Are those NGK8 heat range? They look fantastic for 5K miles. However, pre-mix air cooled motors will most likely need to run rich if run hard and that will also create more deposits even with good oil and gas.
To the OP, for 1.5 hours, that does look like a terrible amount of carbon deposits. I would try a hotter plug and pull the head and inspect the piston after a hard run. If all is well, then run with the hotter plug. Your diagnosis seems correct.
Originally Posted by anotherguy
I disagree,with a heavy load of modern oil,premium pump gas and correct jetting this is what some old plugs (5K miles or so) from my '84 RZ 350 look like. A healthy spark is required though.
But of course this advice comes from some random guy on the internet.