TOPIC:

Still having no luck with the KZ400. Engine still starting reliably but not revving above 1500 rpm with lots of smoke and bogging like it's over-rich.
Carb pistons are bouncing nicely with revs, but idle needle position doesn't seem to matter.
The thing is, gasoline is spritzing out from carb throats with air filters removed.
What would cause the loss of vacuum into the cylinders?
Anything to look at would be appreciated.

Are you using the stock airbox? If so, any spritz of fuel which is not abnormal should be sucked back into the engine. This is a normal airbox function. Ed

Spritz is definitely abnormal, enough to soak a gloved hand, and visible as a spray; the stock airbox drips out gasoline.
It ran great before rebuild. I just need another zebra to chase down.
Have to tear down the carb again I guess? Is there a passageway in there that is normally inaccessible?

I assume you already checked the valve timing and valve clearance. Obviously if that is not right, it could spit air and fuel backward out of the carb.

But even with proper cam timing, I've seen a few bikes do this. I would speculate there is a reverse wave as the valve closes. Air that was moving into the engine now suddenly hits a closed valve and has nowhere to go. The air bounces back like a spring. I believe that seeing the spritz means it's too rich as you already surmised. If I remember correctly, it happened more with the throttle open a bit.
We had this symptom early in the jetting trials on a early Kz400. After we dialed in the carbs (new jets with pods), the spritzing stopped or reduced very greatly such that it was no longer a problem.

OldguyJoe wrote: Spritz is definitely abnormal, enough to soak a gloved hand, and visible as a spray; the stock airbox drips out gasoline...........

That does sound like a lot. For sure fuel shouldn't be dripping out of the airbox. Ed

Have you checked the spark advance?
Steve

I did indeed. In fact, I even advanced the spark timing by a few degrees but to no avail. It's back to factory setting now.
I'm going to try retarding the valve camshaft by a few sprockets to test loudhvx's idea.

I wouldn't alter the cam timing, but it wouldn't hurt to check it. If the timing is set correctly, and the valve clearances are correct, then that won't be the spritzing problem. Things go catastrophic if the valve timing is non-stock without doing some valve-piston clearance measurements etc.

Ok, thank you.
I was pretty careful setting the cam timing to spec upon last re-assembly, but I'll have another look.
It's not too big a job, and I'm getting good at it.

Alright, valve-lash clearance was nominal at .004, camshaft timing shows both T marks (on chain sprocket and on timing unit) aligning perfectly, and advancer point exactly at the centre of coil pickup point.
So I've established timing is not the culprit here.
Am I looking at trying new jets at this point? It only seems unlikely to me because I didn't modify the carb at all during rebuild and the motor purred smoothly (except for the eventual overheat) before rebuild, and now it's running like a 1980 Yugo. I don't think there was anything adjustable inside the carbs that I could have got wrong while thoroughly cleaning. Is this incorrect?
Very open to opinions, and thank you again for all your ideas so far.

What type of choke is it? If there is an enricher, maybe it is not sealing off the fuel flow when you turn off the choke/enricher.

Interesting idea. The only enrichment that occurs is a tab at the back end of the choke lever that advances the throttle slightly. There's no enrich lever or plunger like on my boat motor.
As per your suggestion that it's running rich, perhaps the diaphragms aren't holding down the high-speed-needle pistons (which, as I said, are bouncing freely at idle, but holding down needles firmly with engine off). Double-diaphragm fail? Is that even their function? I just don't know. They appear to be puncture-free.