‘81 KZ440 hanging idle when hot

Hello everyone!
I’m new to this forum, read alot on this forum, and thought it was time to ask for some help.

I am having some troubles with a KZ440a i just bought.

I bought it as a non runner, previous owner said it just died on him and he couldn’t figure out why.

I bought the bike as follow:

Stock engine, stock carbs, stock jetting but with pod filters and open exhausts.

I quickly got the bike running again by cleaning the carbs. They weren’t that dirty at all, but whatever, it fired up right away.

Went for a ride and noticed when the engine all warmed up, the revs would hang if i blip the throttle in neutral, or when holding down the clutch.

I understand this is a lean condition. However...! The spark plugs are black if i let it idle high for a min, then chop it. New plugs.
Also when i cut the ignition, it pops 2 flames from the exhausts.

Also noticed, when it is idling for 10/15 min. It will just stall, when i try to start it again, it barely idles. Most of the times it stalles right away again. Yesterday when i cranked it again, i got backfire coming out of the vaccuum nipple of one of the carbs ( i can tell because my vaccum nipple plug-off is a piece of transparent hose wich i capped off.

It does not react to brake cleaner or other sorts of flammable stuff when sprayed upon the carb holders so doesnt appear to have a vaccuum leak.

Also, i sourced an original air box, complete system, but no changes in this issue whatsoever.

The mechanical ignition advancer is operating really smooth, so i dont think it is hanging at revs. Cleaned and lubricated it anyway.

Valve clearance is spot on.
Distribution timing is spot on.
Ignition is electronic, so no points or timing that can be changed.

Anyone have any idea’s?

A rich condition at some throttle setting can make them black while a vacuum leak will make it idle lean. Being lean at idle won't clean the plugs necessarily if there is a rich condition elsewhere.

If we assume it's carburetion, with the airbox and stock jetting, the most likely culprit is vacuum leak, but it could also be the diaphragm is punctured and not pulling the slide up.

You could have more than one problem. The stalling and roughness while hot could be a pickup or coil.

I assume it is a carburation problem as the first thing i tried when i just bought the bike was to spray some fuel in the carb openings and the bike would then run for a few seconds, before it wouldn’t even fire up.

I cleaned the carbs and installed new o rings etc, bike then fired up and ran. So i think there may have been some dirt left in them, so today i cleaned them again. And leak tested them, with a smoke machine. I did notice just a real little bit of smoke leaking from on of the pilot screws, so i ordered a new carb kit again.

The diaphragms are in really great condition, and everything operates smooth as it should.

Just to be sure i ordered a set of new intake boots as well.

The ones on it right now have little cracks around the end of it,, near the flange. It doesnt react to brake cleaner tho.

But new ones are on the way anyway.

Thanks for the reply, makes a lot of sense about the rich condition at half to WOT. Need to take a look at that next, maybe it’s because it has open pipes on it, i did install DB killers to get some back pressure again.