Specifications

17
5
FIXED MAIN
(MIXTURE SCREW
NOT FUNCTIONAL ON
MOST TECUMSEH BUILT
ENGINES)
IDLE SPEED
ADJUSTMENT
SCREW
IDLE MIXTURE LIMIT
SCREW
Walbro (WTA, WT) and Tillotson (HU) Diaphragm Adjustment
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 come equipped with a main mixture
adjusting screw. To adjust the main mixture, follow the
steps for idle adjustment.
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. 5).
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 contact 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.
IDLE MIXTURE SCREW
IDLE SPEED ADJUSTMENT
SCREW
4
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