Need help reading four stroke plug

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#1
I need some help reading four stroke plugs. I've owned and successfully jetted many two-strokes but I'm having a hard time with the condition if my yz250f plug.Just installed a DRD exhaust-85F temp,high humidity ( FL ) bike ran great,just like stock. I have left stock jetting in ( airscrew 1.5 out from factory ) and the bike has always started easy and runs great. When I checked the plug I can't tell if it is a little lean or a little rich.There is no tan color at all on the plug-the tip of the insulator is white,from the tip down is black-the electrode looks the same-tip is clean,rest is black.rest of inside of plug is black. It doesn't exhibit any bad performance characteristics except it sounds a little fat at the bottom. I probably don't rev it like a typical 125 pilot ( I have yet to hear the rev limiter ) so the fatness is probably OE. I have just heard that white plug=lean=boom and I wanted to pick some experienced four stroke brains.Thanks-
 

Rich Rohrich

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#2
Four stroke plugs should be white on the insulator with a texture similar to a new plug and no obvious signs of overheating on the center electrode or ground strap. There should be a small (approx 1/8") ring of carbon extending from the point where the insulator and the metal body meet at the base of the insulator. You'll only be able to read the plug this way if you start with a fresh plug and a full load WOT run . If the insulator is black with the engine running good it's just a sign that at some point you experienced too much fuel and not enough heat/load. Not an uncommon situation when using an accelerator pump equipped carb.
Reading four stroke plugs is the same as reading two-stroke plugs except for a lack of insulator coloring from oil in the mix. If you are comfortable with reading two-stroke plugs then all you have to do is lose the notion that a white insulator tip is a BAD thing :) If it runs good chances are good it's jetted right. Response is still the best indicator.



Good luck.
 
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#3
So Rich, what you are saying if you ran a plug test on a 4 stroke and the plug is still white the jetting is fine? I posted awhile ago about jetting and you said it should be tan color. Now Im confused??
 

Rich Rohrich

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#4
I've been telling people for years that plugs (2 and 4 stroke) should be white . The exception is two-strokes will develop a bit of a tan appearance with SOME oils, and some leaded race fuels will color the insulator. The common belief that a plug should be a dark brown is a complete load. Just one more moto-myth passed down through the ages.

What I'm saying is you can't read mixture strength by looking at the nose of the insulator. If you don't take a look deep inside at the bottom of the insulator with a magnifier you aren't reading mixture strength, just plug temp. In other words you are wasting your time. Even if you read the fuel ring correctly the only circuit you are reading with all this is the mainjet. You can't accurately read the other carb circuits with the plug.
 
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#5
Thanks, Rich

Thanks for the reply, Rich. I figured if it was running good that the jetting had to be close but I needed some clarification and you provided it .THANKS!!!