A C150 PLT LANDED SHORT OF HWD DUE TO ENG ROUGHNESS.

Narrative

I WAS CLRED TO LAND AT HWD; WHEN I EXPERIENCED ENG PROBS -- THE ENG BEGAN TO RUN ROUGH. NOT WISHING TO PASS UP A GOOD LNDG SITE; AND POSSIBLY ENDANGERING MYSELF AND OTHERS; I PERFORMED A PRECAUTIONARY LNDG AT A VACANT FIELD IN AN INDUSTRIAL AREA OF UNION CITY. THE FIELD WAS FRESHLY MOWED GRASS ABOUT 1200 FT LONG. THE LNDG WAS GOOD AND NO ONE WAS INJURED AND THE AIRPLANE WAS NOT DAMAGED. FURTHER FLT WAS NOT AN ACCEPTABLE OPTION BECAUSE THE FLT PATH WOULD HAVE TAKEN ME INTO AN AREA THAT WAS INCREASINGLY RESIDENTIAL. 2 DAYS LATER; THE MECH CAME TO TAKE THE WINGS OFF TO TRANSPORT THE ACFT OUT OF THE AREA. UPON DRAINING THE WINGS OF FUEL; HE FOUND THAT AT THE TIME OF THE INCIDENT; THERE WAS 5.0 TO 5.5 GALS REMAINING IN THE TANKS. SINCE THERE ARE 3.5 GALS OF UNUSABLE FUEL; THE ACFT HAD 1.5 TO 2.0 GALS OF USEABLE FUEL REMAINING IN THE TANKS. THIS CONFIRMED THAT THE REASON THE ENG RAN ROUGH WAS POSSIBLY CAUSED BY CARB ICING. ONCE THE ENG STARTED RUNNING ROUGH; I CHKED THE FUEL SHUTOFF; CYCLED THE CARB HEAT; AND CHKED THE MAGNETOS. THE ENG CONTINUED TO RUN ROUGH; FORCING A PRECAUTIONARY LNDG.