914 Exhaust Soot

Lots of soot and black plugs sounds like the engine is running too rich. Maybe check the carbs.

Since it began right after your oil change, I am wondering if you remembered to burp the engine before you changed the oil.  If not, you may have added more oil at the change than actually required, and excess oil may now be coming out of the oil-tank breather and making a mess.  Where does your oil-tank breather hose lead?

Is this a new problem that has only recently appeared?

This has been a problem since the aircraft arrived in Cape Town with 12 hours on the engine.  Carbs were re-balanced before first local flight.

-- martin

I would check if the hose between your engine and the intercooler is covered with oil internally.

If it is covered with oil, you have a leak in the oil turbo valve. Due to some time inactivity (could be as less than 1-2 weeks) of the engine, your turbo gets filled in with oil. You will see a black puff when you start your engine and during the first few minutes when this is the case. And if you have oil in the intercooler, this will go to the whole airsystem and make your engine running rough till that the oil is cleaned up.

I always had such issue on my Europa 914 intercooled during winter time when I was running the engine maybe on monthly basis. I changed the ball, spring and case of the turbo oil valve and the issue disappeared.  

Well, you have a new engine, so potentially not the issue. But I still would check if the hose between your engine and the intercooler is covered with oil internally.