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There are two primary methods of adjusting the TillotsonTCT fuel system. The first method
is to adjust only the idle speed through the idle speed screw. This adjustment is done if
engine is hunting or not running steady only at idle. The second is to adjust the air-fuel
ratio through the AFR screw. This adjustment adjusts the amount of fuel being delivered
from idle through wide-open throttle and is used if the engine is hunting or running
abnormally at more than one speed or if the problem is not corrected through adjustment of
the idle screw (possible causes include operation at altitudes greater than 400m (1312 ft)
above sea level or a change in the grade of fuel used).

1. Adjusting the Idle Speed Screw.

If the engine is hunting or running improperly only at idle, you can adjust the amount of fuel
being delivered at idle by turning the idle speed screw with an allen key. Turning the screw
clockwise increases the idle speed. The gauge on the top of the TillotsonTCT shows how
many degrees the throttle shutter is open. If adjusting the idle screw does not solve your
problem, return it to the original position and adjust the AFR screw as shown in the next
step. If the engine still does not operate operate properly, first open the idle screw a few
degrees at a time, then adjust the AFR screw up to 1/2 turn.
On Tillotson vertical engines, the idle speed screw is accessible through a hole in the air
filter cover.
Adjusting your Tillotson TCT
Figure 1.1

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Summary of Contents for TILLOTSON TCT

  • Page 1 If the engine still does not operate operate properly, first open the idle screw a few degrees at a time, then adjust the AFR screw up to 1/2 turn. Figure 1.1 On Tillotson vertical engines, the idle speed screw is accessible through a hole in the air filter cover.
  • Page 2 2. Adjusting the AFR Screw. If the engine continues to hunt or run improperly at speeds other than idle, you can adjust the entire air-fuel mixture range by adjusting the AFR Screw. On models with the EPA limiter screw, the adjustment is made with a small flathead screwdriver. To adjust: A.
  • Page 3 Technical Data How the AFR Adjustment works: The B) AFR screw The AFR screw adjusts the needle C) Fuel needle D) Cam Follower height relative to the cam position. E) Cam Raising the needle richens the mixture (increases the fuel) while lowering the needle leans the mixture (reduces the fuel).

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