TM 5-4240-501-148P
CARBURETION
Automatic Choke
3.
If the engine can be started, run for 2 or 3 minutes,
at a normal operating speed. Check to be sure fuel
tank is 1/2 full of fuel. Then, open the needle valve
to be sure the mixture can be made too rich. Next
close the needle valve to be sure the mixture can be
made too lean. Adjust needle valve to midpoint
between too rich and too lean.
Allow engine to run at idle speed for 3 to 5 minutes.
Again, close needle valve; the mixture should
become so lean the engine will stop. If the engine
continues to run at idle with the needle valve closed,
a fuel leak is occurring at one of the following areas:
Check items 2D, 2H, 2I, 2.J and 2K.
If the choke valve does not react as stated in Steps 1, 2
and 3, the carburetor will have to be disassembled to
determine the problem. (See Repair Procedure below).
The following list is given to aid you in checking the
performance of the Automatic Choke Carburetion
System.
1. Engine Appears to be Under-Choked
A.
Carburetor adjusted too lean
B.
Fuel pipe check valve inoperative (Vacu-Jet
only)
C.
Bent air cleaner stud
D.
Sticking choke shaft due to dirt, etc.
E.
Choke spring damaged or too short (See Repair
Procedure)
F.
Diaphragm
not
preloaded
(See
Repair
Procedure)
2. Engine Appears to be Over-Choked
A.
Carburetor adjusted too rich
B.
Bent air cleaner stud
C.
Sticking choke shaft due to dirt, etc.
D.
Ruptured diaphragm
E.
Vacuum passage restricted
F.
Choke spring distorted, stretched, etc.
G.
Gasoline or oil in vacuum chamber
H.
Leak between link and diaphragm
I.
Diaphragm folded during assembly, causing
vacuum leak
J.
Machined surface on tank top not flat (See
Repair Procedure)
K.
Needle valve seat loose
REPAIR PROCEDURE
Inspect the automatic choke for freeness of operation.
Any sticking problems should be corrected, as proper
choke operation depends on freedom of the choke to
travel as dictated by engine vacuum.
Repair procedures specific to the automatic choke are as
follows:
Remove the carburetor and fuel tank assembly from the
engine. The choke link cover may now be removed and
the choke link disconnected from the choke shaft.
Disassemble carburetor from tank top, using care to
insure diaphragm is not damaged.
CHECKING DIAPHRAGM AND SPRING
The diaphragm is suitable for further use, provided it has
not developed wear spots or punctures. (On Pulsa-Jet
models check to insure fuel pump valves are not
damaged.) Also check choke spring length. The Pulsa-
Jet spring minimum length is 1-1/8" -maximum 1-7/32"
and the Vacu-Jet spring minimum length is 15/16"
maximum 1". NOTE : On Model 110900 and 111900
choke spring minimum length is 15/16"; maximum 1-
3/8". If spring length is shorter or longer than specified,
replace diaphragm and spring.
CHECKING TANK TOP
The machined surface on the top of the fuel tank must
be flat in order for the diaphragm to provide an adequate
seal between the carburetor and tank. If the machined
surface on the tank is not flat, it is possible for gasoline
to enter the vacuum chamber by passing between the
machined surface and diaphragm. Once fuel has
entered the vacuum chamber, it can move through the
vacuum passage and into the carburetor. The flatness
of the machined surface on the tank top can be checked
by straight edge and feeler gauge, as shown in Fig. 10.
A .002" feeler gauge should not enter between the
straight edge and machined surface, when checking at
the shaded areas depicted in the drawing. Replace tank
if gauge enters. NOTE: STRAIGHT EDGE MUST BE
ACCURATE.
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