If you are blocking off the carburetor with cardboard until only a small sliver is showing, is the engine speeding up as you block off more and more of the throat? If so, you are lean. If not, then the mix is correct or rich. Easy to tell with that test if it is lean, not so easy to test for rich.
The sounds you describe (hiccup in sound of air entering carb) sounds like a intake valee leaking. Go by a cheap vacuum gage from Auto zone. Around $10. It will come with a paper detailing about 30 tests you can run with one simple hookup. It will even test your fuel pump.
Anyway,. hook it to the intake at a vacuum source. (Must have vacuum at idle) Needle should read around 18-20 inches of vacuum at warm idle/sea level. If needle has a constant pulse downward you are seeing the results of compression leaking past a valve into the maniifold. Quck, easy, accurate test. Wont fix the valves but will confirm or disprove the idea.
