Spark plug- porcelain tan half way around and white the other?

[Fletch]

I have been trying to find out what this means on the internet with no luck.

I only found one thread somewhere with the picture moved, and someone said the plug was colored on the intake side and white on the exhaust side, indicating that there was not enough gas, and to increase the main jet size.

I'm not sure if I buy that I need to increase the main jet because I already have had it one up and it was too rich. I would think that if the jet was too small, there wouldn't be tan anywhere. Meaning I don't see how that has much to do with the color only going half way around the plug.

I have an expansion chamber if that has anything to do with it?

Here's some pics... Sorry about the lighting.

 

[GearNut]

Judging by that color, you are running too lean for sure. It should be a chocolate brown color to a medium tan color. Speaking for the NT carburetor, the jet controls the fuel from 3/4 throttle on. Any lesser throttle position is controlled by the needle.

 

[Fletch]

Judging by that color, you are running too lean for sure. It should be a chocolate brown color to a medium tan color. Speaking for the NT carburetor, the jet controls the fuel from 3/4 throttle on. Any lesser throttle position is controlled by the needle.

THanks. I moved the needle one richer and it is chocolate brown now where it was white before, and where it was tan before it is closer to black now. The one side is still darker? Do you have any idea why it is richer on one side of the plug? Maybe that guy's theory about the darker side being the intake side is correct? This is also an extended plug- the NGK BP6HS, so maybe it is because of it extending farther into the chamber?

This carb is actually the Mikuni VM 18 which so far is awesome. It's like the CNS but not built like a toy. It comes jetted at 80 and I found that its perfect for my 66cc with tuned pipe. The pilot jet is 30 stock, and I found 25 to be good. I tried jetting 1 larger and 2 smaller on the main, but the stock 80 was the best. I tried 22.5 and 20 on the pilot, but found 25 to be best. It would probably work fine with the stock 30, but I haven't tried it because i was going off another member's recommendations.

For the price I would highly recommend this carb!

I did a lot of reading on carb tuning, and you're right on the main jet from 3/4 up, although I did read that in reality the main jet affects everything and this is often overlooked. For example the fuel getting to the needle jet travels through the main jet. They were saying basically that every component affects everything else, which is true because there is overlap on the ranges that each part affect. By changing the needle position one richer it theoretically shouldn't have changed anything but the lower part of the porcelain. The main jet should only color the lowest part of the porcelain and you should have to have a magnifying glass to see it. So what I've read is kind of contradictory to my experience. I'm still learning about this stuff and it's surprising how detailed plug reading, and carb tuning can get!

This is a cool webpage for tuning Mikuni's and carb theory, but I'm sure it applies to all similar round slide carbs: http://www.iwt.com.au/mikunicarb.htm

This is a cool graph below from that page showing visually how each tuning component overlaps and affects the others in each range.

I also used this tuning guide made specifically for this series carb (VM): http://www.6x6world.com/forums/content/section/288-mikuni-carb-tuning.html

 

[Fletch]

I just had an idea after looking again at that chart and seeing how the pilot jet/air screw effect the entire throttle range. Maybe I will put in the stock 30 pilot jet that I hav yet to try and move the needle one leaner (where it was before). Maybe this will do the trick and I'll get an even color around the entire porcelain? It makes sense that if the stock main jet size was ideal, that the stock pilot may be as well.

 

[GearNut]

The porcelain does not need to be the exact same color all the way around. Flame front travel and combustion chamber characteristics can affect the coloring, as well as refinery fuel additives and the additives in the pre-mix oil. That is why spark plug reading is not the end all of tuning tricks. It is however a really good indicator of overall jetting and generally acceptable in most non professional racing situations.
Don't beat yourself up trying to get a perfectly and evenly colored plug, just make sure that the color that it is is a good color.


Edit: I just read the tuning link that you shared with us and I agree with most everything said except the color of the spark plug. When they say a tan color is ideal, that is in reference to a 4-stroke engine or a 2-stroke which is burning a synthetic reduced residue pre-mix oil such as Opti2. If one is using a standard petroleum or cheapo home depot pre-mix oil, a darker color is desired as it is indicative of a richer fuel ratio which promotes good lubrication of the engine internals.

 

[Fletch]

Thanks GearNut.... I'm running Amsoil Saber: http://www.amsoil.com/storefront/atp.aspx
@ 60:1.

"Saber™ Professional Synthetic 100:1 Pre-Mix 2-Cycle Oil (ATP)
AMSOIL Saber™ Professional (ATP) is formulated with exclusive AMSOIL synthetic base oils and premium additives. Designed for lean mix ratios in two-cycle motors..."

Considering that, would my original tan plug color be ok? Just one notch on the needle clip goes from the color in that picture I posted to a chocolate dark brown where the plug wasn't colored at all, and almost a black color where it was.

 

[GearNut]

Considering that you are using AmsOil Saber You should be fine with the tan color.
Chocolate dark brown seems a little rich to me for a low residue pre-mix.

You can also get a decent idea of the state of tune with the operating temperature of the cylinder and exhaust header.

 

[Fletch]

Considering that you are using AmsOil Saber You should be fine with the tan color.
Chocolate dark brown seems a little rich to me for a low residue pre-mix.

You can also get a decent idea of the state of tune with the operating temperature of the cylinder and exhaust header.

Thanks.... I rode the bike at the original lean setting up the mountain highway from where I live at 500ft above sea level to 3,000 ft. I ran into problems with my carb between 1500-2000 I'd guess. I started turning in the pilot screw and idle screw to make it run richer and finally pulled over to move the needle clip one richer, but it still didn't run at full capacity. I made the same trip with the NT carb with relatively no noticeable change. This mikuni and the CNS I used to use seem more sensitive to temp/altitude changes. Is that just because they are more tuned in? I'm running an 80 main jet on the second to leanest notch. Should I try a 75 at a richer setting? I had to lower the pilot from 30 to 25, so maybe an equal change in main is best?

 

[GearNut]

If the engine is 4-stroking at high rpm's and at or near full throttle opening then yes, reduce the size of the main jet. If it just seems to run out of power and stumble than increase the size of the main jet.

 

[Fletch]

If my plug is colored only half way DOWN the porcelain, is that an indicator of lean high end and wot? I've got Saber at 40:1 this time and I'm concerned it may be leaning out the a/f at wot. i have a shift kit on this one so different gears will give me different wot conditions, but it seems that there isn't too much of a power increase from 3/4 to wot which would indicate a larger jet is needed. It may just be in my head though. After the last piston seizing just after I switched from 60:1 to 40:1, I'm overly concerned about nut running lean AND having enough lube. At 40:1 with the needle at the richest setting, I get NO smoke from the exhaust. This is my second indicator that my main my be too small. What do you think? Increase the jet, or go down to 50 or even back to 60:1?