Regarding the holes above the intake ports -- I don't have any direct 20B
experience, but on the 13B's, similar holes provide some air into the bore
that contains the fuel injector. The fuel injectors spray onto "mixing
plates" (small plastic pieces that I suppose are intended to break up the
fuel spray). The air from these holes also enters above the mixing plates.
The air is sourced from atmospheric pressure, so the effect will be greatest
at low throttle settings (high manifold vacuum). I would "guess" the issue is
primarily light load HC emissions. IIRC, some years did not have the air
bleeds, but I'm not really sure. Those 20B ports sure are large compared with
13B's!!!

Jon Lauter
Rotary Power Marine Corporation
www.rotarymarine.com

 
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