Bill - you pretty well summed it up; some of the "adjustable main jets" are complete (different orifices), others are an "adjuster" to lean the mixture of the main jet (still in place). As I posted, not a one-size-fits-all-works-well-on-nothing.
Additional research with the part number of the carb would be involved; plus one needs to look at the carb in question to see if the original main jet is still in place.
Sometimes, folks will replace the main jet with a LARGER main jet, then add the adjuster.
Jon.
