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While tuning and/or diagnosing any glitches in your powerband
that may arise, the following will provide you with a basic understanding
of which circuits are affected during a particular throttle position.
In most cases, adjusting the number of E-Series Discs will improve and/or
fix most circuit problems. In case additional adjustments are required,
this section will help you locate and solve the problem. It is important
to remember that adjusting carburetion should only be attempted on an
engine in good running condition that is thoroughly up to operating temperature.
Do not make adjustments all at once, and most importantly, do not make
adjustments until first trying the exhaust system with the existing jetting.
Then trying to adjust by varying the number of E-Series Discs. Find the
affected throttle position (circuit) and work on that area only. Doing
one step at a time will ensure optimum performance and smooth efficient
operation.
- Pilot Jet - Up to 1/4 throttle - Controls fuel flow
through the idle circuit - Mixture is adjusted by means of the mixture
screw -There are two types of mixture screws - Fuel Screws and Air Screws
] - A Fuel Screw, as the name implies, meters fuel - A Fuel Screw will
be located on the engine side of the throttle valve, usually on the bottom
of the carb - Turn in to lean mixture - Turn out to richen mixture - The
engine should have a smooth, steady idle with the screw between 1/4 and
3-1/2 turns out from fully bottomed (gently!) - If the engine requires
more than three turns out, the pilot jet may be too lean - If it requires
less than 1/4 turn, it may be too rich. - An Air Screw meters air - An
Air Screw will be located on the air cleaner side of the throttle valve,
usually on the right or left side - Turn in to richen mixture - Turn out
to lean mixture - The engine should have a smooth, steady idle with the
screw between 1/2 and 2-1/2 turns out from fully bottomed - If the engine
requires more than 2-1/2 turns out, the pilot jet may be too rich - If
it requires less than 1/2 turn, it may be too lean. Symptoms and/or adjustment
- With the idle at approximately 1000 rpm, turn the air screw mixture
in slowly until the idle slows or becomes erratic, then begin turning
the screw back out until it again becomes erratic. Count the number of
turns in between the two positions. Set the position of the air screw
between these two conditions, which should also be your smoothest idle
. Finally, adjust the idle screw if necessary to approximately 1000 rpm
again.
- Jet Needle - from 1/8th to 3/4” throttle - Controls
fuel flow through needle jet - Raising or lowering the clip location on
the grooves, richens or leans fuel flow, as well as determines at what
throttle position the tapered part of the needle is to be raised out of
the needle jet - Raising the needle (lowering the clip) richens the mixture
- Lowering the needle (raising the clip) leans the mixture. Symptoms and/or
adjustment - Accelerate to 1/2 to 3/4 throttle, preferably in top gear.
If acceleration seems weak, soft or flat, or the engine responds slow
when the throttle is quickly opened, the mixture is too lean. Lower the
needle clip one position to raise the needle. Conversely, if acceleration
is crisp and strong, but hesitates or shutters as the throttle is closed
from 3/4 to 1/2, the mixture is too rich. Raise the clip one position
to lower the needle. The optimum setting will be crisp acceleration, with
no load up (rich) as the throttle is backed off.
- Main Jet - from 3/4 to full throttle - Controls fuel
flow as the jet needle is lifted far enough out of the needle jet. Until
this point (throttle position), the main jet has no effect on mixture
strength. A larger # jet richens the mixture - A smaller # jet leans the
mixture. Symptoms and/or adjustment - The main jet is the last jet you
deal with and by far the easiest to diagnose. You will only change the
main jet if a problem exists at 3/4 to full throttle. Any other prob-lems
in the carburetion are not affected by the main jet. The two eas-iest
ways to find the optimum setting are: 1) Set-up an area approximately
50 to 100 yards long. Have someone time you through this area. As you
pass the entry, open the throttle fully (on the main jet circuit) and
motor through this area. Essentially, the jet size that provides the least
amount of time to run through this, is the correct main jet. 2) Starting
with a fresh set of spark plugs that have been run for at least a few
miles to gain some color to them, run the engine (under a load) for approximately
a minute or so at 3/4 to full throt-tle. Then, in a safe area, pull in
the clutch, hit the kill switch, and leave the throttle in its wide open
position; all at the same time and coast to a stop. Then remove the spark
plug and inspect its condi-tion/ color. The porcelain should be a reasonably
clean color/condi-tion, light gray to brown is good. If the plug is black
and/or sooty, the main jet is too rich (big) and you should go down in
size to a smaller number. If the plug is white, with no apparent change
in color from brand new, and the engine appears to be running hotter than
normal, it may be lean (small). You need to go up in size to a larger
number.
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