Motorcycle Repair: re plug fouling, kawasaki kx250, oil mixture

Question
The answer to all of your question is yes. The bike did have a fouling prob when I bought it but once the mechanic  changed the pin placement the bike I had no further probs and the bike ran great till this bottle of fuel pre mix oil. I have asked at the moto shop if they changed the oil make up because the oil to me looked thicker from previous bottles and they said no. The oil is Motul 800 factory.The mechanic droped the pin placement because the bike had been rejeted by the previous owner and he thought the excess fuel may have been causing the problem, the bike has run fine scince then for a period of months. But having said that i bought a new bottle of oil and this oil is the same brand and type of oil always used by me.Why does the bike die after a period of riding? For example 2 hours.I agree with your idea of on and off the throtle but you cant ride this bike flat  out all of the time unfortunatly I only use this bike for recreational use and I do a lot of enduro type tracks. Can I afford to drop the oil mixture to say 30:1 instead of 40:1? And another question  from me is , is there any difference between spark plugs marked on the packaging between genuine and  non genuine? One pack had genuine on it and one pack had  Competition /race on it. The plugs were visualy different in the way of the cap pin was srew on on one and not the other. Thankyou for your help.     
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Followup To

Question -
Im having probs with my Kawasaki Kx250 in regards to spark plug fouling. The bike will run fine for 2 or so hours the die due to the plug fouling. I mix my fuel 40:1 and use the plug NGK BR8EG. The carbie pin has been set one notch down from standard on the advise of a mechanic. This problem has only just happened after I bought a new bottle of Motul pre mix oil. I always have used this oil and had no prob in the past. Taking the plug out it looks wet put new plug in first kick it starts. Why and how can I fix this prob please.

Answer -
Ok, here goes.  Was the bike running fine after the mechanic adjusted your needle?  Also, Y did he say 2 do that?  R u on and off the throttle a lot?  2 strokes r most comfortable at full blast (high rpm) or at idle speeds.  If your riding habits haven't changed then how long had u had the bike b4 the fouling issue?  U most definatly have a fuel delivery issue, but I'm trying to narrow down what may b causeing it.

Answer
K, thanks for answering my questions, as it has helped a lot.  The plug differences are critical.  If u r not competitioning/racing then u have a VERY sensitive plug and it WILL foul when riding the bike normally.  Go back 2 the other plug.  If the shop says there's no difference, they're full of shit and r probably out of what u need.  Have them order the proper plug.

If your problem stimmed from purchasing a new oil then I would have to say it's the oil as u think.  Especially if the shop's mixing it themselves.  Hope this helps! Ride safe bro.