On my DA50 with 2.5" standoffs the choke butterfly barly clears the mounting surface. There is about 1/4" between the mounting surface and back face of the carb body. To get it to run properly i've drilled a 1" diameter hole in the mounting surface of my test stand behind the carb to let the engine breath through. Is this a common practice to have to put a carb breather hole through the firewall on rear intake engines? For my aircraft i'm going to run the 3" standoffs to fit my particular installation. This will leave 3/4" clearace between the back face of the carb and the firewall. Will this be enough or should i add the breather hole too? In general, what is the optimum clearance between a rear mounted carb and firewall.

Thanks,
Pete