Let's throw this into the mix .
Exact symptoms as op.
Except mine is fuel injected!
Just completed mcm mod, my ex are in spec , my inlets are out (tight) by 1 shim .
I run no snorkel , everything else is standard.
Mine could be a secondary butterfly issue, can't see why it would be running rich though.
Will pull plug soon after a chop.
New plug btw.
This ol girl has pulled 40k though so there is no telling what could be wrong !

 

Did you try putting the snorkel back on. MCM and no snorkel with no additional fuel may be an issue.