KZ400 runs 60 seconds, dies...

After I brought home my new, old 1975 KZ400 on Labor Day, y'all -- along with my brother, a very fine mechanic himself -- convinced me what I thought was an electrical problem likely was fuel delivery-related, instead, here:
kzrider.com/index.php?option=com_kunena&...ew&catid=4&id=397358 .
You were probably right. However, the bike still isn't.
I've cleaned the carbs twice (no new 'kits', yet, so have not rebuilt them), and after both times the bike has started back up with little problem. But, after running 60 seconds or so, it simply coughs, fits, and stops. Wait anywhere from a few seconds to a minute, and it will restart several times before it finally -- or I, hard to tell which (starter motor doesn't work, so I have to kick it) -- seems to become too worn out to bother anymore.
Today, after the second cleaning and reinstallation, I also tore the fuel tank petcock down completely (again, no rebuilding supplies), blew it out with compressed air, reassembled, and refueled. About the only difference -- likely from the carb cleaning, not the tank/petcock -- is that the first 'firing' resembled idling more than it ever has before. However, it died again almost exactly 60 seconds later.
Obviously, it's getting at least some fuel, or it wouldn't ever start. Spark, too, is undeniably present. My brother opines that any vaccuum leak in the airbox boots (fuel lines and in-line filters are brand new) shouldn't be enough to stop the firing cold if it fires at all, and that simultaneous stoppage of both cyclinders signals fuel-delivery problems, not electrical.
Now, I AM going to change out all the rubber eventually, and rebuild the carbs and petcock. But, are those going to be precursors to getting a test ride, rather than a tuning up after I can evaluate the rest of the bike? Frankly, I'd sure feel better about the expense and time if I knew that the machine would go down the road once it's done.

With no more than a 1/2 tank of fuel, did you leave the fuel cap open and try to get it to keep running? The cap is the vent, no air in= no fuel out.

Motor Head wrote:

With no more than a 1/2 tank of fuel, did you leave the fuel cap open and try to get it to keep running? The cap is the vent, no air in= no fuel out.

Thanks, but, yep, tried it both ways -- cap closed, cap open. Same result.

So if it is still firing, and you think its fuel, have you added some fuel to the carbs through the airbox immediately after it stalls out?

Motor Head wrote:

So if it is still firing, and you think its fuel, have you added some fuel to the carbs through the airbox immediately after it stalls out?

Ummm, no -- and I'll admit right away that that is above my mechanical pay grade (though not that of my brother -- alas, he's in Michigan and I'm in Tennessee).
What, exactly, would that tell me, and how would I do it (just in case I get desperate, I mean )?

Well I was trying to see if when it stalls out, will it re-fire with some fuel added, if so It would be a fuel problem. If not it may very well be electrical.
When you cleaned the tank did you remove the pet cock and clean the screen that's in the tank? Also if you take a container and hold it under the fuel line, with it disconnected at the Carbs, does it flow freely, say 1 qt in a minute or so?

If not already done, could test run with petcock set on RES (reserve) position.

Does problem persist?

Good Fortune!

Motor Head wrote:...When you cleaned the tank did you remove the pet cock and clean the screen that's in the tank? Also if you take a container and hold it under the fuel line, with it disconnected at the Carbs, does it flow freely, say 1 qt in a minute or so?

Yes to all -- though the rate you state might be a little faster than I get, and one line runs more steadily than the other.

Patton wrote:

If not already done, could test run with petcock set on RES (reserve) position.

Does problem persist?

Good Fortune!

Hast hit it! (Though, giving myself a little credit, I didn't see this post till AFTER I'd done it!)
I'd been into the carbs yet a third time this afternoon -- checking/re-checking, and assuring float-fuel level (which was a bit low), and it started right up on the first kick (as previously mentioned, electric starter motor is going to require a rebuild, though relay seems okay -- one click, no ohms). It died EXACTLY 60 seconds later! Okay, I've done everything I can do short of rebuilding the carbs, I thought, so let's try something else, because it's obviously running out of gas. So, what's the best way to get it the most gas possible? Thus, I turned it to 'reserve'.
Darned if it didn't run about 3 minutes -- until I got 'brave', and tried to idle it with the choke off (ran okay that way with a little throttle, though). Repeated the process a couple of times.
Obviously, I've got some adjusting to do yet, aside from the fact that, apparently, none of the contemporaneous Kawasaki service issues -- like the carb-mounting plate -- was ever done. This bike appears to be 'original' in the worst way possible!
Still, it looks like if I can be patient enough to let it run with the choke until I can add some throttle, I might get a test ride attempt in tomorrow. We'll see.
And, I'll start looking for a petcock or petcock kit, either for this bike or the compatible Honda ones (early CBs).
Thanks, guys, for the input.

I know you said you cleaned the carbs, did you remove all of the jets and blow some carb cleaner through everything? did you remove the pilot jets and clean them as well? sorry just had to ask...

Tell me that when you had the carbs apart you took the float needles out and blew air threw them outwards???

It wouldn't be the first time I've seen the inlet full of snot, even seen a couple that required removal of the needle seat to get it clean enough to allow proper fuel flow into the carb bowls.

Just so that I have a clear picture of things...

You can start the bike repeatedly with the choke on.
When you try to turn the choke off it dies?
Or it just dies on its own with the choke still on?

Is it stammering?
Does the exhaust sound smooth, or is it popping?
Do both the exhuast pipes get hot? 60 seconds is deffinately long enough for them to get scorching hot.

Generally, if a bike will idle endlessly with the choke somewhat on, but dies when choke is turned off, it is becuase the pilot jets are not shiney clear and new like.
It will sometimes die when pushed back to full choke, as this can be too much fuel as cylinders get hot.

Remove the pilot jets, and hold them up to a light. you should be able to clearly see light through the jet, and when held at the right angle, you should see the taper inside nice and clean (if your eyes are good enough)

If they're clogged and you cant seem to clear them with spray/air, then poke a single bristle from a wire brush through. The spray some carb cleaner through and let it sit a while to eat the crap in there, repeat several times till shiney inside.

Choke circuit completely by-passes pilot system, thats why it starts/runs for short period.

Best of luck.

I'm pretty much convinced the carbs are fine now -- and, yes, I removed all the jets and examined/cleaned them individually. Both times.
Anyway, once I'd started it with the petcock on reserve it ran each time until I attempted to get an idle without the choke. It idled with the choke, and ran fine without it with some throttle. I just never got it to idle without the choke.
Alas, it doesn't run at all currently, unrelated to the previous problem.
I decided to replace the contact points with newly-ordered ones, and then reset the timing. However, it was only after I'd removed the points and took the new ones out of the box that I realized they weren't the same set. Not even close, really. Either the dealer sent me other than what he'd advertised (and I didn't it fit on the plate securely), or someone has adapted non-original points to the bike. In any case, I reinstalled the used points, but it hasn't started since -- no sign of spark -- so I'm pretty sure I bollixed up the timing. Haven't had much time to fiddle with it since.