Need help reading plug chop/piston crown

Hi,

My name is Ole Knut. I'm new here and I'm from Norway. I was recommended to ask for help here because of lots of twostroke knowledge is gathered here.

We are racing a de-restricted Aprilia RS125 2002 in the Norwegian junior class. No tuning allowed, only removing restrictions. Currently, we are 41 points behind the overall leader with two more races to go the next weekend. We're now doing all we can in a last attempt to have a go at the title. One of the few things left, is to reduce the size of the main jet. We've deliberately been running far too rich mixture all the summer to make sure of as little trouble as possible so we could race instead of fix the engine all the time. We run 98 octane petrol premixed with 3,5% Motul 800. The engine temp is steady at 70 degrees Celcius. The spark plug we're using is NGK BR10EG. Last in the dyno we reduced the main jet from 133 to 128 and got two more horsepower. Now we're looking for more power by reducing the main jet even more. And now the reason why I'm here. I know very little about reading spark plugs. I have attached some photos of our plug after a plug chop at the end of a practice session.

My goal is to try to find the limit for when the engine gets too lean by reading the plug. I have no EGT or detonation counter. Yet.

I have understood that if I want to check the mixture, I must look at the mixture ring. I have no good tools to measure the width of the ring, but it is approx 0,7-0,8mm wide.

How much should I expect to run the engine in the dyno before I can be able to find the mixture ring on a new spark plug?

How do you recommend that I cycle through the gears - Wide Open Throttle through all gears and then keep her at max revs for 10 seconds?

Which width of the mixture ring should I go for? Our main jet is now 122. But factory standard for the street is 120, and that I believe should be more than rich enough. I've heard about people in the Adac cup running main jets sized all down to 110. I plan to start keeping track of Relative Air Density so I can compensate the size of the jet when required.

Picture 1 - mixture ring
http://img199.imageshack.us/img199/1223/73708741.jpg

I noticed the insulator was colored quite differently, depending upon which side of the plug I look at. See picture 2 - 4. Is this normal? Why isn't the color the same all the way around the insulator?

Except the mixture ring, what else can you read from the pictures of the plug? Where do you look and how do you judge what you see?

Picture 2
http://img196.imageshack.us/img196/4738/37795560.jpg

Picture 3
http://img220.imageshack.us/img220/5161/59025890.jpg

Picture 4
http://img220.imageshack.us/img220/4313/43046386.jpg

I've been told three ways of keeping track of the mixture. 1) Listen and judge how the engine performs/look at the smoke it produces. 2) Read the plug. 3) Read the piston crown. I have absolutely no clue about what to look for on the crown, where to look and how I should understand what I find. I would appreciate very much a little introduction into reading the piston crown. And maybe some comments about our piston. It has been used for 5 race weekends and some practice sessions.

Picture 5 - piston crown
http://img220.imageshack.us/img220/2390/46678626.jpg

I hope some of you are able to enlighten me about these issues. We're heading for the dyno Saturday and I very much would like to be able to understand what the plug and piston crown tells me. Thank you very much for helping out.

Regards
Ole Knut

I've been reading up a little. What I find confusing, is that when some people refer to the mixture ring, they say you can judge the mixture by the color of the ring. Other people say the only thing you can judge the mixture from, is the width of the ring. So what gives? What is correct - judge from the color of the ring or the width?

I just read a couple of articles about judging from the width. At rscycles.com I found this regarding carting engines:

"If your engine's mixture is too rich, a colored ring will be present. If this ring continues outward along the insulator to a width of even a millimeter you can be sure the mixture is rich enough to be safe, and too rich for maximum performance. In most engines best performance is achieved when the mixture contains only enough exess fuel to make just a wisp of a 'mixture ring' on the plug insulator.

An air/fuel mixture that yields maximum power is only slightly richer than the one that causes detonation."

Another place I read most engines work best with a ring width between 0 - 2mm and optimum for many engines is around 0,5mm.

So if the above statements are true, it seems I'm a little further.