What causes this build up on 300xs throttle body plate

[LOSTINJ]

My 300XS has total 385 hours which I bought but never ran it . Im getting ready to bolt it on my 20 ft laser and noticed the throttle plate has tremendous oil build up . What causes this and I did notice oil puddle in bottom of plenum . Note I did slightly wipe the plate .

[rock]

Every Opti I have taken apart has had a mess of oil in the plenum. Throttle plate does look a bit nasty.

Rock

[LOSTINJ]

Thanks

[aj06bolt12r]

Its typical but its nasty.. I believe its probably belt dust from the air compressor belt that is being ingested by the motor. Some of the oil being injected into the crankcase ends up in the intake and throttle body, a film of this oil ends up on the throttle plate and causes the belt dust and other environmental contaminants to stick to it... Inside the throttle body is also gross on these engines, I don't know how they last as long as they do, I wish they had air filters on them.

[Jim Speros]

Yes to all above. To be more specific the build up on the top side is usually belt dust. I've seen belt belt inside a 225 plenum so bad if filled all the gaps and cover everything.
On a meticulously cared for engine one might notice the top side of shutter is clean but, on inspection of back side there is build up. It could be from dust and dirt in cowling or if darker blackish it can be from bad reeds. As they begin to frey they allow combustion dust into plenum. Yes the Opti's always have oil in the plenum. It should be clean. If its blackish you should be inspecting reeds.

[LOSTINJ]

Yes I agree to all above . I see the belt dust