Thoughts wanted. Actually just got off from 23 miles runs ok! looks a little rich to me I believe the specs are just from fuel they just wipe off . Everything on this machine is new I mean everything.

A bit rich , but that would be at what ever consistant throttle position you were running at , Dont lean out the main jet if your cruzing at 1/4 throttle . More info needed

Rich

RacerClam is right on, Jetting by just spark plug is tricky. To jet top end you have to run it flat out and then shut it down before letting the motor settle down as the idle/mid range will taint your plug color. Idle to mid range is what most people do on trials/street and for that it is looking rich. If you have not changed anything on the bike I would look at air filter restriction or your timing might have changed. Mid range can be somewhat adjusted with your slide needle position.

I hate to comment too much without knowing the specific conditions of the plug test... as others have said, a plug can soot up very quickly at idle or low rpm.

The missing specs of carbon around the metal shell and the spots on the insulator would make me worry a little about detonation/pinging... I can't really tell in the photo but with a magnifying glass if you see any little "balls" on the center insulator it's a sure sign of some detonation... but again, not knowing if the plug was well-used or brand new it's hard to tell... or if maybe the missing spots were "wiped off"?

Beyond that, if it's running good, I wouldn't lean it out at all.

No shiny spots on the porcelain that I can see, brownish deposits and clean in the center, sorta sooty outside...looks setup right to me.

That was cruising at 45 down a dirt road stomped the brake and slid to stop. Ive already dropped needle one notch. Im not going to mess with main at all because I never see wot down the hiway. Only wot I see is for short burst. I have a new genuine mukuni needle and needle jet coming. figured after 45 years the needle must have some wear. Also I just put in a new plug so can try again that is a 9 and it uses a 8 but that was all I had after getting engine in. I also when building the bike drilled 4 3/8 holes in bottom of filer box to make sure enough air got in. The pilot jet I believe is dead on because screwed in to 1/2 out it starts running rough and at about 1 5/8 it idles perfect

Also its a 75 250 It has no real issues running fine. Just plug kind of worried me its more gray colored than tan
Carb all rebuilt float set towards the high side of specs. boyseen reeds stock pipe unifilter with one layer removed

If it takes an 8 and you were running a 9 that could be why it's a little dark.

Especially since reading plugs at part throttle is not something I'd tune by...

In any case... if it's running good, I'd leave it alone even if the plug is a little dark.