Carbon buildup on throttle body?

[gdeputy]

This is a chrysler 400 big block running with a throttle body, 4 injectors, MS2. I'm getting some uneven carbon buildup in the throttle body, only seems to be around 2 of the 4 passages, the 2 closest to the IAC port. Dunno if thats related. Anyone seen this before or have any ideas as to what is causing it? Not having any problems, just curious what is going on here.

[LAV1000]

crankcase ventilation ?

[BigBlockMopar]

Intake (exhaust mixture) reversion is sometimes a sign of this.
The surrounding engine appears to be 'bone' stock, but is there any kind of hotter cam in there?

And why only the front 2 ports, that's because 98% of time you're only using those ports because of the progressive opening throttle linkage.
Is a vacuum gauge needle steady when hooked up to the engine when idling?
Maybe some leaky intake valves?

[gdeputy]

Engine is stock on the inside, stock cam, etc. Exhaust headers and edelbrock intake. The throttle is not progressive opening, though. All 4 butterfly valves open at the same rate, not like a carb where the primaries are first and the secondaries open up later.

Probably chock it up to old engine, it probably does have leaky valves, etc.. I've got a 440 on a stand i'm slowly rebuilding to replace this.

thanks!

[BigBlockMopar]

Had a closer look at the linkage and you're correct, I presumed it was progressive but it's 4 ports at one.