An engine that won't go rich, if all the fuel passages are clear, could be leaking around any of the gaskets, also. Backplate, head gasket, carb gasket and neck, any carb fittings, glow plug seal (and center post) are all suspect. Many of the modern carbs have little gaskets where fuel nipples screw in, o-rings around the needle valve(s) or in the needle valve housings too.
Also, just the other night, I had an intermittently lean/rich needle setting - pulling the fuel nipple off near the high speed needle, and out of the chamber came a small blade of grass or some other vegetation that must have been floating in the chamber and sometimes obstructing, other times not. Could even be just some old congealed castor in there.