CARBURETION
3.28
Adjustments - Jet Needle and Float Level
Fine Tuning the Mikuni Carburetor
A carburetor with a piston type throttle valve is also called
a variable venturi type carburetor. In this type of carburetor,
the needle jet and jet needle serve to control a proper air/
fuel mixture ratio at the medium throttle valve opening (be-
tween 1/4 and 3/4 opening). Having the proper needle jet
and jet needle has a major impact on engine performance
at partial load.
The jet needle tapers off at one end and the clearance be-
tween the jet needle and the needle jet increases as the
throttle valve opening gets wider. The air/fuel mixture ratio
is controlled by the height of the “E” ring inserted into one
of the five slots provided in the head of the jet needle. The
chart at right shows the variation of fuel flow based on the
height of the “E” ring.
Float Level Adjustment
1. Remove float bowl.
2. With carburetor in an inverted position, float arm (A)
should be parallel with body (B). See illustration at
right.
3. To adjust float arm , bend tang contacting inlet needle.
CAUTION:
Never bend the float arm itself.
NOTE: Mikuni TM-38 carburetor float levels are not adjust-
able.
TMX 34 Venting configuration
2
3
4
4
3
2
15
50 75 100%
Throttle Valve Opening
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