Ryobi Scorpion Whipper Snipper will start but won't keep running?

Engine starts ok, runs for a very short time...(approx 15 to 30 secs) then stalls. I have adjusted the idle mixture screw and throttle screw as per the manufacturer's recs but cannot keep the engine running (there is another adj screw which the manual says is preset and cannot be adjusted). I noticed a small tear in the carby diagphram. Could this be causing the problem? Is it worth replacing the carby or buying a new trimmer? (Trimmer is ~ 14 yrs old)