1990 Mazda Miata converted system R12 to R134

My compressor cluth locked up. So in replacing the compressor and clutch as they are one pc we converted to R134 since the compressor came loaded with 134. Found hole in evaporator and replaced it along with Receiver dryer and expansion valvle. Does not cool below 52 and seems to go between 52 and 56 depending on ambient temp. ANy ideas as to what could be wrong.I was told the conversion would be slightly less effecient but not really noticeable. But now we are all scratching our heads as to how to resolve the problem