Petcock


Definition


Petcock (Noun). Device to limit or stop the flow of fuel. It is named a Fuel Cock in the Suzuki parts inventory system and most service manuals.

Also known as a Fuelcock, Fuel-cock, or Fuel stop in common parlance.


Positions

Petcock.jpeg



Also note that on some later models, if your petcock knob's arrow won't point down, but up- then all the settings are backwards. Arrow pointing Left= Prime, Right= Res, Up= On.


RES


RES stands for Reserve. At this setting the tank will run down to empty. You should only run the bike on reserve when it starts to sputter in the "ON" position


ON


ON is the normal setting. When in the "ON" position vacuum is required to allow fuel to flow. This setting uses the tallest pick up tube.


PRI


PRI stands for Prime. In the prime position fuel will flow freely to the carburetors irrelevant of engine status. Useful if they are dry. You should never leave your petcock on prime because if the float needles are faulty, it may cause fuel to overflow into the crankcase.


How the Petcock Works

Petcock_explanation.gif

Behind the pick-up screens (green in my picture above) are two tubes, one short (the reserve/prime tube) and one tall (the standard ON tube).

The petcock acts as a three position switch comprising two pick-ups and two fuel passages.

In "ON" position, the system selects the taller fuel pick-up tube, and routes it through a passage that only gets opened by engine vacuum pulling a rubber diaphragm open (the rubber diaphragm is normally held closed by a weak spring).

In the "RES" (Reserve) position, the system selects the shorter fuel pick-up tube, and routes it through the same passage with the vacuum diaphragm.

In the "PRI" (Prime) position, the system selects the shorter fuel pick-up and bypasses the vacuum diaphragm by a protrusion on the selector knob pushing up against the metal indent in the center of the vacuum diaphragm (defeating the spring that normally holds it closed).

The engine doesn't produce sufficient vacuum to open up the vacuum diaphragm until the engine is turning at least 800 RPM or higher (varies by bike, ring condition, cylinder wall condition, etc.); should pull healthy at idle. This vacuum is created by the intake valves to the cylinder opening while the piston is moving downwards, which causes the air to be sucked through the carb (which is also where the vacuum pick-up is located). When the engine isn't spinning, there is no vacuum in the line to the petcock; as the starter turns the engine, it produces low levels of vacuum, and once it fires (successfully catching), the vacuum levels climb.

Since the carbs normally had fuel when you shut the bike down, there is normally no immediate need for additional fuel flow at start-up, unless the fuel in the carbs has been evaporated or emptied somehow (such as knocking the bike over). Thus, the fact that the petcock is in the ON or RES position when you start is normally irrelevant because there's enough reserve in the carbs. Once the engine starts, fuel will start flowing through the petcock and refilling the carbs as their levels drop. In cases where the carbs are empty or low on fuel for whatever reason, it makes sense to use the PRI (Prime) setting to flow fuel to the carbs to refill them before trying to start, then to switch back to the ON or RES position.


Purpose


The purpose of the vacuum-driven petcock safety system is to help eliminate two problems that historically plague carburetor-based bikes:

1. Damaged float valves in the carbs permitting excess fuel to flow into the cylinder and potentially causing damage at start-up by having a cylinder filled with fuel (an incompressible liquid for our purposes). As the starter motor turns the engine, a cylinder filled with liquid fuel would over-pressurize, distorting and potentially breaking various parts (including the starter gears, valves, rings or pistons, etc). The petcock in all positions other than PRIME should stop such flow when the engine is not running, thus preventing such damages from occurring at start-up (if functioning correctly).

2. Threat of fire in an accident scenario. By shutting down the flow of fuel when the engine stops running, it stops feeding fuel into a potential fire caused by engine heat meeting spilled fuel. This second reason is the primary reason that vacuum-operated "safety" petcocks became common-place equipment on motorcycles starting in the early 1980's and continuing through the end of the carb era.


Defective Petcock Symptoms


A Katana petcock is said to be defective when any of the following issues occur:

1. The petcock permits the flow of fuel in RES or ON without the engine running; this is the most common failure. This is typically caused by either debris [usually rust] build-up in the vacuum-diaphragm passage that the spring would shut off (debris), or by the spring itself no longer being strong enough to do the job (defective spring), or by the synthetic rubber diaphragm being torn/defective. These issues can normally be resolved with a petcock rebuild kit.

2. The petcock does not permit the flow of fuel in sufficient quantity when in the PRI position to fuel the engine at all RPM's. This is typically caused by blockages in the feeder tube or body of the petcock or by the screens around the feeder tube being blocked. It can much more rarely occur by: the spring rusting (expanding to fill the area) or the spring having been replaced by too strong a replacement (or the spring itself being misinstalled / kinked). These issues may be able to be resolved by cleaning the existing petcock (may or may not need the petcock rebuild kit); in cases of severe rust, replace the petcock.

3. The petcock does flow sufficient fuel while in PRI, but does not flow sufficient fuel to supply the engine at all RPM's in RES or ON. In theory, this problem could lie in the petcock itself, although far more commonly the problem lies outside the petcock, such as a vacuum leak in the vacuum line to the petcock or low vacuum levels at carb #4.

4. The dial of the petcock is frozen in place (binding). This is typically a result of rust from the tank causing subsequent rust in the petcock's PRI diaphragm defeat button. Sometimes this can be fixed by disassembly, and some scouring with either very fine emery cloth or green Scotchbrite, otherwise replace entire petcock.

5. Screens around the pick-up tubes within the tank itself breaking/defective. Although in theory, this should not prevent the bike from running, the screens are critical to keeping larger debris out of the petcock and subsequently the carbs, and pieces from brittle & broken screen can be the source of feeder-tube blockage. If the screen is broken, replace the entire petcock.


Rebuild kit


K&L Supply manufactures a rebuild kit intended specifically for the 1989 - 1997 Suzuki Katana's. Although no kit exists specifically designed for the 1998 - 2006 models, the parts within the kit for the earlier Katana's can be used to rebuild the newer petcocks as well, but will not replace the 98+ spring (so it won't fix spring issues).

The kit contains o-rings, a weak spring suitable for the pre-98's only, and the rubber diaphragm for the vacuum passage. It is useful for situations where the flow does not shut off in the ON and/or RES positions. The kit does not contain a replacement pick-up screen, pick-up tubes nor main knob-assembly; if you need any of these parts to make your petcock serviceable, buy a replacement petcock instead. The kit will normally not correct a situation where the petcock does not flow at all in PRIME (which is typically caused by blocked tubes due to rust & debris, commonly accompanied by a broken screen).

Rebuild how to


See also


Fuel tank connections