If the idle climbs up in RPM and stays there, it typically tells you that the low-speed mixture is too lean. If it idles high after cleaning out the engine, then gradually returns to a normal idle, then the main needle is a little lean. If it does anything beyond that, then it's time to check the fuel system. When the idle fluctuates like you've described, then there's usually a pressure or vacuum leak somewhere. Check that all your fuel lines are properly attached and that there's no cracks or cuts in them. You also want to be sure the fuel filler cap is completely closed and sealed. If the pressure line from the exhaust or the filler cap aren't sealed properly, then the amount of fuel going into the engine will fluctuate. A loss of pressure in the fuel system usually makes the mixture lean, which is accompanied by an increase in RPM.

Less typical but still a possibility is a vacuum leak in the carburetor or around its base. If this isn't sealed well, you can experience the same symptoms. Start by checking all the obvious stuff like making sure the the carb fastener is tight and that the carb is fully seated in the block. If that's OK, then be sure that there's no tears in the bellows seal on the slide valve, and that the main needle assembly isn't loose. You can go through the carb and seal everything with a little silicone sealer, but only do that if you're comfortable taking the carb apart. The only thing I caution here is to use TRACE amounts of silicone sealer. I've seen many "seal" jobs that end up causing other problems because WAY too much sealer is used. Only a very small amount of sealer is needed.

A third possibility is that the carb simply isn't closing all the way when you let off the throttle. If the carb is dirty and you're not using a carb return spring, it might simply be a matter of the carb not closing down complete until a few seconds after letting off the throttle. Vibration will usually cause the throttle to eventually close, so just be sure you haven't let the maintenance go so far that the slide valve is simply unable to move quickly and freely.