TOPIC:

Cylinders 1,2,4,5,6 firing ok on 3.5 turns on air screws. Cylinder 3 will run ok only for about 30 seconds after turning the #3 air screw but #3 won't stay running longer than a half a minute. #3 carb passages blocked?

Just a thought, but I had this happen on another bike, and it turned out to be the sparkplug breaking down when under load / when it got to hot. Fingers crosssed it turns out to be an easy fix for you.
Cheers Tim

Sounds like you may have a blocked passage in there or maybe an issue with you primary circuit diaphram (the small one on the side of each carb body).
Do yourself a favour and strip the carbs completely for inspection and replace anything which looks suspect.
Whilst they are apart, soak the 3 carb bodies in a bucket of deisel for a week or so, moving them around to change their position each day or so. The non aggressive detergents in modern deisel will loosen anything inside without doing any dmamge to the alloy.
Once soaked, blow EVERY orifice out with aa 100 PSI line with a pointed end............you will be surprised what you find in the bottom of the bucket after removing the bodies, and what shoots out of the orifices with the 100 PSI.
I have done this with EVERY jap bike I have owned including many Z1300's and its the only way to be sure the carbs are 100% clear.
Even Sonic Cleaning does not always release ALL the crap from inside............but this method does.

Pete F
UK

Less than a 1000 miles on my spark plugs and the carbon on #3 plug stopped a regular spark. I have been running rich the whole time. Replaced #3 plug and shes happy. Pulling the carbs off again for cleaning...3rd times a charm.

Did you try to change the spark plut to ensure it is not the root-cause ?

Whenever I had problems with a single cylinder it used to be the spark-plug or the plug-connector

Holger

The over-rich condition you're experiencing could be a different anomaly that I've only seen once. During a sync and final set-up on a set of carbs, I discovered that one idle-air screw would not bleed the correct air-mix when set at the typical 3.5 turns. This particular idle-air screw required 4.5-5 turns to add enough air to run the correct mixture. A close inspection of the needle and the carb-body showed nothing unusual. The port at the bottom of the needles "home", the internal threads as well as the needle itself; inclusive of threads and tip showed nothing that could explain this. In this case 3.5 turns was too rich but the 4-4.5 turns made the idle and operation correct.
The only answer to this that I could come up with is that a P.O. had cleaned the port at the base of the needle seat, with a tip-cleaner? tiny #'d drill? or other form of "tool' A mere increase of 1/1000" in the ports diameter would certainly create this condition. More air was required (more turns out on the idle-air needle) to make up for the increase in idle-fuel.
This is why only a strong solvent such as lacquer-thinner should be used (especially in a sensitive "metered" area) to clear a blockage. And of course along with "enough air to supply a planet"
Something else to check: Another set of carbs I was rebuilding had another issue which could have caused an excessively rich idle mixture. I had to remove insect pupae carcasses from the air induction ports. The air required for each idle needle and the emulsion-tube in the high=speed circuit is supplied by it's own port on the intake end of the carbs. There are two such air supply ports for each bore. The upper port supplies air to the idle-circuit. Do not be disillusioned by its substantial diameter. Deep inside this port is a metering jet! It is as equally sensitive to blockage as the idle jet and the idle fuel port & 3 transition ports in the throttle-bore itself. Not enough that these bug-bits were cutting off the air supply to several idle and high speed jets; the tiniest fragment could have blocked these air-metering jets. No air - RICH !
Something to think about, if nothing else.
The '79 I had for merely months before dropping it unceremoniously back in the dealers lap was a good example of how 'soot" from an over rich mixture can render running and starting impossible. The '79 was SO rich ("right out of the box") that a new set of plugs (all 6) would be sooted solid in less than 50 miles. Then it would not so much as fire.
I don't own a "color-Tune" but can clearly see the advantage in setting the idle-air screws to that "perfect" setting.
And YES, My ears are burning. : "Scotch doesn't have a "color-tune" ?!?! It's on a list-somewhere!

Regardless; lets us all know how this turns out (no pun intended)