Plug Chop how-to:

Plug Chops are a good way to test the jetting accuracy of your carb setup.  Simply pulling a plug and looking at the color after running through a variety of throttle positions can be misleading and possibly lead to disaster if jetting is too lean.

The procedure outlined below is intended to test main jet sizing at WOT.  It is strongly suggested to begin with overly rich jetting and work your way down in two size increments to avoid seizure.  The same procedure can be used at marked mid throttle position to test needle jet position.  The distance covered should be at least 1/2 mi, the farther, the better.  It may be best to perform the test in a lower gear to avoid excessive speed.  Throttle position is the critical item.  Variation of throttle position should be avoided and can taint results.

It is possible to examine plugs by using a good light and magnifying glass rather than cutting the threads away.  A single cylinder can be used to hone in on the right jetting to save the expense of new plug consumption, assuming all carbs are jetted and setup the same.

Be aware that changes in fuel type/grade and timing can change results.

1. Get new spark plug(s), but don't put them in yet.
2. Warm up motor to operating temp on old plugs.
3. Install new plugs.
4. Accelerate through all gears to top gear with throttle pinned.
5. When you hit the top of the top gear, hit the kill switch and
pull in the clutch.
6. Remove the spark plug(s) and ride home on the old plug(s).
7. Hold the new plug in a vise and use a hacksaw to cut the threads
away from the center electrode.
8. You should see a brown 2mm "smoke ring" at the base of the
white center electrode if the main jets are dialed in perfect.
9. if the smoke ring is darker than chocolate brown or taller
than 2mm, you're rich on the mains, but if it revs clean to full
throttle, you can leave it there and be on the rich side.
10. If the smoke ring is smaller than 2mm or lighter than chocolate
brown, you're lean on the mains, go up a size and try again.

Chopped plug Illustration:

Below is a series of plug chops ranging from extremely rich to slightly lean.