carbon deposits sticking to rear of car?

[YRUSOSLO]

Okay I have two 240's and they both have this problem. The maroon one is REALLY bad. '89 ka24se I have sprayed a full can of carb cleaner into each of them. I took out the crap Bosch plat's and replaced with good japanese plugs. The red one has had the first two fuel injectors replaced.

When they rev up going down the road I see white smoke out of the tail light. The marroon one has BAD black riding up the back of the car. Genious before me put in a double pipe that faces upwards which makes it even worse. One the red one I've also changed the distributor cap and rotor.

All ideas to stop this dirtying of my car greatly appreciated.