Yes, the Unisyns' sensitivity is marginal for Sprites. There's a very narrow range where that adjuster is closed enough to get a reading but open enough that it doesn't affect the carb adjustment. It will work though--you might have to sync them with that little red floaty thing near the bottom of the tube. I suppose that adjusting the butterflies with a feeler gauge is close enough, but the nice thing about the Unisyn is that it measures the air flow directly.
But as for the original question--covering one carb doesn't tell you much. It just means that you're running on one carb. If the engine speed increases, it might just be that the one carb is closer to being adjusted right. Or, there might be something less obvious going on. I've noticed this phenomenon myself--even on a well tuned car, you can sometimes get a speed increase when you cover one air intake.
To find vacuum leaks, use the well established method of spraying something like carb cleaner on the places where leaks are likely. If the engine speed changes, then you have a leak.