TOPIC:

My KZ400 starts fine (if it's very cold you need choke) but for about the first 15 minutes of riding I have to keep the throttle half open otherwise the engine stalls. Once it has warmed up, it works perfectly, totally stable idling. But if I let it stand still for a while the engine cools down and when starts tends to stall until it warms up again.

I have not given it much importance because I thought it was normal (I restored the bike 3,000 miles ago and I don't know how it worked before this), and because it is also a good way of know if the engine is hot and ready to rev up. But almost everyone talks about "30 seconds on choke, then remove and voila, stable idling, ready to run".

Does anyone know anything about this or has it ever happened to them?

Engine should be hot enough to vaporize the fuel.
I feel the engine cooling fins with my hand.
When the fins near the carbs are warm/hot, the bike is ready.
Your times will vary each time you start the bike.
Others will tell you their methods.

It shouldn't take 15 minutes of running before the bike will run without choke.  More like 5 minutes.

Where are the pilot screws set?  Is the bikes intake system stock?  Airbox installed with a proper air filter?

I only use the choke for starting, for about ten seconds, then continue to open throttle so it doesn't stall.

One turn on pilot screws, both carburettors. Intake system stock. I have to check the filter but I think I put the right one.

1 turn out on the pilot screws is not enough.  Try 2.5 and see if the bike runs better.  And don't be afraid to run the bike on choke longer.  That's what it's for.

Thanks, I'll try with the pilot screw.

I'm also going to investigate how to make the choke non all-or-nothing, as I observe that it is sometimes more useful somewhere in the middle but tends to go up itself.