Saging 3000 Tiger
Don't mean to offend here but it appears that someone must come to this poor engine defense.
1. A 20 X10 is for a ST4500 not a ST 3000 a 18x8 is your prop. Your engine was made to run at 7500 Rpm + Your frying the poor thing.
Lets talk black goo. This is caused one of two thing.
I: Not enough oil causing it to burn away not carring heat off and lubricate the space between piston and liner. In you case ring and liner causing the piston to over expand and wear its surface off against the liner. The black is aluminum.
2: Less likley but possible, some manufactures of glow fuel a used soy bean oil in their fuels. Soy been oils were evaluated by Dave Gerkie in a piece he did for MAN back in the 60's and found it had very good high temperture limits but formed high carbon deposits in the engine making it unaceptable as carbon is an insulator and leads to futher heat build up. No oil should ever carbon your engine when burned. Castor leaves brown varnish and synthetic leaves nothing that why the two in concert work so well.