I run now about 13:1 a/f ratio and the spark plugs are very black and very sooty.

I drove with 14:1 more than 8000 miles without any problems on 440cc, pistons look very good and spark plugs was a bit brown, like it should be.

Now with 720cc and about 13:1 af ratio my plugs are black.

 

I think you should only run at 13:1 at full throttle. And just 14:1 at part throttle...

 

so it is

 

perhaps your partial throttle tuning area is running rich. how did you check your plugs? did you make a hard pass and then shut the engine off completey then checked the plugs or did you drive around and then check the plugs?

 

drive around about 20 miles and checked the brand new plugs.

 

try making 2-3 hard passes then pulling over somewhere to cool down and then recheck your plugs

 

13to1 af is to lean on a turbo car...you want to be around 12.4-12.2 af at full throttle.

 

I have 14.3:1 at WOT and have not 1 problem. Pistons look very fine after 8000+ miles.

 

Dude, 14.3:1 is exceptionally lean for WOT. I predict your motor will die soon. I'm sure eveyone here would agree.