Reading plugs

Post by **guitardude081** » Sat Feb 07, 2009 11:59 am

To get a good read on your plugs, the best thing to do is install 2 new plugs, put 'er up on a stand and give it a wide open pull and hit the kill switch. You'll see a definite black line on both plug electrodes. If the line is passed the bend, you're rich, if it's before the bend, you're lean. If you're rich, it takes longer for the plug to ignite the intake charge so the burn line is farther along the electrode. If you're lean, it ignites quicker and the line is closer to the thread. You have roughly 3 jet sizes on either side of the bend. As Mr. Rainville has stated in the past, it's better to rich and choked than lean and smoked. Remember, premium non-oxy fuel is the best to use whenever available
~Jack

oscar89 · Post by **oscar89** » Sat Feb 07, 2009 11:58 pm

Great info. Can a carb be rich at lower throttle settings but then lean at higher settings? See the Mikuni has a pilot jet-good to 1/4 throttle & then the main & needle jet come into play.

oscar89 · Post by **oscar89** » Sun Feb 08, 2009 12:50 am

My plugs are new w/ about 45 minutes on them. Got very little for deposits on the plugs but there is a light tan deposit on the bottom of the electrode just after the bend. All my running has been less than 1/2 throttle. Just tuned up carb. Am I too lean?

Post by **guitardude081** » Sun Feb 08, 2009 1:52 pm

This test will show how well the main circuit is jetted. You can tune your low end air/fuel mixture with the idle air screw. It sounds to me that you are just a tad on the rich side. If you're unsure, install new plugs and give take a pull on the stand. I have a few plugs on my bench at the shop that are good examples of the burn line. You can safely run a sled a jet size or two leaner than stock as long as the outdoor temps aren't extremely cold. If your burn line is a little before the bend, that's ok. I don't know how well this pic will work, but you may be able to see it.
~Jack

Attachments:

Jetting.bmp (195.18 KB)

oscar89 · Post by **oscar89** » Sun Feb 08, 2009 6:18 pm

Thanks for the great info & the diagram is real helpful. Does all this info apply to 4 strokes too?
Was reading on a website that a plugs color should be read on the insulator. Does that sound right? Sounds like both color & placement of the deposits on the electrode are good indicators. If deposits are whitish does that indicate timing over advanced?

Post by **guitardude081** » Mon Feb 09, 2009 12:49 am

If deposits are white that indicates a lean condition. If you're jetted in the ballpark and it's still white, you may have an air leak somewhere.
~Jack