The diagram illustrates several carburetor basics. The result is a richer mixture. If you blow air across a straw inserted into a container of liquid, a pressure drop is created in the straw column. A float valve admits fuel into the reservoir to replace the fuel consumed by the engine. No flow or low flow. As the throttle is advanced to increase powerhead rpm, the fuel and the ignition systems are both advanced equally and at the same rate.
In almost all cases, the high-speed circuit has fixed high-speed jets and are not adjustable. The fuel for some small horsepower units is gravity fed from a tank mounted at the rear of the powerhead. Timing and synchronization on an outboard engine is extremely important to obtain maximum efficiency. On electronically controlled ignition models, a microcomputer decides when to advance or retard the timing based on input from various sensors. The bottom scribe line on the cam should be aligned with the center of the throttle roller and the roller just barely touches the cam.
At the time the throttle is opened. In order to obtain the proper air fuel mixture for all engine speeds, some models have high and low speed jets. Initial timing and timing advance are both set this way before using a timing light to check the timing. Traditional carburetor theory often involves a number of laws and principles. Even with the gravity feed method, a small fuel pump may be an integral part of the carburetor.
Any time the fuel system or the ignition system on a power head is serviced to replace a faulty part o; any adjustments are made for any reason, powerhead timing and synchronization must be carefully checked and verified. Many units have a filter as an integral part of the carburetor. Accuracy is obtained through proper adjustment of the floats. In this manner, a constant reservoir of fuel is maintainedn in the chamber to satisfy the demands of the engine at all speeds. A restriction in the throat, called a venture, controls air velocity and has the effect of reducing air pressure at this point.
This represents a clogged main jet. When this choke valve is fully closed, a very rich fuel mixture is drawn into the cylinders. Models equipped with electronic ignitions are statically timed by aligning the timing marks on the throttle cam or throttle stopper with timing marks on the flywheel. Using our example, what if the top of the container is covered and sealed around the straw, what will happen? The difference in air pressures at the throat and in the fuel chamber, causes the fuel to be pushed out of metering jets extending down into the fuel chamber. Exact Models Covered Covers All 1996-1997 Models World Wide : F8B F8-B , F9. A choke valve is placed ahead of the metering jets and venture. As the liquid in the column is expelled, an atomized mixture air and fuel droplets is created.
After fuel enters the chamber and the level rises to a predetermined point, a tang on the float closes the inlet needle and the flow entering the chamber is cut off. The throttle cam adjustment cannot be made unless the carburetor throttle shutter is completely closed. The powerhead cannot perform properly and produce its designed horsepower output if the fuel and ignition systems have not been redissolve dusted. This action is necessary because the fuel pump does not have sufficient pressure to draw fuel from the tank during cranking before the engine starts. If the carburetor has more than one reservoir, the fuel level in each reservoir chamber is controlled by identical float systems.
Therefore, there is no link rod between the magneto control lever and the stator assembly. As appropriate, these differences will be explained in detail in the text. Not as much fuel will flow. . If the base of the straw is clogged or restricted what will happen? This is typical of a clogged carburetor bowl vent. Carburetors with single, double or four barrels have individual metering jets, needle valves, throttle and choke plates for each barrel. A throttle valve controls the flow of air fuel mixture drawn into the combustion chambers.
All systems have some type of filter installed somewhere in the line between the tank and the carburetor. After the engine starts, the fuel passes through the pump to the carburetor. This fuel air mixture should then be in the proper proportion for burning in the cylinders for maximum engine performance. If an adjustment is necessary, first loosen the hex bolt and with the punch mark on the eccentric facing down and forward of the hex bolt axis, rotate the eccentric counter clockwise to reposition the cam until the throttle roller is aligned with the scribe line. When fuel leaves the chamber as the engine operates, the fuel level drops and the float tang allows the inlet needle to move off its seat and fuel once again enters the chamber. On the breaker point ignitions, synchronization is automatic once the point gap and the piston travel or timing mark alignments are correct.
At the carburetor, the fuel passes through the inlet passage to the needle and seat and then into the float chamber reservoir. Float Systems A small chamber in the carburetor serves as a fuel reservoir. If the fuel level is raised and you blow again at the same velocity what happens? The throat of the carburetor is usually referred to as the barrel. An alternate method is to check the timing with the powerhead not operating static timing. If the liquid in the glass is lowered and you blow through the straw with the same force what will happen? With the engine shut down, move the throttle cam to check the cam adjustment. Various models have unique methods of checking ignition timing.
A suction effect is created each time the piston moves upward in the cylinder. In simple terms, synchronization is timing the fuel system to the ignition. A float in the chamber rides up and down on the surface of the fuel. The actual ratio of air to fuel differs with engine conditions but is usually from 15 parts air to one part fuel at optimum cruise to as little as 7 parts air to one part fuel at full choke. Fuel level in each chamber is extremely critical and must be maintained accurately. At idle speed, an outboard engine requires a mixture of about 8 parts air to 1 part fuel. A cold powerhead requires a richer fuel mixture to start and during the brief period it is warming to normal operating temperature.