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CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
to run lean and eventually overheat and seize.
If the metering lever height is high, the engine
would run rich.
Inspect the throttle shaft for wear. Rotate the
shaft and move the shaft from side to side. If
the shaft end close to the throttle lever is worn,
there will be lateral (side-to-side) movement. See
Figure 9-41. The throttle shaft is typically worn
from abrasives. The abrasives will also wear out
the moving components inside the engine.
If the carburetor has an accelerator pump,
remove the throttle shaft. Inspect the O-ring on
the accelerator pump piston to determine if it is
flattened, dried out, or broken. Air passing the
O-ring can enter the metering chamber, causing
the engine to run lean.
Record the number of turns on the idle speed
screw, H mixture screw, L mixture screw, or
any adjustment screw on the carburetor. On the
idle speed screw, count the number of turns out
until the tip of the screw just disengages from
the throttle lever. On the mixture screws, accu-
rately count the number of turns required to
gently seat the needle. Write down the exact
number of turns, including any partial turn.
Impulse Circuit Evaluation
Verify that the impulse circuit is working cor-
rectly. Remove the spark plug to make it easier
to turn the engine over. Apply several drops of
oil at the opening of the impulse circuit. Pull the
starter rope. The oil should spit from the impulse
circuit. Another way to test the impulse circuit
is to attach a pressure tester to the impulse cir-
cuit opening, Figure 9-42. Pull the rope. The
pressure gauge needle should rise and fall sev-
eral times, indicating the pressure and vacuum
pulses coming from the crankcase.
Perform a crankcase pressure-vacuum test
before removing the muffler. This test is per-
formed with the carburetor removed. Use a rub-
ber plate covered by a metal plate to seal off the
intake port. Make sure the impulse circuit is
sealed. Loosen but do not remove the muffler.
Z gauge
Metering
lever
Goodheart-Willcox Publisher
Figure 9-40. Measure the metering lever height
to determine how the engine has been running. A
low lever height causes the engine to run lean and
to overheat. A high lever causes the engine to run
rich.
Shaft wear
Goodheart-Willcox Publisher
Figure 9-41. A worn throttle shaft indicates abra-
sives have entered the engine.
Impulse port Pressure tester
Goodheart-Willcox Publisher
Figure 9-42. Connect a pressure gauge to the
impulse circuit output. Pull the rope. The pis-
ton movement creates vacuum and pressure
impulses in the crankcase that can be identified by
the movement of the pressure gauge needle.
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