I second the Promax metering blocks. Work great for me.

When checking that the butterflies are closing all the way, have a friend rev the engine while you're working on the engine. Assuming the rpm's will stay high with this method...when allowing the engine to return to idle, but while still idling higher than wanted, press the two outboard throttle shaft attachment points for the secondaries. If it goes back to normal rpm, that is your problem. This stumped me for a long time. I had to grind a sharp edge onto both outboard carbs butterflies in order to fix. All good now.

Another thing to check...don't over tighten the carbs to the manifold. They only need hand tightening with a ratchet extension. You do not need the ratchet. Too tight and the aluminum bodies will warp. That's likely what happened to mine. Once warped, they'll need the fix in my top paragraph. Good luck.