Evinrude 4906B 4HP 1969, Owner'S Manual

Page: 92 / 422

4
FUEL
4-1 INTRODUCTION
The carburetion and ignit ion principles
of two-cycle engine operation MUST be un-
derstood in order to perform a proper tune-
up on an outboard motor.
If you have any doubts concerning your
understanding of two-cycle engine opera-
tion, it would be best to study the operation
theory
section in the first portion of Chap-
ter 3, before tackling any work on the fuel
system.
4-2 GENERAL CARBURETION
INFORMA TION
The carburetor is merely a metering
device for mixing fuel and air in the proper
proportions for efficient engine operation.
At idle speed, an outboard engine requires a
FLOAT
Fuel flow principle
of a
modern carburetor.
mixture of about 8 parts air to 1 part fuel.
At high speed or under heavy duty service,
the
mixture may change to as much as 12
parts air to 1 part fuel.
Float Systems
A small chamber in the carburetor
serves as a fuel reservoir. A float valve
admits fuel into the reservoir to replace the
fuel consumed by the engine.
Fuel level in each chamber is extremely
critical and must be maintained accurately.
Accuracy is obtained through proper adjust-
ment of the float. This adjustment will
provide a balanced meter ing of fuel to each
cylinder at all speeds.
Following the fuel through its course,
from the fuel tank to the combustion cham-
ber of the cylinder, will provide an appreci-
ation of exactly what is taking place. In
order to start the engine, the fuel must be
moved from the tank to the carburetor by a
squeeze bulb installed in the fuel line.
THROTTLE
VALVE
V ENTURI RING
Fuel flow through the venturi, showing principle and
related parts controlling intake and outflow.