Low compression pistons and incomplete combustion

I am using 9:1 pistons in a sleeved 2zz block. I find that i am getting significant wet carbon build up on the piston crown and suspect that it is due to low combustion temps and incomplete combustion. What startegies could I adopt to minimise this, such as runnin lean under light loads or adding more timing to get temps up.

I doubt the reason is the CR, if wet. Oil from top or bottom, or both, is my bet.

The CR has negligible affect on the burn quality - I would agree with Dom' about the high possibility it's oil entering the cylinders, possibly compounded by an over-rich mixture.

What's the appearance of the spark plugs, and is this a new or old build?

I agree with the other folks. 9:1 is not low enough to cause this. I would diagnose it without CR being a factor.