I have never seen two throttle return springs in a dozen or more carbs. It may be that they added one to overcome some resistance in the cable. The throttle cable will often get hung up from a number of things including kinks, crud and rust. You may need to check the cable and make sure it is not kinked or crudded up. The little piston on the top of the carb that the throttle cable moves up and down can be sticky as well.

Did you knock the emulsion tube out of its seat? That is a necessary part of cleaning it. You need a small dowel or a golf T and tap it out from the top where the little piston seats. Once out check the tiny O-ring that holds it in place. It may need replacing.

To test the richness of your fuel mix you will want to give the bike a hard run for a mile or so and then pull over and pull the plug. I would think think that idling it would produce a misleading, darker plug indicating a too rich mixture.

Hoo