So the reasoning is to eliminate the carbon build up which collects in the air injection passage ways in the cylinder heads. Perhaps capping or plugging the 8mm passage ways from the exhaust port to the main passage would provide you with a better long term result. Just my .02 worth of input.

My opinion regarding this build up: it is a result of the exhaust gases traveling back and forth between the cylinder heads during the normal operation of the engine. The back and forth process would create unusual swirling of the air within the air injection passages. The unusual swirling would better explain the odd way that the carbon accumulates through out the air injection system.

I would imagine during air injection operation it might contribute to some of this build up but it does little to explain all of the accumulated carbon. I would think once the injected air arrived to that area of the engine it would be as warm as the engines rising temperature. If the air injection system was the cause of this build up wouldn’t the 8mm passages be the first to become blocked? Once the passages ways were blocked wouldn’t the collection of carbon behind the blockage be reduced or eliminated? Behind the blockage is the main passages and no further build up would be possible due to a restriction in air flow in either direction.

Others in this forum may provide you with some enlightening ways to reduce the build up by implementing a periodic external cleaning method which might include a solution of top engine cleaner. As of 1996 there have been a number of Mercedes vehicles that must have their cylinder heads removed in order to clear the passage ways in order to extinguish the check engine light. The MIL is illuminated because the Secondary Air Injection test Fails because the reduced air flow does little to assist the catalytic converter during warm up.

Just my thoughts on that accumulated carbon

Happy T-giving all