Ayuh,..... Not sure on yer Volvo, but generally speakin' that pcv should be just an open elbow to vent the crankcase,....
The carbon black is because yer carb is afu, 'n is runnin' waayyy to rich,....
Ayuh,..... Welcome Aboard,..... What is the bolt pattern on the wheels,..??
5 on 4, 1/2" is pretty much the current standard,.....
4 on 4" can be found though,....
Ayuh,..... It's more likely the rings are shot, 'n yer motor needs a rebuild,.....
Do a compression, 'n leak-down test to see what's goin' on with it,....
Ayuh,..... If ya wanta get wet, do it in the water,....
With most boats, at full tilt, the prop is still underwater,....
No reason to pull the drive to check the prop,....
Ayuh,..... Make sure the prop shaft is straight, 'n the prop ain't rubbin' on the lower unit,.....
As I said, Everything but the prop shaft turns in neutral, 'n you said the sound goes away in neutral,....
