think i have a fouled plug

so my i have been working on my friends cr125 getting it jetted to run at the current elevation. he went up 4500 ft in elevation and i changed the main jet from the 360 he had in it to a 340. all he has done to it are fmf pipes. what its doing is when you get on it, it feels like it boggs down and it has no powrband. i think its a fouled plug cuz the tip of the plug was completely black not the tan mocha color that is desired. am i correct or do you guys have any other ideas? also it leaked coolant out of the overflow tube but not much and we made sure we let it cool and put some more coolant in it. when i checked the plug i held it WOT for a couple seconds then checked the plug and it was smoking out of the top of the cylinder when i took the plug out. that is normal right?

Ok I will try that, n it didn't smell like coolant so I'm pretty sure it's good to go there. Does the pilot make a big difference cuz I was always told that's more for like idling