helo! check mo yung vacuum diapragm valve mo and un line.usually rubber ang linyada.kung minsan kasi may leak at kung intact un line without leakage mostly diapragm valve ang diperencya or stucked up.
Normally pag switch on mo ng aircond,mmaintain ng diapragm valve mo by vacuum un RPM ng engine thru a linkage sa acelerator mechanism.Pag defective un diapragm,d stable and ENgine RPM usually pag automatic on & off and aircond.

pag na check mo na at same condition parin,go kana sa advice ni Ianrex carb cleaning and carb nozzle check.

hope this can help you....