there should be a spring of some sort holding the idle screw in tension against the carby housing if this is in place then try putting some plumbers tape on it as the thread may be poor and plumbers tape will give it a bit more bite

if that dont work try draining the carbys by loosing the screws at the bottom of the bowls to drain the crap out, if this dont work check your air filter and give a clean if that dont work maby your carbys need a clean? all else fails find a better mechanic what did your last service include..... fuel injection and intake wise that is?
cheers Tim