So you want to run out of gas do you?


Question: What happens if a fuel injected motorcycle runs out of fuel?  I own a Moto Guzzi Breva 750 (2004) and I bought the Shop Manual for it, which was a total waste of $75.00.  I put the question to my dealer and all he told me is not to run out of fuel.  Same answer from Moto Guzzi Italy.  Sooner or later I will run out of fuel so what will happen?  Thank you for your time. Have a nice day.


Reply:  I will investigate the Fuel injection system on the Moto Guzzi in greater depth for this reader, and reply to him personally, in case this system is drastically different than most.  Generally, the short answer is nothing.  Fuel injection systems have a high pressure (50-80 psi) fuel pump typically mounted in the fuel tank.  From there, a high-pressure fuel line delivers this fuel from the pump to the delivery manifold mounted to the back of the injectors.  This manifold is called the fuel rail.  The fuel rail, much like an airbox is designed to have enough capacity to supply the injectors at full demand without restriction, at the same time supporting and locating the back side of the injector.  The high-pressure line is identifiable by its very rigid nature, larger outside diameter and it will have banjo style bolts or mechanical quick disconnects to secure both ends.  The unused fuel (at low engine speeds and demands) will overwhelm the relief valve, pushing open the spring that is set to system pressure and return to the fuel tank via a soft rubber traditional style fuel hose.  This hose is typically ¼” or so in diameter, and secured using only spring clamps, as the pressure in this hose is negligible being open to the fuel tank at one end.  The system is constantly recycling in this nature with fuel always in motion whether it is being used or not.  An advancement on this system, as on the new Kawasakis, puts the relief valve in the tank after the pump but before the supply line, venting into the tank itself, simplifying the hosing under the tank.  This system has only the one high-pressure line off the bottom of the tank to the rail, with no return system required.


In contrast, a carburetor style fuel delivery system uses a low-pressure pump that purely has to move fuel from the bottom of the tank up to the carburetor float bowls.  When the bowls fill and the carb floats rise up to their top level, the fuel needle and seat close, stopping incoming fuel flow.  The pump will then stall against this very moderate pressure and wait until the engine has used some fuel to start up again.  This style of pump is commonly heard when you turn on your key.  The pump will make a soft dud., dud.., dud…, dud …..sound, that slows and then stops when the bowls are refilled.  This start up cycling heard when you turn on your key is purely making up for float volume lost to evaporation.  A multi-meter will confirm that this diaphragm style pump common on most carbureted engine system, uses no electrical power when it stalls like this.  It only cycles as fast as demand requests, with the battery current following proportionately. 


The global move to fuel injection, has seen the vast reduction of race bikes that run ‘total loss’.  Bikes that run ‘total loss’ operate without any charging system whatsoever.  This loss of your charging system allows you a benefit from the weight reduction of the stator coil and the rotor, which typically sits right on the crank, slowing engine as well as overall acceleration.  Ditching the charging system also exempts you from the loss caused by the magnetic drag of turning the rotor through the stator.  The only electrical drain on ‘old school’ carbed race bikes that ran total loss was the modest requests from the ignition, and a little bit of power for the low-pressure pump.  Race bikes could easily compete for 45 minutes to an hour on an often under sized battery (compared to stock) with a good charge, devoid of any charging system whatsoever.  Now, on an F.I (Fuel Injected) bike, the battery would have to supply the ignition, the injectors, and a high-pressure pump that runs constantly, typically not worth chancing a DNF.


So, after a little bit of background, back to the reader’s question.  Your fuel-injected bike is about to run out of gas.  Your pump starts to cavitate and then runs dry when fuel is no longer present, the bike quickly stalls as the injectors have nothing to spray and you would turn off the ignition.  Every F.I bike I have had experience with has a fuel pump shut off system built into the ECU to keep the pump from working while the ignition is turned on but the engine is not running.  This same system is what you hear prime, then shut off the pump after you turn on the key but before you start your bike.  If you do run the tank out of fuel (as we do on the race team all the time to measure fuel remaining after practice sessions), just cycle the key switch a couple of times to re prime the system.  If the area is quiet, you will hear the air purge back into the tank from the fuel rail and the pump load a little as it re primes.  I have run a number of fuel injected bikes out of fuel on the dyno, both on purpose and while hoping to do 'just one more run' with zero side effects.  The odd time you will hear of an old snowmobiler (usually) that claimed to have seized an engine while running out of gas.  The design of the float bowl in a carburetor would allow you to ‘dry up’ the pilot jet for an instant before running the main jet out fuel, stalling the engine.  For this to be the cause of seizing an engine, you would have to be riding a carbureted two stroke, on the ragged edge of detonation already, with EGT’s through the roof for that brief moment of a lean condition to be the reason for an engine failure.  Chances are, the engine was on it’s last tank of fuel anyway, you just had a different excuse to blame the seizure on.  So go ahead, run that tank as low as you dare, just have a buddy around with a siphon hose… eh Mark?