Cranking slower and slower has nothing to do with the fuel pump or spark plugs. It has to do with the battery and the generator recharging it. Start by measuring the battery voltage with an inexpensive digital voltmeter. A fully charged battery will read 12.6 volts. A good but discharged battery will be near 12.0 volts. If you find less than 11.0 volts, a cell is shorted and the battery must be replaced.
Measure the voltage again with the engine running. It must be between 13.75 and 14.75 volts. If it is low, suspect the voltage regulator which lives inside the generator.
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Tuesday, August 9th, 2011 AT 7:18 PM