I assume you mean the metering diaphram's inlet valve (the little one with the rubber tip, small lever and spring). This can only be changed by slightly bending the little lever or teh spring. Some folks think this setting is a tuning secret, and they go to great lengths setting and measuring "pop-off" pressure. Truth is it's not that critical, and if the lever is sitting as per spec it'll do it's job.

See this:
http://www.geocities.com/farellus/idle_adaptation.html

Hope it helps,

J