Good advice so far but I will add my 2 cents worth,
An engine with a too rich low speed setting will load up fuel in the crankcase. So when you open the throttle the engine sucks all the excess fuel and sputters or dies. Also if you are wide open no fuel will build up until you reduce throttle and the build up begins causing rough running or dying.
So set the High speed as instructed. Let the engine idle a while then open it up (not to fast) if it sputters and goes rich or dies turn the low speed setting clockwise a LITTLE BIT like a 1/16 of a turn or less. restart and retest. The closer you get to where yo uwant it the smaller the turns, this process is a PITA. But doing it right will give you a great running engine. Also be aware that an engine that has not been run in adequately may not take the leaner iddle settings yet. Some engines need 3 tanks or so some need a gallon.

Make any errors on the rich side! Because it is hard as heck to put aluminum or chrome back on a cylinder wall![:@]

A day at the field when it is to windy to fly is a great time to run in and dial in a new motor. Running one in while others are flying is sometimes not so well appreciated. go to the far end of the pitts if others are flying. Some fields have designated run in areas.
Do a search here on engine break in and ead up.