Carb Floats...again

If the mixture is rich on one side I do not think it will show on the balance Gauges.

You could pull a plug from each side and there MIGHT be a noticeable difference.

It should also show up in the Exhaust Gas Temps as a colder exhaust.

If you do not have a full set of EGT gauges a handheld IR Temperature Gun MIGHT Work.

Just throwing ideas at the wall to see what sticks.  

You don't specify the kind of aircraft you're dealing with but checking the carbs' float weights, verifying the absence of float rubbing, and ensuring that the carb bowl gaskets are properly seated are relatively easy and cheap to do (assuming you are legal to do such things yourself) -- that would at least get rid of one possible issue.  Is there any fuel odor or evidence of leakage out of the carb bowl vents?  Remove the air cleaners and reach in the carbs' throats to feel if the carb pistons move up and down smoothly without any sticking.  Check that your two throttle linkages are coordinated in their movement throughout their full range of motion.  Same for the starting carbs (i.e., chokes) -- are the starting carbs fully closed and sitting against their stops when the "Choke" is Off?  Is the carbs' crossover tube secured and leak-free?  What idle speed are you using?  On my RV-12 / 912 ULS the idle is set at is 1,650 rpm but I endeavor to keep the idle at 1,800 rpm or more to minimize gearbox issues -- too low an idle rpm can be problematic.

So I think I found the problem: neither enrichment levers are hitting the stops in the off position. I'm guessing that they are "leaking" during low power operations. Has anyone found a stronger return spring that will overcome the friction of the cables?

I do not think you should rely on those wimpy little springs to fully close the starter carbs' enrichment valves.  The majority of installations I have seen use the "Choke" actuation cables themselves to push the valves closed -- the springs may add a little extra closing power but not enough to overcome normal cable friction.  I suggest that when your in-cockpit "Choke" control is OFF the cables should push the valves fully closed. In addition, I add a little extra cable push length to ensure that the cable has truly pushed the lever closed against its stops.  Also, make sure the levers' movement is the same for both carbs and their action is coordinated.

Now after a whopping 27 hours( and 4 months) of use on a new set, I have to change the floats again. Left side weighed 9 grams, right side 11. Has anyone experimented with drying them out, maybe in a warm oven and reusing them?