SHA 14:12 is a normal straight exchange carb for the Gurtner, for a stock AV7 you need a 53 jet.
Check for leaks at the carb to intake union, the shim there should only be to get the carb to grip properly, for a proper seal throw in a 5/8 x 13/16 x 3/32 oring.
Give the motor a leak down test (not by spraying aerosol shyte around the motor but a proper one) to check if the spi seals or gaskets on the engine are leaking. Get a new crush gasket for the exhause and remove the exhause and give it a goodly clean out.
Check the timing, best running is with it set at 0,4mm at index and initiation at 1,5mm BTDC.