KZ750N1 dies under any sort of load

I've been working on this KZ750 Spectre for two weeks now, and I thought I had it just about ready for the road! But now, under any sort of load, the bike chokes and dies.

Some background: I rode the bike when I first got it, two weeks ago, on a 45-minute ride. It was in rough cosmetic shape but ran and shifted fine, up big hills and at interstate speeds as well as around town. I took it home and got to work.

Since I got it, I have:
Changed the engine oil and filter with Rotella 15W-40.
Replaced the air filter.
Replaced the battery.
Replaced the airbox inlet ducts with brand new ones.
Cleaned the carburetors.
Replaced the fuel lines and inline fuel filter.
Changed the handlebars, and lubed the clutch and throttle cable.

Now, the bike idles great in neutral, and in gear with the clutch pulled in. As the clutch is released (regardless of how much throttle is fed in or how slowly the clutch is released) the bike will sputter and die before it moves. On level ground, I can get it to move just slightly before it dies, but even holding the clutch barely engaged will kill it.

On the centerstand, the bike has no issues and will spin the rear wheel all day long in gear.

My initial thought was that it was starved for fuel, and so I replaced the inline fuel filter and ran all new line from the petcock to the carbs. This made no difference.

Any ideas? Thanks for any suggestions!

Is your kickstand up or down? There is a kill switch on the kickstand/sidestand that will kill the engine when you let clutch out if the stand is left down.

The kickstand is up, and with the bike on the centerstand the rear wheel spins with the clutch engaged, so I don't think it's a malfunctioning switch.