Spark plug color

PeteH · Post by **PeteH** » Wed Jan 25, 2012 1:44 pm

Here's a hopefully-not-too-silly question for the mechanics out there. We talk at length about the desired color of the spark plug electrode being a nice mocha brown, which indicates combustion is happening at the right temperature: not too rich, not too lean.

In the 2T world, tuners do plug chops to get a more-or-less instant read - running the engine up to temperature, riding WOT, then killing the engine to get an up-to-the-second look at the electrode, including cutting off the threaded section of the plug with a tubing cutter to get a look at the electrode all the way down to the base. Obviously this means treating the plugs as 'disposable'.

But here's my question: can you pull out your plug, make a mixture adjustment, reinstall the plug, drive for a while, then re-examine the plug for results? Do you need to start with a new 'clean' plug each time, or will the electrode change color over time to reflect its current temperature?

I'll be adding a Prima pipe with some rejetting (dealer install) this winter and I want to make sure the mix is right, but must I start with a new plug each testing cycle to verify it?

MGM · Post by **MGM** » Wed Jan 25, 2012 5:17 pm

It's my belief that a plug chop is done with a new plug. That's how a motorcycle tuner taught me. Plug chops are routine with any motorcycle, 2 or 4 stroke that has had intake or exhaust mod's.

That said, I don't think the plug is unusable after it's brief use to get the bike dialed in. If they're not all crapped up, just keep them for future maitainence. Plugs are usually about $3.00 so I naver bothered to keep them, but I don't see why you couldn't.

PeteH · Post by **PeteH** » Wed Jan 25, 2012 5:26 pm

Yeah, I've seen plug chop examples where the thread is left intact, but some others where the threads are cut away, obviously making them unusable.

Since we can't really do a true plug chop run on a Buddy (can't really cut off engine rotation and fuel delivery), it's not the exact science that it is in the manual-transmission / fuel cut-off world.

My question is more like since we're not looking for an instantaneous read like a true plug chop, can the same plug be examined over time to see a variation in mix results, or will it always assume the darkest color?

agrogod · Post by **agrogod** » Wed Jan 25, 2012 10:29 pm

If I understand you correctly it would not be possible. It would entail cleaning the plug to pristine newness which just isn't going to happen. This is one of the reasons new plugs are used for each test.