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## Fuel Plunger Primer 

The optional fuel plunger primer is used to draw fuel from the tanks to supply it directly into the engine prior to starting. This is particularly helpful during cold weather when engines are hard to start because there is not enough heat available to vaporize the fuel in the carburetor. For some aircraft, it is the only way to deliver fuel to the engine when first starting. After the engine starts and is running, the fuel pump pushes fuel to the carburetors and begins normal fuel delivery. To avoid overpriming, read the priming instructions in the POH. 

## Choke 

A choke or fuel enriching system is an alternate method to provide additional fuel to the engine for initial cold starting. Actuating the choke control allows more fuel to fl ow into the carburetor. 

## Fuel Bulb Primer 

The fuel bulb primer is manually actuated by squeezing the bulb to draw fuel from the fuel tanks. This charges the fuel lines and carburetor float bowls before starting the engine the first time on a given day. After the engine starts, the fuel pump is able to deliver the fuel to the fuel bowls. An electric auxiliary fuel pump can also be used to charge the fuel lines and carburetor fuel bowls before starting. This auxiliary fuel pump is also used as a backup pump of the engine driven fuel pump fails. 

## Fuel Gauges 

The fuel quantity gauge indicates the amount of fuel measured by a sensing unit in each fuel tank and is displayed in gallons. Do not depend solely on the accuracy of the fuel quantity gauge. Always visually check the fuel level in the tank during the preflight inspection, and then compare it with the corresponding fuel quantity indication. It is also important to track inflight fuel consumption. Be sure to consult the POH and know the approximate consumption rate to ensure sufficient fuel for flight. If an auxiliary electric fuel pump is installed in the fuel system, a fuel pressure gauge is sometimes included. This gauge indicates the pressure in the fuel lines. The normal operating pressure can be found in the POH. 

## Fuel Filter 

After leaving the fuel tank, the fuel passes through a fi lter before it enters the fuel pump or carburetor. This fi lter removes sediments that might be in the fuel. [Figure 4-18] 

## Fuel 

Aviation gasoline (AVGAS) is identified by an octane or performance number (grade) which designates the antiknock value or knock resistance of the fuel mixture in the engine 

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cylinder. The higher the grade of gasoline, the more pressure the fuel can withstand without detonating. Lower grades of fuel are used in lower compression engines because these fuels ignite at a lower temperature. Higher grades are used in higher compression engines, because they must ignite at higher temperatures but not prematurely. If the proper grade of fuel is not available, use the next higher grade as a substitute. Never use a lower grade. This can cause the cylinder head temperature to exceed its normal operating range, which may result in detonation. Unfortunately, AVGAS 100 Low Lead (LL) may not be recommended by two-stroke engine manufacturers and may not be preferred by the four-stroke manufactures. Even though the “LL” stands for low lead, 100LL contains more lead than the old leaded gas dispensed at automotive filling stations. The lead in the fuel leaves deposits in the piston ring grooves, freezing the rings in position and reducing engine performance. Spark plugs are also very susceptible to lead fouling. This is especially true in two-stroke engines that use cooler ignition temperatures than standard aircraft engines. 

AVGAS does have some advantages. It degrades slower than auto gas, maintaining its efficiency for a full 3 months. AVGAS 100LL has no seasonal or regional variations and is manufactured according to a standardized “recipe” worldwide. If the airport has only 100LL available, it is permissible, absent any limitations of the engine manufacturer, to mix 100LL and auto gasoline for use in two-stroke engines. A 50–50 ratio will boost the octane rating and limit the amount of lead available for fouling. Generally speaking, this is a 

