THE PORTAL HUB - Pinzgauers, Haflingers, Volvo 303s and more!

Here one for the carb tuning experts out there!
Last night I tuned the 710M and noticed something strange.

The problem:
The idle mixture on the barrel that feeds cylinder #1 seems to be really rich. All the other mixture screws adjusted as expected around 2 turns out but the #1 screw needed to be set at only about 3/4 turn out. It also seems to be extremely sensitive in comparison to the other three.

Here is some background info:
Carbs were rebuilt about 1.5 years ago, civilian ignition & wires, plugs new, air filter new. Prior to setting the carb idle mixtures I checked the timing (set slightly advanced), carb balance is good, and idle is at 850 rpm. The engine runs well, very good power and idles smooth. There is a slight stumble sometimes of idle. The plugs read consitant on each cylinder, they are slightly lean when read after a mid to high RPM run and slightly rich when read after a extended idle period.

Ok, so why is the cylinder #1 idle mixture so sensitive and needs to be set so lean (3/4 turn out)?

Thanks in advance to all comments!

The gasket between the bowl and the base may be coming loose, or the trottle shaft bore may be worn causing a vac leak, or (if equiped), the throttle shaft seals may be deteriorating.