Fuel Plunger Primer Choke Fuel Bulb Primer Fuel Gauges Fuel 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. 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. 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. 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. 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] 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  Fuel Filter 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  4-14 Carburetor Fuel Bowls FUEL F LOW F U E L F L O W Fuel Filter Fuel Pump Figure 4-18. Fuel system showing fuel filter to fuel pump to  carburetor float bowls.