i would say after the engine is totaly break in ...after the first galon ,,,yes you can...i aways remove a shim after 3-4 ltrs---...but i stay at 25 fuel ,,,i wouldnt recomend someone to remove a shim while using 30 fuels...and ofcourse after removing a shim you always start with checking how it responds ..looking after the temperature and hearing for strange diferent noises ,,,..there are sometimes that after removing a shim you realise that you need a new rod or a new main bearing...in some engines you need a diferent type of plug...litle colder..sometimes its all superb ...just at the begining after the shim take out you pay attention ,,,,,,,,,then you enjoy ...a more alive engine ,,,and always better botom end...