a. Incorrect ignition timing due to a malfunc-
tioning ignition component.
b. Improperly adjusted valves or worn valve
seats.
c. Low engine compression.
d. Clogged carburetor jet(s).
e. Fouled spark plugs.
f. Incorrect spark plug heat range.
g. Oil level too low or too high.
h. Contaminated oil.
i. Worn or damaged valve train assembly.
j. Engine overheating. See
Engine Overheating
in this section.
6. If the engine knocks when accelerating or when
running at high speed, check for the following:
a. Incorrect type of fuel.
b. Lean carburetor jetting.
c. Advanced ignition timing caused by malfunc-
tioning ignition component.
d. Excessive carbon buildup in the combustion
chamber.
e. Worn pistons and/or cylinder bores.
Poor Idle Speed or
Low Speed Performance
1. Check the valve clearance. Adjust the valves as
necessary.
2. Check for damaged intake manifolds, or loose
carburetor, air filter housing, or surge tank clamps.
3. Perform the spark test described in this chapter.
Note the following:
a. If the spark is good, perform to Step 4.
b. If the spark is weak, test the ignition system as
described in this chapter.
4. Check the ignition timing as described in Chap-
ter Three. If the ignition timing is incorrect, trouble-
shoot the ignition system as described in this
chapter. If the ignition timing is correct, check the
carburetor and fuel system.
Poor High Speed Performance
1. Check the ignition timing as described in Chap-
ter Three. If the ignition timing is correct, perform
Step 2. If the timing is incorrect, test the following
ignition system components as described in Chapter
Nine:
a. Pickup coil.
b. Ignition coils.
c. Ignitor unit.
3. Perform the fuel pump operational test described
in Chapter Eight. If the fuel flow is acceptable,
check for one or more of the following:
a. Clogged fuel line.
b. Stuck or clogged carburetor float valve.
4. Remove the carburetor assembly as described in
Chapter Eight. Then remove the float bowl(s) and
check for contamination and plugged jets. If there is
any contamination, disassemble and clean the car-
buretor(s). Also pour out and discard the remaining
fuel in the fuel tank and flush the tank thoroughly. If
there was no contamination and the jets were not
plugged, perform Step 4.
5. Incorrect valve timing and worn or damaged
valve springs can cause poor high-speed perfor-
mance. If the valve timing was set just before the
onset of this type of problem, the valve timing may
be incorrect. If the valve timing was not recently set
or changed, remove the cylinder head and inspect
the valve train assembly.
6. Check the carburetor and fuel system. Pay par-
ticular attention to the following:
a. A faulty diaphragm in the carburetors.
b. Improperly set fuel level.
c. Clogged or loose main jet.
d. Faulty fuel pump.
e. Clogged air filter.
Engine Overheating
1. Incorrect spark plug gap.
2. Improper spark plug heat range.
3. Faulty ignitor unit.
4. Incorrect carburetor adjustment or jet selection.
5. Incorrect fuel level.
6. Clogged air filter.
7. Heavy engine carbon deposits in combustion
chamber.
8. Low oil level.
9. Incorrect oil viscosity.
10. Oil not circulating properly.
11. Valves leaking.
12. Dragging brake(s).
13. Clutch slipping.
Engine Runs Roughly
1. Clogged air filter element.
40
CHAPTER TWO
