Gas leak from carbs

When Ilet the bike sit without turning the petcock off
any suggestions as to what is wrong?

weak float valve springs

It has generally been stated here (especially if you don't have a vacuum operated petcock) that this is normal due to the pressure of gravity. It is recommended that you shut off the fuel when the bike is not running.

But as TB pointed out, the spring in the float valve is probably weak from age and use.

When float valves and float mechanism are both functioning perfectly, fuel should remain at the service level inside the float bowl without reaching the overflow circuit.

Some gruesome details ---

Float valve consists of a seat (with an orifice) and the pointed needle that fits into the seat.

Fuel entering the carb goes first through the float valve, and is supposed to stop filling the bowl when the float lifts the needle which presses into the seat.

The least failure of complete seal between the needle and seat will allow fuel to keep flowing through the float valve into the bowl. And eventually the fuel level rises far enough to reach the overflow circuit.
Such failure to completely seal often results from crud or speck of grit at the interface between the needle and orifice.
Good reason to install an aftermarket in-line fuel fuel filter, necessary to supplement the coarse screen filter in the petcock.

Typically, the overflow circuit exits from a nipple on bottom of the bowl where a hose attaches and runs over the swing arm to discharge underneath the bike.

Some carb designs do not include an overflow circuit.

Good Fortune!

Here's an example of manual slide carb.

[Click on image to enlarge view.]

Here's an illustration showing how the float (float tang) works with the float valve to control fuel flow into the bowl.

[Click on image to enlarge view.]

My carbs do this too, but I plan to go through them thoroughly this winter and replace those assemblies and check the jetting. I just turn off the petcock after riding.

All of the above but lets not forget about filling the motor completely with gas.:ohmy: Keep an eye on the oil level and give it a sniff.

Thanks a lot for the valuable info , I'll take these suggestions in consideration when trying to fix this situation , probably dirt in the float nneedle orfice.
If I didn't mention before,my bike is a stone stock 1975 Z-1 B


Thanks>>>>>>>RonBo