Carb idle issues as car warms up

So, I've got a Pierburg 1B3 carb on a 1.5 JB engine. It's been running okay, I went out in it last week.

Went to start her today, perfectly mild morning. Pulled the manual choke out, got her started idling around 1500 as the engine warmed up.

Then as the engine warmed, the rpm dropped. I pulled the choke out more until it was fully shut, but the rpm still dropped. Tried adjusting the fuel mix screw and the idle bypass to no affect. Eventually she dropped too low and was stalling.

When applying my foot to the accelerator in neutral, it'll rev up to 3000 rpm fine and stay there, though does sound a bit sputtery/lumpy, maybe because it's not warmed up.

But it will not idle with full choke in neutral.

Honestly confused as it ran fine a week ago and I've not changed anything. Tried again this evening, same deal. Cold, choke full out, idles around 1000.

Used to be idle when warm with choke full in was 900, and at cold I only pulled the choke half way and it'd idle at 2000 rpm.

Really confused what's changed with it since it's just sat on the drive for a week!

So it idles too low when cold and when warmed up? How about when you drive it when warmed up with the choke pushed in?

I'm thinking some debris (eg rust particle) may have got stuck in idle bypass jet.

Hmmm - rust particles getting drawn from the fuel tank ?
Vacuum leak ?