Intake Manifold Carbon Build Up?

[olddog]

Shined a light down ports 2, 3, 6, & 7 and I can see the intake valves (Single plane intake). They are clean and dry. From where the injector sprays on is clean and directly under the injector still looks as polished as the day I installed the intake.

I rubbed my finger all around the intake. On the floor of the intake under where the PCV valve port is there is a hint of an oily film. The rest is a very thin layer of dry powdery carbon just like you would find in an exhaust manifold. I could rub it off.

I sprayed some carb cleaner on a rag and it wiped off. So I wiped the entire intake down until I had it clean. Then sprayed the cleaner directly on it and wiped it off with clean rags a couple times and it is just like the day I installed it again.

This intake has an air gap between the lifter gallery and the manifold. I don't think it gets hot enough to carbonize oil. Do you still think it is oil? I'm still thinking exhaust gasses sucking into the intake from the valve overlap, but it is everywhere. Even on the bottoms of the throttle plates. I wouldn't think reversion would go that far.

I'm interested in more opinions, as I'm not sure what it is.

[Gaz64]

Olddog,

If it is only a dry film of "fuel carbon", I wouldn't be too concerned.

The reason you're getting it is because you don't have carbs to keep the runners clean AND you have reversion from a higher overlap cam.

Technically your system would be sequential, but as to when the injector fires in relation to the valve opening period is something to discuss with the injection manufacturer.

Even if the injector truly fires only during inlet open period, you will get some air/fuel mixture blown back into the manifold at low to medium engine speed.