805161 Mercruiser STUD (4-3/4 IN.)

Fuel System
Fuel Flow
Fuel System Schematic
(1) Fuel injection nozzle. (2) Fuel injection lines. (3) Fuel return line. (4) Constant bleed orifice (part of the elbow). (5) Fuel injection pump housing. (6) Fuel priming pump. (7) Check valves. (8) Fuel transfer pump. (9) Fuel tank. (10) Primary fuel filter. (11) Secondary fuel filter.Fuel is pulled from fuel tank (9) through primary fuel filter (10) by fuel transfer pump (8). From the fuel transfer pump the fuel is pushed through secondary fuel filter (11) and to the fuel manifold in fuel injection pump housing (5). Fuel pressure in the fuel manifold is determined by the fuel transfer pump spring. A constant bleed orifice is in the fuel return line elbow. Constant bleed orifice (4) lets a constant flow of fuel go through fuel return line (3) back to fuel tank (9). This helps keep the fuel cool and free of air. The individual fuel injection pumps get fuel from the fuel manifold and push fuel at a very high pressure through fuel lines (2) to fuel injection nozzles (1). Each fuel injection nozzle has very small holes in the tip that change the flow of fuel to a very fine spray that gives good fuel combustion in the cylinder.Fuel Injection Pump
The fuel injection pump increases the pressure of the fuel and sends an exact amount of fuel to the fuel injection nozzle. There is one fuel injection pump for each cylinder in the engine.
Fuel Injection Pump
(1) Spill port. (2) Check valve. (3) Pump barrel. (4) Bypass port. (5) Pump plunger. (6) Spring. (7) Fuel rack. (8) Gear. (9) Lifter. (10) Cam.The fuel injection pump is moved by cam (10) of the fuel pump camshaft. When the camshaft turns, the cam raises lifter (9) and pump plunger (5). The pump plunger always makes a full stroke. As the camshaft turns farther, spring (6) returns the pump plunger and lifter to the bottom of the stroke.
Pump Barrel And Plunger Assembly
(1) Spill port. (2) Check valve. (3) Pump barrel. (4) Bypass port. (5) Pump plunger. (11) Orificed reverse flow check valve. (12) Spring. (13) Spring. (14) Scroll. (15) Slot.When the pump plunger is at the bottom of the stroke, fuel at transfer pump pressure flows through spill port (1) and bypass port (4). Fuel fills pump barrel (3) in the area above pump plunger (5).
Pump Barrel And Plunger Assembly
(1) Spill port. (2) Check valve. (3) Pump barrel. (4) Bypass port. (5) Pump plunger. (11) Orificed reverse flow check valve. (12) Spring. (13) Spring. (14) Scroll. (15) Slot.After pump plunger (5) begins the up stroke, fuel will be pushed out bypass port (4) until the top of the pump plunger closes the port. As the pump plunger travels farther up, the pressure of the fuel increases. At approximately 690 kPa (100 psi), check valve (2) opens and lets fuel flow into the fuel injection line to the fuel injection nozzle.
Pump Barrel And Plunger Assembly
(1) Spill port. (2) Check valve. (3) Pump barrel. (4) Bypass port. (5) Pump plunger. (11) Orificed reverse flow check valve. (12) Spring. (13) Spring. (14) Scroll. (15) Slot.When the pump plunger travels farther up, scroll (14) uncovers spill port (1). The fuel above the pump plunger goes through slot (15), along the edge of scroll (14) and out spill port (1) back to the fuel manifold. This is the end of the injection stroke. The pump plunger can have more travel up, but no more fuel will be sent to the fuel injection nozzle.
Pump Barrel And Plunger Assembly
(1) Spill port. (2) Check valve. (4) Bypass port. (5) Pump plunger. (11) Orificed reverse flow check valve. (12) Spring. (13) Spring. (14) Scroll. (15) Slot.When spill port (1) is opened by plunger (5) the fuel nozzle closes and spring (13) closes check valve (2) as the pressure above plunger (5) drops below 690 kPa (100 psi). At the same time the orificed reverse flow check valve (11) opens.Orificed reverse flow check valve (11) closes when the fuel pressure in the fuel injection lines is 6900 kPa (1000 psi). This keeps the fuel in the injection line and above the reverse flow check valve at 6900 kPa (1000 psi). Orificed reverse flow check valve (11) prevents rough idle by stopping any secondary injection of fuel between injection strokes. This valve is only effective below 8250 kPa (1200 psi) and has no effect above that pressure. When the engine is shutdown, the pressure is gradually released through a small groove on the bottom face of orificed reverse flow check valve (11).When the pump plunger travels down and uncovers bypass port (4), fuel begins to fill the area above the pump plunger again, and the pump is ready to begin another stroke.The amount of fuel the injection pump sends to the injection nozzle on each pump stroke can be changed by the rotation of the pump plunger. Gear (8) is attached to the pump plunger and is in mesh with fuel rack (7). The governor moves the fuel rack which turns the fuel pump plungers according to the fuel needs of the engine. When the governor turns the pump plunger, scroll (14) on the plunger changes the distance between the top of pump plunger and the point where scroll (14) uncovers spill port (1). The longer the distance from the top of the pump plunger to the point where scroll (14) uncovers spill port (1), the more fuel will be injected.To stop the engine, the pump plunger is rotated so that slot (15) on the pump plunger is in line with spill port (1). The fuel will now go out the spill port and not to the injection nozzle.Fuel Injection Nozzle
The fuel injection nozzle is installed in an adapter in the cylinder head and extended into the combustion chamber. The fuel injection pump sends fuel with high pressure to the fuel injection nozzle where the fuel is made into a fine spray for good combustion.
Fuel Injection Nozzle
(1) Carbon dam. (2) Seal. (3) Passage. (4) Filter screen.