S&S Cycle MGL Carburetor - Manual (Page 5)

(The size is stamped on the end of the tube.) This
spitting is a lean condition and must be corrected. The
smallest idle tube that eliminates this condition should
provide the best gas mileage. For better throttling
characteristics and performance. a richer idle tube, one
or two sizes larger, may be necessary. This is especially
true if the bike is to be used exclusively on a drag strip.
Larger jets or richer mixtures will enable one to run a
colder engine which is sometimes desirable. This is best
determined by experimentation with the particular
engine to be used. Gas mileage is oftentimes determined
by an individual’s riding habits. Snapping the throttle
open in the lower RPM ranges is an inefficient way of
operating the bike. S&S
® carbs are not equipped with
accelerator pumps. Consequently, poor throttling
characteristics may occur since the intermediate system is
effectively bypassed. The engine will respond better if
the throttle is rolled on.
Intermediate Air Bleed
The intermediate air bleed, Part No. 151, is located
directly adjacent to the intermediate jet. All carburetors
have an intermediate air bleed because it is necessary to
correct the air/fuel ration over the whole intermediate
range. For most practical purposes, a .040 intermediate
air bleed is used to do this. Larger (more air or leaner
fuel mixture) or smaller (less air or richer fuel mixture)
air bleeds are available, but caution must be exercised
when changing these. Example: a larger air bleed could
cause a leaner fuel mixture in a riding range that over a
period may destroy the engine due to detonation or
seizing. The best advice is to leave the air bleed alone.
Main Jet - from 3500 RPM or 55-60 MPH on up:
Main jetting is best determined by testing at the drag
strip. Jetting should always be done at operating
temperature. After a run is made, the engine RPM and
final speed should be noted. A second run should be
made with a .004 larger (richer) jet installed. Again, RPM
and final speed is noted. This procedure should be
continued until the miles per hour falls off. The main jet
should then be made smaller (leaner) to gain the best
maximum RPM and MPH. When making these runs, do
not strive for ETs, but for consistent miles per hour. We
have found that maximum miles per hour and RPM have
been the best indicators of proper jetting.
If the engine is buttoned off after each run, the
sparkplugs may be inspected for richness or leanness. Plug
color is not always a good indication of carb jetting on
quarter mile tests. We have found that different brands of
gasoline, gasoline additives, engine heat (due to ignition
timing), and type of plugs and heat range used alter
readings drastically. New plugs usually require the time
involved in a road test to properly develop the plug color.
It is best to use a proven combination and to consult the
sparkplug manufacturer,for more specific information if
one desires to become proficient at plug reading.
Champion Racing Division has a very informative
booklet. For details write:
Champion Spark Plug Co.
PO. Box 910
Toledo, Ohio 43601
If main jetting tests cannot be conducted at the drag
strip, the “RPMing” method can be used. This procedure
would be to run the bike through the gears noting how
quickly and smoothly the engine reaches the RPM level
where gears are shifted. Under racing conditions this
level is where horsepower peaks and begins to taper off.
The main jet that makes the engine pull the strongest or
RPM through the gears best is the correct one. Our
experience is that a jet about .006 smaller (leaner) in size
will make the engine pop and sputter or “break up” in
the carburetor and die. A jet about .006 larger (richer)
will make the engine run flat and sluggish or “blubber”.
5