I had inky black exhaust with Davis fuel when I started running a Tower 40 with an RJL head. As I ran the engine more and started flying it, the exhaust gradually changed to clear. I don't think it was a matter of being overcompressed at first (although that would be the usual cause for dark exhaust.) Try letting the engine warm up for maybe 30 seconds before you set the compression.

The contrapiston in my RJL 40 head came apart. The shelf below the O-ring groove was about 1/32 in thick, and this entire shelf came off and welded itself to the top of the piston and the face of the contrapiston. Major damage was bent rod and distorted case. Made a new contrapiston with a 1/16 in thick shelf and no further problems with the head.

Jim