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Engine cranks but will not start. Good spark to spark plug. Eventually the battery wore down from cranking so I jumped it from auto. Still no start. Then I started auto and lawn tractor fired right up. Then I shut the tractor and the auto off and tried restarting. No start. Put jumper cables back on, still no start. Then I restarted auto and tractor again fired right up. Seems it won't start with 12 volts but will with 14 volts.

The amount of voltage shouldn't make any different seeing the mower engine runs off a mag and not voltage to the engine. The only thing I can think of is that with the auto running it is turning the mower engine over faster. You should check for bad battery cable connections or bad battery cables.

The battery cables and connections look good. No corrosion at either end. Is there any way to test them?

DanHogCVO said:

Engine cranks but will not start. Good spark to spark plug. Eventually the battery wore down from cranking so I jumped it from auto. Still no start. Then I started auto and lawn tractor fired right up. Then I shut the tractor and the auto off and tried restarting. No start. Put jumper cables back on, still no start. Then I restarted auto and tractor again fired right up. Seems it won't start with 12 volts but will with 14 volts.

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You probably already know this. You need compression, spark and fuel.
I would put a small amount of fuel in the intake, (under the air cleaner). This will give it some gas, even if the carb or fuel system
is not operating. If it fires up for a few seconds, you have isolated it to a fuel or carb related problem. This includes bad gas, fuel pump, hose problems or a dirty clogged carb (or one that needs other type of service).

I have been told 50 times that a motor was "blown". I pull it through, feel some compression, check spark.
Put that gas in the intake. It fires up. So common it is unbelievable. hahahahah (I have brought a lot of blown motors back with a carb rebuild and fresh gas, hahahah)

reynoldston said:

The amount of voltage shouldn't make any different seeing the mower engine runs off a mag and not voltage to the engine. The only thing I can think of is that with the auto running it is turning the mower engine over faster. You should check for bad battery cable connections or bad battery cables.

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X2. Disconnect cables and clean contacts. Retry.