Over the years with these little engines I've found that whats common about them is a VERY relative term. things like temeperature humidity and fuel quality can effect operation from one day to the next also if it's a new engine and not yet broken in performance can be erradic the best advice I got is reset the high speed needle to the number of turns suggested in the instructions and try again.

Some of the best days of my life were spent fiddling with a Cox .049 in a field... go figure