187
CHAPTER 9 Two-Stroke Engine Troubleshooting and Evaluation
Copyright Goodheart-Willcox Co., Inc.
These tests help identify if the fuel delivery sys-
tem has a fault or the engine has a fault.
The lower-end pop-off test allows fuel to
bypass the fuel delivery process of the carbu-
retor. To perform this test, remove the air filter
cover and element. Make sure the throttle is held
wide open and the choke remains open. Also,
make sure the stop switch is in the RUN position.
Apply about a dozen drops of fuel down through
the carburetor throat and pull the rope several
times. The liquid fuel is immediately pulled into
the crankcase and should provide enough fuel
vapors for the engine to run. Pull the rope until
the engine “burps,” indicating it wants to start.
Pull just one more time to see if the engine will
start and continue to run on the rest of the fuel
in the carburetor throat. If the engine begins to
run, you know that the integrity of the crank-
case is good, as the air-fuel charge in the crank-
case experienced primary compression and was
transferred to the combustion chamber. You
also know that secondary compression, spark,
and spark timing were good enough to cause the
engine to run. In such a case, an upper-end pop-
off test is unnecessary.
If the engine fails to pop-off, the problem
could be lower-end problems, such as bad crank-
shaft seals or leaks in the crankcase. However,
the engine could also fail the lower-end pop-off
test if there are problems with the upper end
of the engine. The next step is to perform the
upper-end pop-off test.
The first step of the upper-end pop-off test r r
begins immediately after a failure in the lower-end
pop-off test. Remove the spark plug to see if it is
wet. If the plug is wet, fuel was transferred from
the crankcase and into the combustion chamber.
Dry off the end of the spark plug. Place sev-
eral drops of fuel into the combustion cham-
ber and reinstall the spark plug. Reattach the
high-tension lead. Make sure the switch is in the
RUN position, the choke is off, and the throttle
is held wide open. Pull the rope several times
to see if the engine will pop-off. If the engine
fails the upper-end pop-off test, you will need to
look for problems with secondary compression
or the exhaust system.
The pop-off tests identify very quickly if there
are problems with the engine itself. Failure of
either test indicates that additional time will be
needed to perform a thorough evaluation of the
engine.
Two-Stroke Engine Evaluation
When the troubleshooting process does not lead
to the immediate repair of the two-stroke equip-
ment, the engine must be thoroughly evaluated.
The faults found during the evaluation process
will be added to the faults discovered during
the troubleshooting process. The engine evalua-
tion process should reveal the fault or faults that
caused the engine to run poorly or not run at all.
When evaluating the engine, start on the out-
side of the engine and work your way to the inside.
During this process, certain tests must be per-
formed before parts of the engine are disassem-
bled. For example, the crankcase pressure-vacuum
tests should be performed before the cylinder is
removed. If the cylinder-to-crankcase gasket is
leaking, removing the cylinder could destroy this
evidence. After the crankcase pressure-vacuum
test, the cylinder can be removed to inspect the
cylinder bore, the piston, and the connecting rod.
Likewise, some tests on the carburetor must be
performed before the carburetor is removed from
the engine for internal evaluation.
Troubleshooting questions are questions
the technician should ask when evaluating an
engine. These questions have been strategically
placed in the following sections to help explain
why certain tests are performed in a certain
way and at a particular time. These questions
are printed inside quotes, bold, and italicized.
The following list divides the two-stroke engine
evaluation into logical areas for inspection:
• Engine exterior.
• Air intake system.
• Ignition system.
• Fuel delivery system.
• Exhaust system.
• Internal engine components
The evaluation process starts on the outside of
the engine, with no components removed. The air
intake system is next because it can be evaluated
by itself by removing the components and evalu-
ating them. The fuel delivery, ignition, exhaust,
and engine systems each have tests that need to
be performed before components are removed.
When these areas are inspected, perform the
appropriate tests first before removing or disas-
sembling components. For example, the engine
crankcase pressure-vacuum test needs to be per-
formed before the muffler is completely removed.