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Questions & Answers:

Have a 63M2AWE -10 outline 12206 carb on a Chris Craft B-20 4 Cyl  marine engine.

Having severe carb flooding. The carb has been rebuilt with a new kit. The carb has an acceretor pump.
The ignition sys has been changed. The compression is fine. The fuel pump has been changed and is in the 5-6 PSI range.

The fuel that collects in the throat is discoloured with a dark powery substance. The fuel in the float bowl is clean and clear. The float is set properly and I believe the floodng is not coming from the float needle valve sticking. Does the accelerator pump have anything to do with the flooding?

The answer to his question is “no”.
As I see it, one of six possibilities exist (in no particular order):
1)  Viton needle tip not cleaned with non-residual carb cleaner so as to be compatible with gasohol.
 2)  Fuel quality so poor that needle won’t seal.  It sounds as though fuel is rusty, which suggests water is present, which suggests that the gasoline and ethanol have separated — you get the picture.
3)  Debris stuck in needle seat holding needle open.  Did he assemble the fuel system with teflon thread tape?  I bet he did.
4)  Float is leaking.
5)  Fuel pump is generating more pressure than owner has measured.  In my opinion, 5 – 6 PSI itself is too much for that carb, and is certainly more than a 4-cyl. engine requires.
6)  Needle seat threads were not dressed with teflon thread paste, and fuel is leaking past them.