated the next type of fuel valve.  The wear that goes with age and miles reduces the carburetor float valves’ ability to shut off the gas (we are replacing a lot of float valves in the shop) makes turning off the gas even more imperative.

      2.) Vacuum Operated

         This is the most common valve found in use today.  It uses engine vacuum createdwhen the engine is running to pull back a small piston that allows the fuel to flow. When the engine shuts off, the vacuum is removed and the fuel shuts off.

         This valve also has three positions: ON – RESERVE – PRIME

         ON is the position for normal operation and the position you return to after refueling the bike.

           RESERVE see above

           PRIME is the position used when the automatic vacuum shutoff is to be bypassed.

            The most common use is after a bike has been sitting a week or longer and the gas has evaporated from the bowls, or the bike has run out of gas in the ON position.  Turn the valve on for about 15 seconds and then turn it to the position it was in before (ON or RESERVE). Do not leave the valve in this position after the bike is started or two possible problems could arise. First is the likelihood that the carbs will overflow (see OFF above) or because the PRIME position uses the RESERVE  fuel level.  If you continue to operate the bike at the reserve level and run out of fuel you will be OUT OF FUEL (see pushing the bike).

         The failure of this shutoff system necessitates the rebuilding or replacement of the valve.

3.)    Remote Vacuum Valve

This is used where the space constraints do not allow the incorporation of the Automatic Vacuum Shutoff into the fuel valve itself.  This configuration is used on both Sportbikes and Cruisers, so to determine if your bike has one, some investigation may need to be done.  While the configuration is different from the valve described in #2 above, the operation is the same.

4.)    Electric Shutoff

Rarely used (thank goodness).  Most common the 1979-83 KZ-1300, a truly memorable occasion pushing one of those when the fuel is OFF.


In the normal course of operation the owner of a bike needs to have a general idea of the amount of fuel left in the tank.  This can be accomplished in a number of ways. 

The first and most common is the TRIP METER.  This is the smaller second odometer in the speedometer assembly.  It is accessed and/or zeroed by a knob on the meter or a button on the newer electronic speedos.  Every time the tank is filled up return this trip odometer to zero.  You can then anticipate the timing on refueling as you run out of gas in the ON position the first time and look at the trip meter to see the miles traveled.  If you watch the trip odometer you can avoid the inconvenience of restarting the bike by anticipating point at which the bike will run out of fuel.

The second is using the gas gauge if provided on the bike.  This can be used in concert with the trip meter to monitor fuel level and relative consumption rates.

The third is opening the gas cap and looking in.  This is not always a reliable method as the unusual shapes used in the floors of motorcycle gas tanks can greatly affect the amount of gas available you can see.

© Bill Whisenant 2007
updated 11/26/2012


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