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78/83/90 Series Horizontal Shaft Engines
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VI. Check valve timing/actuation.
VII. Check exhaust.
3. Starts, runs poorly
3a. Starts, then dies
I. Run the engine with a spark tester in-line between the spark plug wire and the spark plug or use an
oscilloscope and see if the spark goes away at the same time the engine dies.
II. Check choke operation.
a. Black smoke?
b. Wet plug?
III. Prime test immediately after engine dies. If it restarts; this may indicate a problem with fuel flow to
the carburetor. Check the gas cap, fuel line, fuel filter, and the float in the carburetor.
3b. Runs with low power output.
I. Look for unusual exhaust color (smoke).
II. Unusually hot muffler (may glow red).
a. Retarded ignition
b. Exhaust valve opening early (lash too tight)
III. Mechanical bind
a. A slightly bent crankshaft. In some cases the drag may increase and decrease as the crankshaft
rotates. This produces a pulsing feeling that is different than a jerk back.
b. Parasitic external load. A bind in the equipment the engine is powering.
c. Internal drag from a scored piston or similar damage.
IV. Low governor setting or stuck governor.
a. Check RPMs using a tachometer.
b. RPMs should not droop under moderate to heavy loads.
V. Low compression
a. Check valve lash
b. Check compression
c. Check leak down to identify the source of the compression loss.
VI. Flow blockage
a. Exhaust blockage, usually accompanied by an unusual exhaust sound.
• Just as a throttle on the carburetor controls the engine RPMs by limiting the amount of air an
engine can breathe in, an exhaust blockage will limit engine performance by constricting the
other end of the system.
• The muffler itself my be blocked.
• The exhaust valve may not be opening fully, possibly because of extremely loose valve lash
settings.
• The exhaust valve seat may have come loose in the cylinder head. This may cause a loss of
compression, a flow blockage or it may randomly alternate between the two.
NOTE: The cause of an exhaust valve coming loose is usually over heating.
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