PLT OF SMA DECLARES EMER TO AVOID TSTMS BUT HE FAILS TO SQUAWK ASSIGNED CODE AND ATC IS UNABLE TO IDENT HIM.

Narrative

I WAS DEPARTING FROM THE FORT PIERCE; FL; ARPT (FPR) ON AN IFR FLT PLAN. I WAS PIC OF AN EXPERIMENTAL LANCAIR (MODEL 360; MARK II). I ERRED IN ENTERING A XPONDER SQUAWK CODE OF XYZZ WHEN IN FACT MY IFR CLRNC DIRECTED ME TO ENTER A CODE OF XYYZ. TSTMS WERE FORECAST FOR THE AREA AND ARE; OF COURSE; EXPECTED IN THE AFTERNOON IN FLORIDA. THE SKY CONDITION WAS SCATTERED CUMULUS CLOUDS WITH BASES ABOUT 2000 FT AGL AND VISIBILITY IN LIGHT HAZE OF ABOUT 10 MI. MY IFR CLRNC WAS TO DEPART FORT PIERCE ARPT; RECEIVE RADAR VECTORS N TO THE VERO BEACH VOR AND THEN CONTINUE ON N ON MY FLT PLAN; AS FILED. I DEPARTED RWY 9. AS I WAS BEING TURNED OVER TO ZMA; I SAW THAT I WAS APCHING A LARGE DARK CUMULUS CLOUD OFF THE COAST AND I ANNOUNCED TO ZMA THAT I WAS READY TO TURN N TO VERO BEACH. ZMA TOLD ME TO MAINTAIN MY CURRENT HDG AND CLB TO 4000 FT. TO AVOID FLYING INTO LARGE DARK CUMULUS CLOUD IN THE AFTERNOON IN FLORIDA WHEN TSTMS ARE NOT ONLY EXPECTED; BUT WERE ALSO PREDICTED; I DECLARED AN EMER AND LEVELED OFF BEFORE ENTERING THE CLOUD AND TURNED S TO FURTHER SEPARATE FROM THE WX. I EXPLAINED MY REASON FOR DECLARING AN EMER (TO AVOID FLYING INTO A CUMULUS CLOUD BUILDUP) TO ZMA. I CANCELED MY IFR FLT PLAN AND DECLARED MY EMER SIT RESOLVED WHEN CLR OF THE WX. A SHORT TIME LATER; ZMA ASKED ME TO RESET MY XPONDER TO XYYZ. I CORRECTED MY XPONDER SETTING AND ZMA ANNOUNCED I WAS RADAR CONTACT. SHORTLY THEREAFTER; ZMA TOLD ME THE REASON FOR MY LONG STRAIGHT-OUT DEP LEG FROM PORT PIERCE ARPT (RATHER THAN A TURN N) WAS DUE TO THE CTR NOT HAVING RADAR CONTACT WITH ME DUE TO IMPROPER XPONDER CODE. I APOLOGIZED FOR ANY INCONVENIENCE I HAD CAUSED THE CTR AND CONTINUED ON VFR.