Generac GT-990 — Check and Adjust Ignition Magnetos
Adjustments
For the Generac GT-990, from the Generac service manual.
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Before you start
- Magneto air gap on single cylinder engines is non-adjustable.
Procedure
- Disconnect Wire 18 from terminal by oil cooler.
- Disconnect spark plug wires from spark plugs.
- Measure primary winding resistance: approximately 300K ±10K ohms.
- Measure secondary winding resistance: approximately 14K ±3K ohms.
- Check diode isolation between magnetos; INFINITY should be measured.
- For V-twin, rotate flywheel until magnet is under laminations.
- Place 0.008-0.012 inch thickness gauge between magnet and laminations.
- Loosen mounting screws and let magnet pull magneto down against gauge.
- Tighten both mounting screws.
- Remove gauge by rotating flywheel.
- Repeat for second magneto.
- Retest for spark.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Primary winding 300K ±10K ohms
- Secondary winding 14K ±3K ohms
- Air gap 0.008-0.012 inch (0.20-0.30mm)
Source: Generac service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.