Bill
I had a homelite do about the same thing .it had something in the hi speed needle
passage blocking it it would run for a short time on hi speed then act fuel starved
and quit,if I ran it on 1\2 choke it would run ,then quit as soon as you opened the choke.
Icant believe it would be a timing isue because it runs ok for a brief period of time,
try a smaller pitch prop,and see if it is still the same. clean the carb pull it apart blast
everything metal with carb cleaner blow out needles with compressed air ,reassemble
and try it again.(please take appropriate safety measures safety glasses,gloves etc.)
to check for an air leak ,spray gasket joints with carb cleaner as engine run ,listen for
change in rpm's, the rpm's should jump up breifleyif you have a leake, it doesnt sound
like one but if all else fails start at the begining .