The Pilot Circuit - Ducati 900SS Workshop Manual

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CARBURETOR
The pilot circuit.
The pilot circuit meters the air-fuel m•xture at idle and at small throttle openings. The pilot
circuit consists
of
a senes
of
passages in the carburetor body, o pilot jet to meter fuel.
and o pilot screw .
In some models, the pdo t screw meters the air flow through the pilot circuit. This system
is used o n the sl•de-typc corbu•eto•s
In othe1 models the p•lot setew controls the amount of awfuel mixture flowing through
the pilot circuit. lhis version is no•mally found on CV carburetors.
. . . NOTE: On some carburetors the air jets are located in the top part of the
. . . carburetor body, underneath the diaphragm. When cleaning or replacing
them, care should be take n that they are not interchanged. Poor pe rformance will
result.
TIPS:
l)A p 1 lo1 circu11thot 1 S 1 00 leon wd1 cause lower than normal CO readings and low-lo-
no rmol HC. If the leon condihon 1 s severe enough to cause leon misfire, the HC
readings will be very high. A machine w ith this condition will usua lly experience o
hesllotion just off
or
idle.
2)A pdol circuit that
IS
rich will cause higher than normal CO readings and normal-to-
high HC, depend1ng on how severe the condition is. A machine with this condition
w1ll g•ve poor fuel economy and . usually. poor performance os well.
Fuel fa• the pilot cucu1 t 1 s drown thro ugh the pilot jet. which protrudes into the float bowl.
Air for the pilot circuit enters through o small passage in the inlet side of the venturi and,
in some models. is metered by the p ilot screw . This screw is tapered to provide a g1adual
tncreose in a ir flow os the screw is backed out.
MIKUNI uses two different types
of
pdot jets in its carburetors. One is the primary type.
wh1ch has only the metering hole drilled through its entire length.
The other is lhe air-bleed type. which has o series of holes drilled through its sides These
air bleed holes correspond wirh the pdot circuit air passage. As air flows through these
holes. the fuel bubbles. ond becomes part ially atomized before entering the venturi air
stream
c
TIPS:
NOTE: Although these jets are the same physical size, they should not
be
interchanged.
lnterchang1 ng the pdot 1 e1 s •esulis
•n
p001 pcdo1 rnonce .
If
the primary type is 1 eplaced
with the air-bleed type a very rich cond ition w ill exist at idle.
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Spurgo ouo
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Pnmo11n
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spurgo
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Prrmory (non-oir bleed}
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