"K. Smith" wrote A 101 so that even you may get a
rubimentary understanding;

(i) The only way it seems a crankcase transferred 2 stroke can viably
get through the EPA rules is by running extremely lean mixtures at low
to medium revs. In the early days OMC dealers were claiming 40-1
mixtures but once the failures started all hard technical material was
withdrawn.

Mixture has nothing to do with meeting the EPA requirements in any direct
sense.
The problem with the conventional ported two stroke outboard is emissions of
unburned hydrocarbons. Since the incoming fuel/air charge is used to
displace the exhauste gases. Approx. 15% of the fuel/air mix passes in thru
the intake port and out thru the exhaust port without the benefit of having
been in the cylinder during a combustion phase.
Hence ways to significantly reduce the amount of unburned hydrocarbons in
the exhaust
a) seperate the airflow/scavenging from the fuel.
b) switch to a four stroke process where the piston scavenges the cylinder.
c) post burn the unburnt fuel in a catalytic converter.