c. 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".
d. On certain engines, particularly Sportster
®
models, an indication of a rich main jet
condition will sometimes appear as a slight
miss at transition from the intermediate system
to the main jets.
e. If this occurs while running hard through the
gears at 3000-4000 RPM, try smaller main jets.
f. We have run:
1. 900cc Sportster
®
models with .050 main jets.
2. 1000cc Sportster
®
models with .052's.
3. Stock 74" panhead with .050's.
4. 80" shovelhead with .052's.
5. 98" shovelhead with .056's.
g. The above was warm weather jetting at 600 to
1000 feet above sea level elevation.
h. Obviously, like the intermediate jetting, the
main jet requirements will vary somewhat
with conditions.
Trouble Shooting Tips
Engine will not run at steady speed:
● 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.
● Faulty ignition system - fouled plugs, worn points,
defective coil.
● Incorrect intermediate and/or high speed jetting.
● Foreign material in air and/or gas passageway in
carb causing flow restriction. Remove and clean
intermediate jets and bleeds.
● Valve train defect - bad valves, sticky valves,
bad or broken springs, improper clearances for
high lift cams.
● Bowl vent obstructed.
Engine will not RPM:
● Restriction in fuel system.
● Faulty ignition system.
● Incorrect high speed jetting.
● Foreign material in air and/or gas passageways in
carb causing flow restriction.
● Too much gear. (Not enough horsepower to pull
the gearing.)
● Valve train defect.
● Bowl vent obstructed.
7
(12 pages)
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