Plug fouling

[lc4]

I recently fully restored my Xl 250 Motosport that I have hardly put more than 10k on it due to a tyre issue (another story) and that it will quickly foul plugs. I had put a new jet kit through the carby ,full rebiuld on the motor etc. What I find is it will foul the plug (with carbon not oil ) as quickly as it takes it to get up to temperature. I had changed the plug  (which is the correct heat range )from a "R" to a standard non resistor plug as I thought maybe the coil is a bit weak but still the same problem. I would like to take it out and trash it to "clear it" but with only 10k on a complete rebiuld I don't want to invite more problems. The only other non standard thing is that I have added a valve saver additive to the fuel. When the plug is cleaned the bike will start and run clean without blowing black smoke so I don't think its a rich fuel mixture. Thought I would put it out here for a couple of suggestions

[matcho mick]

try a hotter plug,like a 7 just to run it (in),you didn't overdo the valvesaver??,piston ring in upside down,(like,wheres the carbon coming from on a new built motor??)

[lc4]

The carbon is defenitly not from burning oil and I added the valve saver as per instructions , I did give a hotter plug a thought but came to the conclusion that they (XL250) have always run a 8 plug.

[oldfart]

Tell me why are you using valve saver ?     
I would have a guess , and say it's unburnt fuel

Put a pic up of a spark plug for us to view ..........

[490 tez]

you havent put a new plug cap on with a resister built into cap as some of the old coils dont have enough gtrunt to push through them

[bazza]

never had problem with my motorsport and have owned for 20 years now.Try 1 thing at a time
fist no fuel additive
2nd new plug no fuel additive
3rd check float level

[pmc57]

Just a guess, is the air/mixture screw the correct turns out?

[lc4]

Thanks for the ideas to date, I will start with the simple things first (fuel) as suggested and work from there