Specifications

11
IDLE MIXTURE
SCREW
IDLE SPEED ADJUSTMENT
SCREW
4
Carburetor Pre-Set and Adjustment
Both the Walbro and the Tillotson carburetors used
on TC engines have non-adjustable main mixture
jets. Only the idle mixture is adjustable by turning
the idle mixture screw. Use the following procedure
to pre-set the idle mixture screw. Turn the idle
mixture screw (clockwise) finger tight to the closed
position, then turn the screw counterclockwise to
obtain the proper preset (diag. 4).
Walbro Model WTA, WT 1 - 1-1/8 turns
Tillotson Model HU 1-1/4 - 1-3/8 turns
Final Idle Mixture Adjustment
Start the engine and allow it to reach normal operating
temperature (after 3-5 minutes). As the speed control
is set at the idle position, turn the idle mixture screw
slowly clockwise until the engine R.P.M. just starts
to decrease. Stop and note this screw position. Turn
the idle mixture screw slowly counterclockwise, the
engine will increase in R.P.M. Continue to slowly
turn the screw until the engine R.P.M. starts to
decrease. Note this position and turn the mixture
screw back clockwise halfway between the two
engine R.P.M. drop off positions. The idle mixture
adjustment is complete.
Some carburetors came equipped with a main
mixture adjusting screw. To adjust the main mixture,
follow the steps for idle adjustment.
Walbro (WTA, WT) and Tillotson (HU) Diaphragm Adjustment
4a
Emissionized Tillotson
Similar in design and operation, the Tillotson
emission carburetor uses a fixed main jet with an
adjustable idle. The idle circuit has a limiter cap to
prevent over richening. The cap is locked onto the
adjustment screw in a rich position, allowing only a
leaner adjustment. The main is fixed on these, which
means that the main mixture limiter is non-functional
on Tecumseh built engines (diag. 4a).
In compliance with E.P.A. and C.A.R.B. regulations
the following procedure must be followed.
NOTE: These caps can be removed for servicing
of the carburetor. Follow these steps.
1. Turn the caps clockwise until they hit the stops.
2. Remove the caps with a pointed instrument
such as an awl.
3. Then turn the screws in until softly seated, note
the number of turns. The screws must be reinstalled
to this same static setting. Replacement of the
caps is required to maintain E.P.A. and C.A.R.B.
emission compliance.
FIXED MAIN
(MIXTURE SCREW
NOT FUNCTIONAL ON
MOST TECUMSEH
BUILT ENGINES)
IDLE SPEED
ADJUSTMENT
SCREW
IDLE MIXTURE
LIMIT SCREW