Chapter 2: Basic Troubleshooting
13
IV. Bent crankshaft (unrepairable)
V. Internal binding, crankshaft, connect-
ing rod or piston (unrepairable)
2. Crankshaft turns, no start.
2a. Most gasoline engine diagnosis involves iso-
lating problems in the four critical factors an
engine needs to run properly:
I. Ignition- sufficient spark to start com-
bustion in the cylinder, occurring at the
proper time.
II. Compression- enough pressure in the
cylinder to convert combustion into
kinetic motion. It also needs sufficient
sealing to generate the vacuum
needed to draw in and atomize the
next intake charge.
III. Fuel- correct type and grade of fresh
gasoline; in sufficient quantity, atom
-
ized (tiny droplets) and in correct fuel/
air proportions.
IV. Flow- if all of the above conditions are
met, but the flow of air is constricted
on the inlet or exhaust side it will
cause the engine to run poorly or not
at all. This also includes ensuring the
valves are timed to open at the proper
time.
2b. Isolate the ignition system and compression
from the fuel system by performing a prime
test.
I. Burns prime and dies. This would indi-
cate a fuel system issue.
II. Does not burn prime. Not a fuel sys-
tem issue. Check for an ignition, com-
pression or flow problem.
2c. Compression or ignition problem
I. Check the engine stop and/or ignition
switch.
II. Test the ignition system using a proper
tester.
III. Replace the spark plug with a new one
or a known good one.
IV. Check compression or leak down.
V. Check valve lash.
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. Test for invisible damage to the air fil-
ter by starting the engine with the air
filter removed.
IV. Prime test immediately after engine
dies. If it restarts; this may indicate a
problem with fuel flow to the carbure
-
tor. 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 pow
-
ering.
c. Internal drag from a scored piston
or similar damage.
IV. Low governor setting or stuck gover-
nor.
a. Check RPMs using a tachometer.
b. RPMs should not droop under
moderate to heavy loads.
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