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MACK—303S-304S
September, 1936
Reprinted December, 1944
MACK JR. SYMBOL IA254
CARBURETER No. 303S 936—Models IMA-IOMA-IOMB-IOMX 937—Model 2M
(Starting Serial 1651) 1937—Model I I M (Starting Serial 2201)
3-3/16" Bore; 43/8" Stroke 1936—Models 20MA-20MB-20MX 1937—Model
2I M (Starting Serial 2160)
3%" Bore; 41/4" Stroke
MACK JR. SYMBOL IA255
CARBURETER No. 304S 936—Models 30MA-30MB-30MX-90MA-90MX 1937—Model
3I M (Starting Serial 2330) 1937—Model 9I M (Starting Serial 1561)
3%" Bore; 5" Stroke 1936—Model 90MX-90MSX
1937—Model 9I MX (Starting Serial 1561) 35/8" Bore; 5" Stroke
MACK, J R.
TRUCKS
1936 -1937
Carbureter No. 303S—List Price $22.05
Carbureter No. 304S—List Price $29.40
(303S Superseded by 5915)
A $3.75 exchange allowance is deducted from the list price if buyer turns
in the carbureter which is replaced by a 303S; a $5.00 exchange allowance
when replaced by a 3045.
CARBURETER SPECIFICATIONS
Dimensions: (303S) Flange size, 11/4 inch (31.75 mm). Throttle bore, 1-7/16
inch (36.51 mm).
(304S) Flange size, 11/2 inch (38.10 mm). Throttle bore, 1-11/16 inch (42.86
mm).
(303S) Main venturi, 11/4 inch (31.75 mm) I. D.
(304S) Main venturi, 1-9/32 inch (32.54 mm) I. D.
Float Settings: (Both models) Top of float 1/64 inch (.397 mm) below top
surface of carbureter body casting. (Plus or minus 1/64 inch (.397 mm).
Vent: (Both models) (Extends into air horn) inside. Size, 7/32 inch (5.56
mm) I. D. tubing.
*Gasoline Intake Needle: (303S) Triangular, horizontal No. 44 (2.18 mm),
drill in needle seat. (304S) Triangular, horizontal No. 42 (2.37 mm) drill
in needle seat.
Idle Orifice Tube: (Both models) Jet size, .0276 inch (.70 mm). (303S) Idle
passage air bleed (through plug) size, .061 to .063 inch (1.55 to 1.60 mm).
(304S) Idle passage air bleed (through plug) size, .049 inch (1.25 mm).
Idle Ports: (Both models) Upper port, length .200 inch (5.08 mm). Width .030
inch (.762 mm). Opening .008 to .012 inch (.203 to .305) with valve tight
closed.
(Both models) Lower port, size .053 to .057 inch (1.35 to 1.45 mm) die. (For
idle adjustment screw).
*Set Idle Adjustment Screw: (Both models) I/4 to 3/4 turn open. Do not idle
engine below 300 rpm. or 6 miles per hour.
Main Metering Screw: (303S) Calibrated to flow 240 to 244 cc per minute.
(304S) Calibrated to flow 268 to 272 cc per minute. (Do not gauge for size.
If in doubt, replace with new part.)
Vent Tube: (3035) Air bleed through plug in diffuser, size, .0394 inch (1.00
mm) drill.
(304S) Air bleed 'through plug in diffuser, size, .0315 inch (.80 mm) drill.
Main Vent Tube:
Side Holes: (3035) 2-.0354 inch (.90 mm). Approximately 21/64 inch (8.33
mm) from end. 2-.0354 inch (.90 mm). Approximately I-I/64 inch (25.8 mm)
from end. 2-.0354 inch (.90 mm). Approximately 1-33/64 inch (38.5 mm) from
end. (304S) 2-.0354 inch (.90 mm). Approximately 21/64 inch (8.33 mm) from
end. 2-.0354 inch (.90 mm). Approximately I-1/64 inch (25.8 mm) from end.
Step-up Jet: (303S) (Power orifice) Size, .0335 inch (.85 mm) drill. (304S)
(Power orifice) Size, .0354 inch (.90 mm) drill.
Accelerating Pump: (Both models) Type, delayed action.
(Both models) Stroke, summer setting (inner hole, short stroke), intermediate
setting (center hole), winter setting (outer hole, long stroke).
(Both models) Pump intake passage, size I/8 incn (3.18 mm). (Both models)
Pump needle seat size, No. 52 (1.61 mm) drill. (3035) Pump (discharge) jet
size, .0315 inch (.80 mm) drill. (304S) Pump (discharge) jet size, .0354
inch (.90 mm) drill.
Choke: (Both Models) Manual—Butterfly type, with pressure re-lief poppet
valve.
MOTOR TUNE-UP—MACK, JR.—Be Accurate! Always Use Feeler Gauges!
Caution: Change worn or leaky flange gaskets. Tighten manifold bolts and
test compression before adjusting carburetor.
Use Timing Light and Time by Flywheel Mark
Use Gauge Idle Adjustment
Set Flush With Screw Setting
to 1/64" Below 1/4 to 3/4
Edge of Casting Turn Open
tr=y
Breaker Points to Open 10° Before U. D. C.
CARBURETER
If carbureter loads up after considerable service, check float level. Wear
on lip of float lever will raise float level. Before adjusting float, be
sure float lever pin plug is firmly seated. Reset float level by bending
lip of float lever away from needle to raise level, or bend lip toward needle
to lower float level. Bend vertical lip of float only.
If motor stalls while idling, set throttle lever adjusting screw so motor
runs approximately 300 rpm. Then set idle adjustment screw as instructed
above. If these adjustments do not correct the trouble, remove idle orifice
tube and plug xsselmhly and clean with compressed air.
If acceleration is not satisfactory remove pump jet and clean with compressed
air. Also examine pump link setting. Pump link has three holes giving short,
medium and long strokes. If link is set to short stroke, reset to give longer
stroke to pump piston. In hot weather, high altitudes, or when high test
gasoline is used, connect pump link in inner hole, giving shorter stroke.
VANE CLt5 NC=
Set Valves
Models 2M-11M:
Intake .007"
Exhaust .010"-.012"
All other models:
Intake .007"
Exhaust .008"
ADJUSTMENTS
Maximum economy is secured only when breaker points, spark plugs, valves
and motor timing are set to manufacturer's specifications. After motor is
properly tuned, the following should also he done to insure satisfactory
performance and economy:
(a) Float level must he set as above.
(h) Ball check in step-up valve assembly must seat and move freely. When
re-assembling, valve housing must be screwed in tight against seat.
Step-up push rod must move freely in upper and lower guides.
(d) Step-up piston in upper casting should not bind and must he free of dirt.
(e) Main metering screw can be replaced with leaner than standard metering
screws, for altitude or high test fuel.
Caution: Do not attempt to gauge metering jets with drills These jets have
been flow-tested, to insure correct fuel flow.
Spark Plug Gap .025"
Set
Breaker Points
.020"
Copyright 1936
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