S&S Cycle MGL Carburetor - Manual (Page 6)

Troubleshooting Tip-— Engine will not run at Steady
Speed:
1. Restriction in fuel supply system - gas tank vent
plugged, gas petcock too small (stock Harley-
Davidson
® petcock is adequate, but might require
running on “reserve” to provide adequate supply for
big inch engines), needle and seat assembly not
working properly.
2. Faulty ignition system - fouled plugs, worn points,
defective coil.
3. Incorrect intermediate and/or high speed jetting.
4. Foreign material in air bleeds, intermediate jet, main
jet, and/or passageways in carb causing flow
restriction.
5. Air cleaner is other than S&S ® , or no cleaner is used.
Some air cleaner designs are such that the carb
cannot draw air freely as it is needed.
6. Valve train defect - bad valves, sticky valves, bad
or broken springs, improper clearances for high
lift cams.
Engine will not RPM:
1. Restriction in fuel system.
2. Faulty ignition system.
3. Incorrect high speed jetting.
4. Foreign material in air and/or gas passageways in
carb causing flow restriction .
5. Air horn being used without shrouding bowl vent. (If
the bowl vent is not shrouded, the air pressure in the
bowl is different from atmospheric pressure.)
6. Incorrect air cleaner being used.
7. Too much gear. (Not enough horsepower to pull
the gearing.)
8. Valve train defect.
General Information
1. Do not chrome the carburetor as extensive damage
will result.
2. The carburetor has several drilled passages that are
permanently sealed with three drive plugs. Do not
attempt to remove these plugs.
3. Do not over-tighten the idle mixture screw,
intermediate jet, or intermediate air bleed.
4. Do not attempt to remove the small 1 ⁄
8
” diameter
tube near the choke in the carburetor body. This is
the main jet air bleed and is permanently pressed
into the body.
5. The float level is 3 ⁄
4
” when measured from the top
of the float to the scribed line on the bowl gasket
surface. This is done with needle in the closed
position.
6. When the motorcycle is not running, the fuel shutoff
valve should be turned off.
7. The throttle plate, Part No. 55, and throttle shaft,
Part No. 83, should be checked annually for signs of
wear. Replace, if necessary. If they are removed, be
sure that the throttle plate is reinstalled correctly.
See Figure A.
8. If the throttle arm, Part No. 48, is removed from the
throttle shaft, it must be reinstalled in the wide open
position with the butterfly at 90-degrees to the fully
closed position. See Figure B.
9. If an air cleaner other than ours must be used, make
sure the bowl vent hole on the inlet end of the
carburetor is not covered and sealed off.
10. Cams and exhaust systems make some engines
difficult to carburate. A combination of cam overlap
and back pressure, or lack of back pressure will cause
mixture dilution at certain engine RPMs. This
dilution will cause engine roughness or misfiring
when the engine is held in this range. To attempt to
carburate for this condition usually destroys the
carburation over the rest of the range. It is best
under these conditions to change the cam and/or the
exhaust system.
The MGL has an adjustable idle mixture screw, a fixed
intermediate jet, a fixed intermediate air bleed, and a fixed
main jet.
The idle mixture screw, Part No. 54, is located on top of
the carburetor. Turning the screw in, clockwise, will lean
the mixture. A counterclockwise motion, out, will richen
the mixture.
The intermediate jet, Part No. 153, is screwed into the
carburetor on the top of the body. It is made in sizes .028,
.029, .030, .031, .032, .033 a .035; .035 being the richest.
The intermediate air bleed, Part No. 151, works in
conjunction with the idle screw and intermediate jets
and is located adjacent to the intermediate jet. The air
bleed required on all of our test engines was .040. For
those who must have larger or smaller ones, they can be
ordered on special request.
The main jet, Part No. 72, is made in increments of .002
and range in size from .060 to .104. It can be reached by
removing the bowl assembly.
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