Open the HSN 1/4 and run another laps and see how long the idle will be high until it drops again. If does not make any difference you might be having the self turning HSN syndrome which happens quite frequently in (.21 and above) engines when the HSN seat is worn out or the o'rings are square (too old). When this condition happens although you might always set the needle right, after running the engine high revs the HSN starts closing spontaneously due to very high vibration frequency and leaning the engine overall fuel flow and makes you think the LSN is wrong. If you want to know if you have a self turning HSN just mark the needle position with a pen marker and run feel laps. if the needle is moving even a bit, replace either the o'rings or the whole needle seat. The best way to test this is set the carburetor to the factory settings or any settings you remember the engine will be rich enough to start easily and check the engine behavior from cold to hot and see if the HSN will turn. This is one of the reasons most airplanes have a ratchet on the HSN, it is easy to do small increments and also prevents the needle to turn spontaneously under high vibration.