Fix Fault Mcculloch B25ps

mcculloch strimmer used straight petrol in two stroke engine went for 5mins then cut out & now wont start after i've put in right mix the strimmer is brand new

i cannot get my strimmer to start,we hav stripped it down,cleaned everything,put it all back together,it starts for about 5 seconds then stops,then wont do a thing,just dont know what to do next,pleas ... more