Gasoline mixed with air flowing past the intake valves helps keep them clean and avoids carbon build up in most cases. There was a time in the 90’s when I saw many engines with carbon tee pees on the stem of intake valves. At that time it was a shell gasoline issue so bad gas can certainly cause the issue. Direct injected engines have no fuel in the intake mix, any oil from the guide can burn on the stem is not washed off so over time the build up gets larger. I have used top end cleaners or water to successfully clean the head chamber or piston heads to lower Knox emissions in cars but zero success getting carbon off the back of intake valves. If a known and proven product is out there the industry would like to know.
In the OP case he clearly states the engine runs great, leave it be.

 

Circa late sixties when I was a grad student at the U of Wisconsin Engine Research Center, I would sometimes take the SWC out on "the beltway" in the wee hours of the morning after the bars closed and do an "Italian Tuneup" getting it up to at least the top end of third gear, which was about 130 MPH as many as two or three times.

One night I looked out the rear view mirror as I approached 6500 in third and saw a trail of glowing orange particles behind the car that were actually still glowing as they hit the pavement and bounced off. I immediately backed off and drove slowly back to my apartment thinking that something inside the engine was disintegrating. The next several times I drove the car I "heard noises", but finally realized it was just my paranoid imagination.

My final conclusion was that the 15-20 seconds of 1500 degree exhaust blowing through the exhaust system at very high velocity was simply heating up and knocking off hot carbon particles from the combustion chamber to manifolds and upper exhaust system and out the tailpipes. At the time I had straight through glasspacks, so there was nothing to impede the flow of hot glowing particles. The OE type reverse flow mufflers probably would have trapped them or cooled them enough so they were no longer glowing as they exited the tailpipes.

Jim - Yes, I have no doubt who gave you the Chevron Richmond research facility tour.

Duke

 

Perhaps I over embellished and should have said the deposit buildup won't "hurt" airflow, but there is a chance that it could actually improve it.

Studies of inlet port flow using smoke or other markers show a large dead area between the valve head and stem at both low and high lifts. That's because the mixture follows the path of least resistance from the port to the annular opening between the edge of the valve and the seat, which leaves a volume of "dead air" in the shape of a cone from the top of the valve head up the stem, and this is one reason why deposits build up in this volume because there is no high velocity fuel air mixture to wash them off.

The exhaust valve situation is similar as the flow must go around the edge of the valve leaving the top of the valve with dead air and since the exhaust valve runs hotter than the inlet any deposits, like from oil down the guides, carburizes.

This brings up an interesting point about Rochester FI system. The nozzles are aimed directly at the top of the inlet valve, so there is a continuous flow of liquid that modern detergent gasoline should keep the top of the inlet valve clean compared to a carbureted or direct injection engine.

Duke