Emissions, timing?

[rhughes@domain.elided]

Harvesters,

It's that time of year again.

A couple of days ago, I failed my emissions test badly.  Just like last
year, I passed the HC with flying colors, and failed the CO miserably -- at
both 15 and 25mph. (In SL county, they recently changed to a dyno setup
where the tech actually drives the wheels to specified speed, rather than
just revving the engine to certain rpms.  Creeping Californization.)

From everything I learned last year from failing four emissions tests,
excessive CO means a rich mixture.  Only problem is, I'm already jetted
really small (50s on a 2210c -- the jet sizes do go down into the 40s, but
I would have to special order them).  A guy at a local speed shop suggested
my power valve is blown, and sold me a new one.

Aside from installing the new power valve, is there anything else I should
do to lean out the mixture?  Screw in the idle mixture screws, obviously.
New air filter?  Is higher or lower octane gas better?  One of those fuel
system cleaner products?

Also, should I retard the timing back towards spec?  It's at 12btdc now
(with no pinging).  I can't see how timing would affect richness, but it's
really the only change I've made since last year (except for driving it for
a year.)

Thanks for any help.

Rob H.