The vacuum systems have proved to be one of the important inventions pertaining to successful motor operation. They are self contained, simple in construction and automatic in operation. They do away with the troublesome power and hand pressure pumps and their connections.

Fig. 50 shows the top arrangement and connections. R is the air vent over the atmospheric valve. The effect of this is the same as if the whole tank were elevated, and is for the purpose of preventing an overflow of gasoline, should the position of the car ever be such as to raise the fuel supply tank higher than the vacuum tank. D shows the pipe connection from the fuel supply tank. C shows the pipe connection to the intake manifold. W shows a tap or vent through which gasoline may be fed into the upper chamber, in case the fuel supply tank is damaged or put out of commission. R shows the air vent connection from the lower tank.

Fig. 51 shows a general diagram of vacuum system installation. One of the chief advantages is that it allows the carburetor to be placed near the head of the motor and does away with the long manifold connections required with the gravity feed systems. This also reduces the frictional resistance, gives a richer mixture and greater volume of flow.

Fig. 52 shows a sectional view of the Stewart Vacuum System and explains the operative value of each part. A is the suction valve for opening and closing the connection to the manifold through which a vacuum is extended from the engine manifold to the gasoline tank. B is the atmospheric valve, and permits or prevents an atmospheric condition in the upper chamber. When the suction valve A is open and the suction is drawing gasoline from the main supply tank, the atmospheric valve B is closed. When the suction valve A is closed, the atmospheric valve B must be open, as an atmospheric condition is necessary in the upper tank in order to allow the gasoline to flow through the flapper valve H into the lower chamber. C is a pipe connecting the tank to the intake manifold of the engine. D is a pipe connecting the tank to the main fuel supply tank. E is the valve control lever and has two coil tension springs S attached to operate the short valve lever F. G is the metallic air-containing float, which controls the action of the valves through the spring and lever arrangement. H is the flapper valve at the outlet of T, and it closes by suction when the vacuum valve A is open. When the vacuum valve A closes, the atmospheric valve B opens and relieves the suction in the upper tank, the flapper valve H opens and allows the fuel to flow from the upper tank into the lower chamber.

J is a plug in the bottom of the tank which can be removed to clean or drain the tank. This plug can be removed and replaced with a pet-cock for drawing off gasoline for priming or cleaning purposes. K is the line to the carburetor. It is extended on the inside of the tank to form a pocket for trapping water and sediment. L is a channel space between the inner and outer shells and connects with the air vent R, thus admitting an atmospheric condition to exist in the lower chamber at all times, and thereby permitting an uninterrupted flow of gasoline to the carburetor. R is an air vent over the atmospheric valve; the effect of this valve is the same as if the whole tank was elevated. It is also for the purpose of preventing an overflow of gasoline should the position of the car ever be such as would raise the fuel supply tank higher than the vacuum tank. Through this tube the lower or reservoir chamber is continually open to atmospheric pressure. T is the outlet at the bottom of the float chamber in which the flapper valve H is located. U is the float stem guide. V is a strainer which prevents foreign matter from passing into the vacuum chamber. W is a tap or vent through which gasoline may be fed into the upper chamber if the fuel tank is damaged or put out of commission.

The simple and durable construction of this system makes it unlikely that the car owner will ever need to make internal repairs. Before attempting to repair this tank make sure that the trouble is not due to some other cause.

Air Vent.—A small amount of gasoline may escape through the air vent occasionally. This will do no harm and no adjustment is needed. However, if the vent tube continues to overflow, one of the following conditions will be responsible: 1. The air hole in the main supply tank is stopped up, or the hole is too small. Enlarge the hole or clean it out. 2. If gasoline leaks from the system except from the vent tube, it can only do so from one of the following causes: a. A leak may exist in the outer wall of the tank. If so soldering it up will eliminate the trouble. b. The carburetor connection on the bottom of the tank may be loose. c. There may be a leak in the tubing at the head of the tank. d. The cover of the tank may be loose.

Failure to Feed Gasoline to the Carburetor.—This condition may be due to other causes than the vacuum system. Do not tinker with it until you are sure that the trouble is not elsewhere. Flood the carburetor. If gasoline runs out of the float chamber you may be sure that the vacuum system is performing its work properly.

To Remove Cover.—To remove the cover for inspection, take out the screws and run a knife blade carefully around the top to separate the gasket without damaging it. Shellac the gasket before you replace it to make the tank air-tight.

Faulty Feed.—If faulty feed is traced to the vacuum tank, one of the following conditions may be the cause. The float valve G may have developed a leak. To repair, remove the top of the tank to which it is attached. Dip the float into a pan of hot water. Bubbles will show the leak. Punch two small holes, one at the top, and one at the bottom, and blow the gasoline out. Then solder up the holes and the leak. Use solder carefully in order not to add too much weight to the float. A small particle of dirt may be lodged under the flapper valve. This trouble can usually be remedied by tapping the side of the tank. In order to determine whether or not the flapper valve is working properly, plug up the air vent tube and remove the pipe extending from the bottom of the tank to the carburetor. Start the engine and place a finger over the opening (from which you removed the tube). If continual suction is felt, it is evident that the flapper valve is being held off its seat. If tapping the side of the tank will not remedy this condition, remove the cover and withdraw the upper chamber. The valve is attached to the pipe projecting from the bottom.

Strainer.—Remove and clean the strainer screen located at V, Fig. 52, every five or six weeks. This screen collects all the dirt and foreign matter in the gasoline, and often becomes stopped up.

Filling the Vacuum Tank.—To fill the tank after it has been cleaned or repaired, leave the spark off, close the gas throttle, and crank the engine over a few times with the starter or by hand. It takes less than ten seconds to create sufficient vacuum to fill the tank.