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How to do a Plug Chop on a Motorcycle


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If you plan on changing things on your engine – like the exhaust or air filter, you’ll likely need to test out new jet combinations to get your carbs running correctly.

The best way to see if your air/fuel mixture is correct at certain throttle positions is to do a plug chop.

What is a Plug Chop?

A plug chop is a method of testing your motorcycle’s air/fuel mixture by reading the spark plug color after cutting the engine at various throttle positions.

This allows you to identify which jets you’ll need to change or adjust.

You’ll essentially ride the bike at the throttle position you want to test, stop the bike, and look at the spark plugs to decide whether it’s too rich or lean. Take a look at the charts that show which jets control which throttle positions and tune from there.

You’ll want to get the bike up to operating temperature when you tune your carbs, as a cold engine does not vaporize gas as well and will require a richer mixture to get going.

To decide if you need to rejet your carbs, you should start with a plug chop.

With any good controlled experiment, it’s a good idea to change one thing at a time and retest to get it perfect.

A plug chop will help you tune your carbs if you’ve changed things like the exhaust or air filter, or even top end components.

How to Do a Plug Chop

To perform a plug chop on your motorcycle:

  • Put fresh, new spark plugs in the engine.
  • Bring the motorcycle to operating temperature.
  • Find a flat, long, empty road.
  • Ride for about a mile at the throttle position you want to test.
  • Pull in the clutch, kill the ignition, and coast to a stop.
  • Remove the spark plug and inspect the color and note the engine’s performance.
  • Make changes as necessary.

Once you’ve got your readings, you’ll be able to determine how to adjust your carbs if they’re rich or lean or rejet them if necessary.

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