Spark plug pictures

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#1
Hi,

as some of you may know I had a few jetting problems. I have a 99 yz250. I've leaned out the pilot 1 size and raised the clip 1 position and leaned out the main 1 size. Here is what my plug looks like after a weekend of racing





Any comments would be appreciated. Thanks
 

Jaybird

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#2
My spodely observation says you are still a bit fat on the main.
Can't see how many threads are black.
 

bwalker

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#4
how does the bike feel after the changes? What fuel and oil are you using? Remeber that plug color is a poor indicator of jetting.
 
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#5
Bwalker

The bike feels good, maybe a little faster. I am not very good at telling. I am just the pilot!! I run Yamalub R2 with Pump gas(Supreme it called in Canada) @ 32:1
 

bwalker

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#6
You plugs look rich ,but I could not tell you which circuit based on the pictures. I would suspect you pilot and needle are still rich. Get your pilot right, then your needle, and then your main. Makeing more than one change at a time tends to make the process more dificult.
 

Jaybird

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#7
I've heard a good indicator of correct plug heat is the blackening of the first three or four threads. What plug are you using?
You know, I don't like to look at the top of the insulator when I'm working on the main....but is the insulator not a bit different looking on the thread picture, than it is in the first two? May be an optical conclusion. :)
 
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#8
I believe plug readings are only reliable for a WOT test to determine where the main should be. Pilot and needle clip should be done by feel. This is what I've been told by people that know allot more than me (which may not be saying much).
 

bwalker

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#9
I've heard a good indicator of correct plug heat is the blackening of the first three or four threads
Jaybird, The method you described is not really accurate. Try this link for the correct method. http://www.strappe.com/plugs.html

believe plug readings are only reliable for a WOT test to determine where the main should be. Pilot and needle clip should be done by feel. This is what I've been told by people that know allot more than me (which may not be saying much).
KxKen, You are right.
 

Jaybird

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#10
bwalker,
In that tutorial it's mentioned that the correct way to tell if you have the correct heat range is if the insulator is nearly cleaned of deposit. Thing is, when we add our two stroke concoctions to the mix, it seems a fine line between needing a hotter plug or a main jet tweek, and how do we distinguish between the two? I was always under the impression that if your plug wasn't reaching optimum temperature it would not blacken but one or two threads. The insulator method would be great once we know that our jetting is correct.
 

bwalker

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#11
In that tutorial it's mentioned that the correct way to tell if you have the correct heat range is if the insulator is nearly cleaned of deposit. Thing is, when we add our two stroke concoctions to the mix, it seems a fine line between needing a hotter plug or a main jet tweek, and how do we distinguish between the two?
To get a proper reading on a main jet you must examin the base of the insulator where it joins the metal shell not the insulator tip that as is mentioned in the article. A two stroke jetted to the edge and running a high quality fuel/oil combo will have an almost white insulator. As far as the heat range things go, the factory speced plug is way conservative in MOST CASES so useing a colder plug is not necessary. In many cases a hotter plug can be used. My advice would be to stick with the stock plug and jet it correctly then check to make shure you have the correct heat range.
 

Jaybird

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#12
I have the base of the inlulator concept down, but how does one tell if there jetting is correct if the same procedure to check both the main jet, and the heat range is the same? Would you say that if your heat range isnt proper, then you won't be able to get the main jet right? BTW...I wasn't judging his tuning when I mentioned the tip of the insulator earlier, I was simply mentioning it looked different than his first two photos.

I'm having a hard time with this one....I've always thought my upper circuits would run even with a slight discrepancy in heat range from the plug...it's the lower slower circuits that need the range correct. Am I off base here?

I would like to hear someone weigh in on the thread black thing...have I been wrong to think that heat range has something to do with the blackening of the threads?
 
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bwalker

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#13
I have the base of the inlulator concept down, but how does one tell if there jetting is correct if the same procedure to check both the main jet, and the heat range is the same?
The procedure for each is different. For the main jet the bottom of the insulator is examined, for heat range the tip of the insulator and the ground strap is examined.

I'm having a hard time with this one....I've always thought my upper circuits would run even with a slight discrepancy in heat range from the plug...it's the lower slower circuits that need the range correct. Am I off base here?
If the heat range of the plug is too hot it can cause pre ignition. This can occure at part and full throttle, but it is most likely to occure at large throttle openings.
I would like to hear someone weigh in on the thread black thing...have I been wrong to think that heat range has something to do with the blackening of the threads?
I think so.
 
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#14
CR250 plug chop picture

For setting the main jet, a plug chop test seems to work well.
Install a new plug, ride it around for a while to give it some color.
Find a long uphill (sand if possible to provide engine load) section, get into 4th or 5th gear wide open for say 10-15 seconds.
Simultaneously pull the clutch, hit the engine kill button, and close the throttle.
Coast to a stop.
Pull the plug and look WAY DOWN the plug where the porcelain meets the metal shell. You will see a mixture ring as noted in this link http://www.strappe.com/plugs.html

Here is what my '00 CR250 plug looked like. The plug was cut down on a lathe.

Word from DRN moderators is that it looks pretty close to correct main jetting.
Remember that this test is ONLY for the main jet circuit.

Here is another from my kids '01 CR80.

I thought this looked a tad lean so I went one larger on the main.

Is this of any help?
Good luck!