A MOONEY PLT EXPERIENCED A FAILURE OF THE FUEL MIXTURE WHILE IN N90'S AIRSPACE AND RETURNED TO TEB.

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ACFT WAS IN CRUISE AT 5500 FT. THE ENG FUEL MIXTURE HAD BEEN LEANED TO 50 DEGS RICH OF PEAK EGT. WHEN BEGINNING DSCNT TO MY DEST ARPT; I BEGAN TO ENRICH THE MIXTURE. I NOTICED NO DROP IN THE EGT. AS DSCNT PROGRESSED EGT BEGAN TO RISE. I MOVED THE MIXTURE CTL LEVER TO FULL RICH BUT EGT STILL CONTINUED TO RISE WITH DSCNT. I ARRESTED THE DSCNT AND PERFORMED A SLOW CLB BACK TO 5500 FT WITHOUT ADDING PWR. EGT RETURNED TO 50 DEGS RICH OF PEAK. THIS WAS CONSISTENT WITH A BROKEN MIXTURE CABLE. I DECIDED TO RETURN TO TEB. I TURNED TO A NEW COURSE DIRECT TO TEB AND DSNDED TO 4500 FT. PRIOR TO ENTERING THE CLASS B AIRSPACE OVERLYING TEB; I CONTACTED NEW YORK APCH; EXPLAINED THAT I HAD LOST MIXTURE CTL WITH THE ENG FUEL MIXTURE LEANED FOR CRUISE AT 5500 FT. I REQUESTED ENTRY INTO THE CLASS B AT 4500 FT AND A LOW PWR DSCNT WHEN CLOSER TO TEB. I ALSO ASKED IF NEW YORK APCH COULD ARRANGE WITH THE TEB TWR CTLR; A STRAIGHT-IN APCH TO RWY 24 WHICH WAS IN USE AT THAT TIME. THIS HE WAS ABLE TO DO AND SHORTLY I WAS TOLD TO CONTACT TEB TWR. THE APCH AND LNDG WERE ACHIEVED WITHOUT ANY PROBS OR EXCESSIVE EGT. SOME MIGHT ARGUE THAT I SHOULD HAVE LANDED AT THE NEAREST ARPT. I DON'T THINK THAT THE PROB PUT ME OR THE ACFT IN ANY IMMEDIATE DANGER. I FELT THE LONG; LOW PWR APCH WOULD NOT POSE ANY RISK OF DAMAGE DUE TO EXCESSIVE EGT.