dd is on the money. It's condensation from blow by gasses. Re-fit the PCV valve to the bottom side of the carby, plumb that to the block line and vent the rocker line to the air cleaner or fit it with a filter (air goes in at the rocker cover). All the deets are in the tech wiki.

You'll also see this with clapped out engines since the excessive blowby prevents effective venting but even then it clears up with a long drive, longer the better.