GPZ 900R Fuel tap

GPZ 900R (XZ 900A) Fuel Tap

The fuel tap on the GPZ 900R is the same across all models and can be temperamental.

It is important that all riders are aware of a) How it works, b) How to get home if it misbehaves and c) How to fix it when you get home.

HOW IS IT DESIGNED TO WORK ?

There are 3 lever positions on the tap. RUN. RES and PRI.

RUN - With the lever in the RUN position, Fuel will not flow through the tap until the engine is started. This is the only position that the tap should be in during normal operation.

RES - This is the Reserve position, to prevent the tank running dry. If you run out of petrol on RUN, turning the lever through to the RES position should give you at least 20-25 miles range. When on reserve, fuel will not flow through the tap unless the engine is running.

PRI - This is the Prime Position, and is generally used after the carburettors have been drained, or assumed to be dry. Turning the tap to this position will align drillings within the tap and gravity will feed fuel directly to the carburettors. **Use this position with care.**

The fuel tap is operated by a vacuum which is provided by the engine. The No 2 cylinder inlet tract has a small spigot attached to it, (which also used to balance the carbs) this is connected to the fuel tap by a narrow rubber pipe.

When the piston is on it’s induction stroke (suck) a tiny amount of vacuum is diverted to the tap. This vacuum then pulls on a diaphragm (I) which lifts a plunger and allows fuel to be delivered to the carburettors. When the engine is turned off, a Spring (J) pushes the diaphragm, and shuts off the fuel supply.

Given that the inlet tract and fuel tap are connected to one another by a pipe, an in-line “blow-back” valve is used to prevent flame from a backfire tracking up the pipe, and into the fuel system. This valve is a tiny 2-3 mm diameter disc fitted behind the brass spigot connecting the vacuum pipe to the tap. and is the cause many running problems.

It is a simple floating disc, and designed to be “pulled” open by the vacuum, and “slammed” shut by any back pressure. Do not be surprised to discover that your fuel tap does not have a blowback valve fitted, it may well have been removed by a previous owner, as many owners consider that the improved reliability by discarding it, is more important than the risk of explosion. Clearly the risk exists, but is considered by many to be almost negligible.

WHAT’S WRONG WITH A SIMPLE ON - OFF TAP ?

There’s nothing whatsoever wrong with an on - off design. It’s people like you and I that are the weak link. we tend to forget to turn them off. The consequences of which can be disastrous.
Once fuel is delivered to the carbs they have to regulate its delivery into the engine, and can only hold so much in their float bowls. Once the float bowls become full, their fuel shut off valves stop the fuel delivery from the tap. That is providing that everything is set correctly. However, if they fail to shut off the fuel and the fuel tap (and gravity) is still delivering fuel, the petrol has only two places to go. Forward or backwards.

If it goes backwards into the airbox it will quickly find its way down into the sump via the crankcase breather, and contaminate your engine oil. If it goes forward it may fill up a cylinder with fuel.

Then, the next time you try and start the engine, the piston will come up on its compression stroke expecting to find a nice fuel / air mixture to ignite, and instead will find a cup full of petrol. Having been thrown round at speed and being unable to compress or expel the fuel, it's likely to bend conrods and things! Mr Kawasaki fitted a vacuum operated tap in order to protect you from yourself. Never be tempted to fit after market, in-line fuel filters they restrict fuel flow too much, and can lead to running problems.

Never leave the fuel tap in the Prime position.

WHAT GOES WRONG, AND HOW DO I FIX IT ?

A number of complaints are common to this type of fuel tap, and all of them must be rectified at the earliest opportunity, to ensure reliability, and avoid internal engine damage. All faults will manifest themselves by either poor running / cutting out, or by the tap delivering fuel when it shouldn’t.

FUEL IS LEAKING, FROM THE TAP

Fuel dripping onto engine casings from the tap is due to an O ring or gasket failure.

FUEL COMES OUT OF THE TAP WHEN THE ENGINE IS NOT RUNNING.

Check that the tap is not on PRI. If the tap is on RUN or RES, and fuel is issued, it is almost certainly due to a stretched, or perforated diaphragm (I). Or, the blowback valve has seized, and is “holding” a vacuum between it, and the diaphragm.
Replace the diaphragm (I), and return spring (J), and if this fails to rectify the complaint, investigate the blowback valve. It often gets a corrosion build up, and will benefit from a clean out,and a squirt of WD 40. You may wish to consider discarding it, but this is not an endorsement, be aware of its role. Try slitting it from edge to centre, this will help the pressure equalise without detracting from its function too much.

THE ENGINE FADES WHEN IN THE CRUISE.

“Sudden death syndrome.” normally strikes when your sitting at a steady 60 - 80 mph. Quite simply the carbs run dry, and the engine is starved of fuel. If you’re quick, you can spin the tap round to PRI, just as you would when switching from RUN to RES when running low. This should deliver fuel quickly enough for the engine to pick up, and get you home. If you do end up at the side of the road, turn it to PRI wait 5-6 seconds, and the machine should start. If it doesn’t and your convinced its fuel starvation, try opening the fuel filler cap. It may be that the fuel tank is not venting correctly, and is creating its own vacuum and holding the fuel inside the tank.

Once you’re up and running again try to turn the tap round to RUN, it may well continue to function normally with no further problems. Check also the quality, and integrity of the connections of the vacuum pipe feeding the tap. If in doubt overhaul the tap and/or renew the vacuum pipe.

CONCLUSION.

If its not broken don’t fix it. Just be aware of how the fuel tap operates and learn to identify it’s shortcomings.

Avoid aftermarket overhaul kits, (Tourmax) they are rarely effective, for a higher rate of success, only use genuine Kawasaki parts for fuel tap repairs.