Choke Question 1981 KZ750 LTD

I hope this isn't a dumb question, but when I use the choke on my bike, it seems like it is working when starting the bike, but doesn't seem to increase the idle speed. When I first start the bike on full choke, it fires up and then runs on a really low idle (about 1200 rpm). Then when I turn off the choke the rpms don't change. Just wondering what might be going on.

I have an '81 KZ750 and the same thing happens to me. I think it has to do with the fact that it is an actually choke (cutting off air supply) vs an enricher which adds additional gas to the mixture. I have two other bike with an enricher and the idle increases with the enricher on.

I've had this problem also, but found that by bending the stop tab a couple of mm gives me choke and some enrichment by raising the throttle butterflies about 1 mm.
This meant I had choke and also about an extra 1200-1500 ( so about 2000-2500 in total) rpm when the choke is fully engaged.
Cheers
Ben UK