The MGA With An Attitude
SUPER ECONOMY MODE - CF-115
How to drive without fuel (almost).
OOPS !!! Did you ever run out of fuel a mile or two short of a gas station? (Haven't we all)? You might kick yourself for being absent minded. You might walk to thumb a ride and come back with a fuel can. Or you might push the car if it's not too far. These days I suppose you might make a cell phone call to have a friend bring you a can of gas (although I don't have a cell phone or auto club card). This article may give enough information to allow you to continue to drive the car to the nearest gas station without having to walk or push.
This starts with an understanding of how the fuel system works in the MGA. The car has dual carburetors with manual choke, and an electric fuel pump. Fuel flow arrives at the carburetors via flex hose from the heater shelf to a banjo bolt at the rear carburetor float chamber, then another hose loops slightly upward and forward to the front carburetor float chamber. Fuel pressure is moderately low, intended to be around 3 psi in the supply hose plus or minus a bit. The carburetors work and the engine will run well with very low fuel pressure at the carburetor inlet fittings, like 1/2 psi or less. It will even work without a fuel pump with siphon action and/or gravity feed from a fuel can just a foot or so above the carburetors.
One day you may be driving casually along without noticing low fuel level, and the engine suddenly sputters, loses some power, and seems to run on two cylinders. It may take you two seconds to realize it just ran out of gas, but all is not lost, and maybe you won't have to walk. First order of business is safety, so check for other cars around you, and see if there may be a shoulder on the road wide enough to drive on at slower speed. When you have space and time for this maneuver, get off the throttle as soon as possible, put it in neutral, shut the engine off, and coast. Take safe haven on a wide smooth shoulder if possible, or continue in the traffic lane if there is not much traffic, and switch on 4-way flashers if you have them. When the engine has stopped turning, switch the key back on to make the electric fuel pump run while you continue to coast. When you have a few more seconds to think, figure out where the nearest gas station might be, then see of you can coax the car to run that far. At this time there may be one cup of fuel left in the tank the you might be able to retrieve, and that might get you up to three more miles if you can nurse out about 60 mpg fuel economy.
While coasting with the fuel pump running, the pump may be sucking up a lot of air and a little dribble of fuel. The fuel pickup pipe is on the right side of the fuel tank, so you may take advantage of a slightly sloping road shoulder to tilt the car slightly to the right to bring any remaining fuel into contact with the pickup pipe. You might also rock the car or zig-zag the the car slightly left and right to slosh any remaining fuel in the tank to be in momentary contact with the pickup pipe. Any fuel the pump manages to pickup can be delivered to the carburetors along with copious amounts of air bubbles.
When the car has coasted down to about 20 mph, depress the clutch, select 3rd gear, press throttle pedal half way down and hold it steady, and let the clutch up to start the engine. When the engine starts keep the throttle about half down and be patient while it picks up some speed. You may be pleasantly surprised that it actually runs. When the car gets up to about 45 mph, get off the throttle, shift back to neutral, switch off engine, and coast again. When engine has stopped turning switch on key to run fuel pump. Continue to repeat this coast and drive cycle as long as necessary (or as long as possible). Keep the electric fuel pump running while you are coasting.
With time the fuel recovery rate drops, as there was not much left in the tank to begin with. What little fuel is available will run into the rear carb until the float chamber is full. When the float valve shuts off flow into the rear float chamber more fuel will flow over the second hose loop to the front carburetor. With enough fuel available the front float chamber will fill and shut off flow, in which case the fuel pump might actually build up a little pressure in the fuel line (as it may be compressing some air bubbles). With some luck, next time you pop the clutch to restart the engine the carburetor float chambers may be full of fuel, and away you go.
This works as long as you can continue to nurse a few more dribbles of fuel out of the tank. When there is not enough fuel to feed the dual carburetors, the front carb will run out of fuel first as the rear cab takes the lion's share (everything available). As fuel level in the front carb drops it will get to a point where mixture leans out enough to cause misfire, in which case the engine will suddenly seem to be running on two cylinders (a fairly close description of the real situation). For fuel economy reasons it is not good to run on two cylinders, because what little fuel is going into the the front two cylinders without burning is wasted without producing any power. Also since there is a crossover balance pipe in the intake manifold, the rear two cylinders will be running just a little lean and will also not make best use of available fuel. As such, it is desirable to keep it running on all four cylinders whenever possible. If you're only up to 35 or 40 mph when it starts to sputter, so be it. Get off throttle, switch off, coast, and switch on after engine stops to keep fuel pump running.
You haven't lost the fight yet. If it looks like you're coming up a little short of where you want to go, there is another trick. When you're not getting up to a reasonable speed (less than 30 mph tops), the front carb is going low on fuel and running lean (sputter, sputter), then you can pull the choke out some as necessary to enrich fuel mixture. The lower the fuel level in the float chamber the more choke will be required, but only give it the amount of choke it needs to run, not too much choke to waste fuel. In this manner you can keep it running on all four cylinders for a short while, as long as there is some fuel in the float chambers.
Of course this cannot go on forever, but you might make two or three miles out of it. Eventually with very little fuel left there will be no fuel in the front carb and low fuel in the rear carb, and you may be using full choke and half throttle just to keep it running on two cylinders. When that no longer works you need to find a safe place to park it. Maybe you can actually get to a gas station this way. If not, at least you have somewhat less distance to walk or push. If you happen to be driving up a long hill (or a mountain) when this happens, you might consider making a U-turn to coast a long way down hill, depending on your judgment for best shot at making a gas station or getting off a busy road. One thing is for sure. Newer cars with high pressure fuel injection need not apply for this technique. Sometimes there is a certain charm or advantage in old technology.