Reserve lights

Fuel petcocks and reserve lights

(BTS, November 2004 )

"Why can't new Beemers have fuel petcocks like the old ones?  With them, I know exactly how much fuel is left. So simple, no electronics needed."

I actually do hear this from time to time. Those of you who are incredulous need read no farther. And for those who don't know what I'm referring to, the old carbureted BMWs have mechanical fuel valves which let the engine run out of fuel with about a gallon remaining in the tank, at which point the rider turns the valve to allow the remaining fuel to be used.  That way the rider knows that he is on his last gallon and needs to find a pump.

As usual, the new technology really is better.  In this case there are two reasons why the reserve light is a good idea.

First of all, safety.  "So there I was, in the left lane right before the two lanes merged into one for the construction, at the top of the hill, nearly past the five fully-loaded tractor trailers running nose-to-tail in the right lane who were full on the throttle trying to carry momentum to the top, and right as I'm abreast of the driver's door of the first one, with the concrete barriers a hundred feet ahead in my lane...."  or maybe it's"So I pull out for a pass, and just as I cut back in front of the cement mixer at the bottom of the downhill..."    OK, get the point?  Sudden power loss isn't the best way to notify the rider that he has only 40 miles of fuel remaining, if it happens at a moment when he wishes that he has 100 feet more fuel NOW, never mind what happens in forty miles.  A light on the dashboard does the job with a much higher level of cooperation.

The main hurdle, however, is technical.  Old bikes fed fuel via gravity to the carburetors and didn't have any complicated wiring for float sensors and lights.  Yes, the fuel petcock is a very simple device that works well aside from the obvious safety issues.  The fuel-injected bikes, however, already are committed to having lots of wiring, including wires into the tank for the fuel pump.  That pump doesn't like to run dry, either, and the catalytic converter doesn't like the fuel mixture that it gets when the pump runs out of fuel. In short, there is no good reason at all, in this day and age, why the engine should be deprived of fuel merely to send a warning message to the rider, and BMW has provided a much better means for that
warning with the reserve light.

Some old-timers do turn the petcocks to 'reserve' before putting themselves at risk the way I've described.  Once they're on their own again, it goes back to 'main' and the low fuel can manifest itself at a time of the rider's choosing.

If your fuel light comes on too early or too late for your liking, you can bend the float arm as a means of calibration.  If it comes on inconsistently, the float arm might be rubbing on the vent hoses in the tank.  Your dealer can help you in either case, but if you want to dig into it yourself feel free to contact me.