Problem solved!

It rained on and off all day today, so I had plenty
of down-time. The cowl came off and I retorqued the
intake bracket; loosened the intake hose, slid it
up the pipe a fraction of an inch and retorqued it;
and pulled the manifold pressure tap(*) from the
primer port and plugged the port. Started it
up and waddaya know, it idled really well on
either and both mags. Put the MAP line back
in the primer port and tried it again. Still
smooth, but *maybe* a little rougher than before.
Capped and stowed the MAP line and replugged the
cylinder. I'll put a restrictor fitting in later
just in case. I don't know which thing fixed it,
but it's fixed. It was much easier to set the
idle mixture when one cylinder wasn't sucking air.
Now I'm wondering which cylinder will be the hottest
now that #3 isn't leaner than the rest. Just have
to fly it and see :-)

Thanks to everyone for the suggestions, on and offline.

Dave 'Hoover' Hyde


(*) Forgot to mention this was on the problem
cylinder too. #1,2, and 4 are primed.