i would try using compressed air, and shoot it into the orifice of the jets.
also, barrymans b12 chemtool can dissolved things like that if you let the entire carb (rubber bits removed) sit and soak overnight.
when you say oscillate, you mean it revs up and down?
sometimes oscillating idle can be caused by the carb gasket leaking as the carb shakes on it's mounts.
to check the carb gasket for leaks, try spraying a little starting fluid right at the gasket seam with the engine running.
if it revs up, then that gasket leaks.
also, does this thing have a governor? sometimes the governor can cause things like this to happen.
to your original question, once i did just that, drilling through the casting where i knew a jet was concealed.
it didn't really help much though, because once i got in there, it was obvious that it was a non adjustable jet.
alot of times, these carbs don't even have a needle valve, so they aren't at all adjustable.