There is no real "factory setting " every carb is different ,what they do is ajust it on a flow bench so that they know it works to a pre determined setting ,thats it. Does the used 421 have really tight compression ? And does it have a 2 needle or 3 needle carb ? need to know this . Mine has a 2 needle and the high speed needle is about flush with the outer body while the low speed needle is about 1/32 in from flush. But mine has well over 13 gallons through it !
I would start with both needles flush , and then tune the bottom and idle screws so it will at least idle for a minute and take the throttle with out stalling , it will be loaded up and cough and sputter but not stall . Be sure your plug is new and at least a #6 NovaRossi plug (factory plug) I use #4 personally . Next you will need to SNEAK UP on a top end setting ! DON'T TUNE it while it is cold !!!!!!!!!!!! You will just make it too lean !!! Get it up to temp -200-240 and tune it at half tank , after all this you pinch the fuel line at an idle 2" away from carb and the idle races your too lean ! it should barely change , use needle nose plyers and squeeze the line just hard enough to close it ,and do this for .5 a second . This engine has incredible power so be prepaired ,usually most guys like it modded so it is smoother from bottom to top. Don't get a EB mod or Hot Mod , they both make the bottom end weak ! RB Mods does a better job on this engine .