I'm assuming you have the stock vacuum petcock, and the clean air system stuff is still connected.
If you check the air cleaner diagram drawing at Partzilla & look at the smog lines, notice there is a T fitting & also a Y fitting closer to the fat air switch valve. I think the line on the Y fitting is for the vac port on the petcock. The lines from the T fitting go to the vac ports on the carburetors.
1981 KZ650-CSR. Competition, Sports & Road.