Burning spark plugs

[orestis]

Hi 
I’m new here. It’s great to see this forum 
it will be nice if the moderator could delete the spam 
so i bought a husky boy 2001 with a Morini s5gs motor
i have done a lot of things on it 
now my son has start riding it and the problem is that after 1/2-1 hours of riding (not continually) it burns the spark plugs. I use 20% oil on the gas. It’s true that my son is riding on low rpm. 
is the low rpm a reason for that kind of problem. Do I have to check something else 
thanks in advance 
chris

[orestis]

I have clean the carb. 
it’s running the standard jetting 
I think that is running rich and I have tried to close the idle mixture screw. I started at 1+1/2 turn and I tried a different Positions between 1-2,5 turns. I got the needle from the middle position, one step lower. It’s feels it’s not running perfect and also does the same with the spark plugs 
I measured the coil and I get 6,8 κΩ
any idea would be appreciated