I have a Craftsman, Model # 917270821; 19.5 HP B&S; turbo Colled; Auto-42' cut. The mower was running and mowing great. I turned it off to add gas and now it will not start. The engine will turn over, but there is no "fire" from the plugs. Need help getting it started.

Thank you
Paula

I don't have all the numbers here for your engine.
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video_player
THe ingintion coil may have gone bad os just a wire problem.

Lets check some things out: Possibilities here
Loose ar bad plug wire
Ignition coil wire is bad and grounding
Ignition switch went bad
Manegto
Coil
Diododes

YouTube - Lawn Mower Repair Ignition Coil Test

Check for a flywheel that is loose and not seating properly on the taper. This could result in no spark if the air gap between the flywheel magnet and magneto core is then incorrect. However, due to the close spacing, you would probably feel and hear serious scraping in this case.

Check for bad connections or defective wiring including faulty or water logged insulation

Electronic ignition - There is likely a single potted module which includes the circuitry and ignition coil. If anything goes wrong with this module, replacement is the only option. Once the wiring and resistance of the secondary has been checked, there are really no addition tests that can be performed on an electronic ignition module without special equipment. A defective ignition module will have to be replaced.

Breaker point ignition - Possibilities are bad, dirty, corroded, or loose points or points that are grossly out of adjustment, a bad condenser, or a bad magneto coil.

The magneto, like the ignition coil on an automobile, contains two windings:

A primary with a few turns of heavy wire.

A high voltage secondary with thousands of turns of super fine wire.
In an automobile, the battery supplies the primary current; in a magneto, the magnet on the flywheel moving past the core at high speed acts as a generator and induces current in the primary.

As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center (TDC), the current (and flux) should be maximum and at this instant the points open. The flux then collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at the spark plug.

(For EE types, this is somewhat similar in basic operation to the flyback converter in a switchmode power supply except that the moving magnet supplies the input power instead of the rectified AC line and the points act as the switch instead of a power transistor.)

The secondary will always be accessible for testing but the primary of an electronic ignition may be not be due to the electronic components:


Secondary: 3 K ohms (maybe a little higher but not open). Much lower would indicate a shorted winding.

Primary (if non-electronic and accessible): very low - guessing less than an ohm.
Wires can break due to corrosion or vibration. This would result in an open winding - infinite resistance. Shorts can develop between adjacent windings or to the core. This may be detectable as reduced resistance but without knowing exactly what it should be, there is no way of knowing if a slight discrepancy represents a problem or just slight variations in design or manufacturing.

A more complete test would involve checking the 'Q' or doing what is called a 'ring' test and even more for an electronic ignition. This requires special equipment. Therefore, it is best to swap in a known good unit. They are not that expensive.