ENGINE IS HARD TO START
When the engine is difficult to start, but finally does get going, the most common causes are usually similar to those that cause complete starting failures, namely:
1. Partly clogged fuel line, fuel filter or fuel system vent hole (usually in
gas tank cap)
2. Low compression
3. Fouled spark plug
4. Worn breaker points
5. Loose connection at magneto or points
6. Deteriorated spark plug wire
7. Choke’s failure to close completely
8. Defective but not completely failed magneto coil
9. Water in gasoline.
Conditions 4, 5, 6, and 8 will result in a weak spark, one that will jump across the electrodes in the plug or to an electrical ground intermittently. Number 1 will show a virtually dry spark plug. Number 9 is a likely possibility if you didn’t drain the tank and refill with fresh gasoline at the start of the season. As an extreme possibility, water in the gas tank might result in what seems to be a complete starting failure if you happen to give up a bit early.
The possibility of a plugged fuel system vent deserves some explanation. When the fuel pump or engine vacuum draws gasoline out of the tank, air must fill the gap. If the vent (usually a pinhole in the gas tank cap) is plugged, then air cannot enter. Result: There is a vacuum in the gas tank that keeps the fuel there. If the vent is partly clogged, only a limited amount of air can replace the fuel, and therefore only a limited amount of fuel can be drawn into the carburetor, causing the engine to suffer from fuel starvation. To check for this possibility, try to start the engine with gas cap removed.

ENGINE STALLS
The most common cause of engine stalling is a clogged air filter (something
you can test by simply trying to start the engine with the filter off). An
other is failure to open the choke once the engine has warmed up. There
are other causes, however, such as the plugged fuel system vent and water
in the gasoline, and many others.
ENGINE VIBRATES
Many people panic when the engine vibrates, assuming the worst. In most
lawn mower cases the problem isn’t in the engine at all; here the blade is
probably bent or damaged. Another common cause for vibration is that the
engine mounting bolts are loose. Still another is a worn flywheel retaining
key. See the appropriate troubleshooting chart at the end of this chapter
for all the common possibilities.
POOR PERFORMANCE
The chart on page 62 shows many common internal engine causes of poor
performance, but here are others you can check for:
1. Choke partly closed, or clogged air filter causing an overrich mixture
2. Poor compression
3. Inadequate lubrication
4. Dirty carburetor
5. Retarded ignition timing: The combustion pattern is altered, reducing the completeness of the combqtion process.
6. Improperly adjusted carburetor: This is most likely to occur after the carburetor has been rebuilt.
7. Engine running too hot: The most common reason for a small-gas-en
gine’s overheating is that the air flow is restricted by leaves, wood
shavings or ice; the c&se and equipment relationship is obvious.
8. Mower or blower air vane governor packed with leaves, preventing
the throttle from opening