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Four Most Likely Causes of a Weak Gas Engine

The causes for lackluster performance between a gasoline engine and a diesel engine are as different as the fuels they use.

A gas engine traditionally becomes lazy due to an ignition advance issue, worn timing chain, incorrect air-to-fuel ratio, or restricted exhaust.

ignition advance

Ignition advance is the number of crankshaft rotational degrees in relation to the piston being at top dead center (TDC) when the spark plug is arced.

For example, a 10° BTDC (before TDC) means the plug is arced when the piston is 10 crankshaft rotational degrees before TDC. ATDC (after TDC) identifies a firing when the piston is heading downward.

The reason for advancing the engine is to allow the flame to keep up with the piston. The piston travels faster than the flame expands across the bore. It is ideal to time spark plug arcing so as to allow fuel to be completely burned and all of the energy to work against the piston. As load and engine speed vary, the ideal amount of advance changes, too. This is due to different burn characteristic in the bore.

worn timing chain

A common mistake is to set the initial timing but not to confirm the mechanical advance by raising the rpm of the engine. 

The rate of advance (along with the total advance curve) is responsible for the engine’s power.

It’s not unusual to find an older engine with rusted closed centrifugal weights. In this case, there is little to no advance. Just as common are weak or broken advance springs. This has the mechanic unknowingly setting the base timing to negate any advance since it is being evoked at idle. Modern fuel requires a different advance curve, so play with the setting for the best performance.

incorrect air-to-fuel ratio

A low float level, air leaks around the base of the carburetor and intake manifold, or stuck metering rods or power valve will result in a lazy engine due to a lean air-to-fuel ratio. A weak accelerator pump will cause sag and/or hesitation as the throttle is opened.

SF Engine Man: Carburetors

As the timing chain wears, the camshaft timing retards and influences the valve opening and closing events along with the ignition timing (the distributor is run from the cam).

Retarded camshaft timing will kill low rpm performance while not affecting or actually improving high rpm use.

restricted exhaust

When exhaust is restricted, the engine can’t breathe, and it may not run. Connect a vacuum gauge to the intake manifold and register the reading with the engine at idle. Raise the engine speed to 2,500 rpm and keep it there. The vacuum signal should become stronger. If it stays the same or drops, the exhaust is plugged.

(Note: A plugged exhaust is common with a light-duty engine that’s factory-equipped with a catalytic converter.) 

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