Yamaha F50 Service Manual

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Features and benefits1
Newly designed four carburetors
Newly designed four carburetors have been based on the current F50A. Prime Start has been
adopted for the starting system to further increase starting performance and serviceability. Further-
more, the acceleration pump and the dashpot have been integrated to simplify construction and to
ease serviceability. The four carburetors can be accurately and easily adjusted due to the simple
construction of the links.
1Prime Start
2Acceleration pump
Identification / Features and benefits 
						

							GEN 
INFO
General information
1-762Y3A11
CDI unit with microcomputer
The ignition system consists of a flywheel, stator, pulser coil, thermoswitch, oil pressure switch, CDI
unit, and ignition coil. The CDI unit contains a built-in microcomputer that determines the ignition
timing separately for acceleration and for normal operation, based on signals received from the
pulser coil, thermoswitch, and oil pressure switch.
.
1CDI unit
2Flywheel
3Stator
4Pulser coil
5Thermoswitch
6Ignition coil
7Oil pressure switch
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Lower unit
Higher class pinion and gears
The lower unit of the new T50/FT50B, FT50C uses the same type of large gears that are used in the
F100. The use of the same type of gears that are used in a higher class model provides ample dura-
bility, making it possible to accommodate a wider range of applications throughout the market.
Pinion Forward gear Reverse gear
Gear ratio
ModelNumber 
of teethDiameter 
(mm)Number 
of teethDiameter 
(mm)Number 
of teethDiameter 
(mm)
F50/F50A 13 45 24 74 24 75 1.8
T50/FT50B, 
FT50C13 46 30 95 30 95 2.3
F50/F50A T50/FT50B, FT50CS62Y1220
Features and benefits 
						

							GEN 
INFO
General information
1-962Y3A11
Shift slider type shift mechanism
The T50/FT50B, FT50C has adopted the shift slider type shift mechanism. This shift mechanism
enables a prompt engagement of the dog clutch regardless of the operating speed of the shift lever.
Thus, smooth and positive shift operation has been made possible.
1Shift rod
2Propeller shaft
3Dog clutch
4Shift slider
ÈForward
ÉReverse 
						

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Technical tips1
Carburetor
Starting system (Prime Start)
To facilitate the starting of a cold engine, an air-fuel mixture that is richer than normal is required.
For this reason, the Prime Start system has been adopted in the F50/F50A, T50/FT50B, FT50C. In
the Prime Start system, the thermo heater plunger is in a position that fully opens the fuel enrich-
ment valve while the engine is being started. Thus, fuel enrichment is achieved during the starting of
the engine, and continues while the engine is being warmed up. Once the engine starts, current
flows from the lighting coil of the to the thermo heater, allowing the wax in the Prime Start unit to
expand. The expanded wax moves the thermo heater plunger in the direction to close the enrich-
ment valve. As a result, the volume of fuel that passes through the fuel enrichment valve decreases.
A few minutes after the engine has started, the thermo heater plunger completely closes the fuel
enrichment valve, thus ending the fuel enrichment by the Prime Start system.
1Prime Start
2Intake manifold of the engine
3Thermo heater plunger
4Fuel enrichment valve
5To carburetor #2 or #46Carburetor
7Intake silencer
ÈAir
ÉFuel
ÊPlunger movement
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Features and benefits / Technical tips 
						

							GEN 
INFO
General information
1-1162Y3A11
1Prime Start
ÈThe enriched air-fuel mixture is delivered to
cylinders #1 and #2 by the Prime Start system
attached to carburetor #1.
ÉThe enriched air-fuel mixture is delivered to
cylinders #3 and #4 by the Prime Start system
attached to carburetor #3. 
						

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Carburetor operation
1. Idle and low-speed operation
Since the vacuum at the venturi is low when the throttle valve is opened slightly, the main nozzle
does not supply any air-fuel mixture to the engine.
When the engine is operating at idle, the fuel that passes through the pilot jet and the air that passes
through the pilot air jet mix, enabling the air-fuel mixture that has been regulated by the pilot screw
to be fed through the pilot outlet.
Because the throttle valve opens slightly when the engine is operating at low speeds, air-fuel mix-
ture also starts to be fed from the bypass holes.
1Pilot screw
2Pilot outlet
3Bypass holes
4Venturi
5Main nozzle
6Pilot jet
7Main jet
8Pilot air jet9Main air jet
0Throttle valve
ÈAir
ÉAir-fuel mixture
ÊFuel
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Technical tips 
						

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INFO
General information
1-1362Y3A11
2. Medium speed operation
When the throttle valve opens further, air-fuel mixture is fed from the pilot outlet and all the bypass
holes. In addition, air-fuel mixture is also supplied from the main nozzle in accordance with the
opening angle of the throttle valve.
1Pilot screw
2Pilot outlet
3Bypass holes
4Venturi
5Pilot jet
6Main jet
7Main nozzle
8Pilot air jet9Main air jet
0Throttle valve
ÈAir
ÉAir-fuel mixture
ÊFuel
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3. High-speed operation
When the throttle valve approaches its fully open position, the fuel that has been regulated by the
main jet and the air that has been regulated by the main air jet are mixed in the main nozzle. The
resultant mixture is then sprayed by the main nozzle into the venturi. The air-fuel mixture that is
sprayed through the venturi is then fed into the engine.
1Pilot screw
2Pilot outlet
3Bypass holes
4Main nozzle
5Pilot jet
6Main jet
7Main air jet
8Throttle valveÈAir
ÉAir-fuel mixture
ÊFuel
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Technical tips 
						

							GEN 
INFO
General information
1-1562Y3A11
Acceleration pump
The function of the acceleration pump is to ensure a smooth acceleration by preventing the air-fuel
mixture from becoming temporarily lean in case the throttle valve is opened suddenly. When the
throttle valve is opened suddenly, a large volume of air is introduced into the engine. However,
because fuel is heavier than air, it is not possible to supply the volume of fuel that is necessary for
the large volume of air that has been introduced. Thus, because it is not possible to achieve the air-
fuel mixture that is required by the engine, bucking or hesitation results. For this reason, the acceler-
ation pump temporarily increases the fuel volume in order to adjust the air-fuel mixture to a ratio that
is necessary for the engine in case the throttle valve is opened suddenly.
1Bypass hose
2Acceleration pump
3Pump chamber
4Diaphragm 1
5Throttle lever
ÈAir flow
ÉThrottle valve opening direction
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