Plug colour

[morris]

What do you guys who master the black art of plug reading think of this plug.
I have had it in the plunger for about 300 miles now.
In my opinion it shows a bit of a lean mixture, which would explain the engine misfiring at full throttle like it's running out of breath.
I'm running a concentric 900, 240 main, 106 needle jet, 20 pilot.

[East_Coast_BSA]

I would agree it's lean.  There is no color in the center section, it should be a light/medium brown.  Also the sharp edges of the center electrode are going way, an indication of too much heat.  The first I would do is pick up the needle a notch.  Unless you're running wide open a lot, I wouldn't change the main just yet.  See what the needle change does for you.

[stu.andrews]

I would recommend exactly the same action to start off with. If you are only running on very light throttle settings, then richen the pilot air screw slightly.

[Topdad]

Hi Morris, I'm running the same carb on mine and had the same problem/colour musky suggested before doing anything else to turn the idle mixture in a qtr turn, mine is now a lovely brown colour however just realised I don't ride as hard has you . I was concerned about the pipes blueing suddenly not at full bore, poss might help, best of luck, Bob

[morris]

Thanks lads, that confirms what I suspected.
I remember that thread Bob. Combined with the exhaust blueing it already gave me a hint. I'll try that one first as it's the least trouble, but a recent post (Hate of the day) by Musky set me thinking. I also did away with the plastic float in favour of a stayup float, so I'm afraid I may need to check/adjust the fuel level in the float chamber.
I'll have a look at the needle position to, as I can't remember which position I've put it in. Really need to learn to take notes while assembling... 
And about the hard riding, I'm sorry, just cant help it... Every time I set off, I tell myself to take it easy this time, but inadvertently after a while I notice the needle approaching the first three figure mark on the speedo... 
Let you guys know when I hit the ton... 

[East_Coast_BSA]

What's the number on that Spark plug?  It's tough to tell from the picture, but it looks like an NGK B6ES.  You should probably be running a B8ES (crosses over to a Champion N3), which is two steps cooler.  That will help things, at least give you a better read on plug color.

[morris]

What's the number on that Spark plug?  It's tough to tell from the picture, but it looks like an NGK B6ES.  You should probably be running a B8ES (crosses over to a Champion N3), which is two steps cooler.  That will help things, at least give you a better read on plug color.

Yes it's an NGK B6ES. I have a box of B8ES on the shelf, so will give it a try. But something is definitely not quite right in the mixture department. I'll have to dig into that. There also was the odd spit back when opening the throttle from idle, but adjusting the pilot air screw cured that. I will now raise the needle a couple of notches and see what happens.

[morris]

In the end changed the main jet from a 230 to a 250 and lifted the needle one notch with this as a result;

[muskrat]