Points or Carb??

I'm working on my 1978 KZ650B. The bike sat for a while until I got tired of it sitting and put it up on the worktable last week. It did indeed 'run great when parked' about two years ago, but just last week I got it going again, and it did run great except for a little 'coldness' when starting - but give it a little choke, crank it a few times and it fired right up. So fresh off my successes getting some other bikes to run, and thinking about how it hesitantly fired up, I decided to go over the points gap and timing...I set the gap at .35mm and got it static-timed perfectly. It's got fresh gas, a fully charged battery (13.47 volts). I hit the starter button, and cranked it until it ran down the battery, but wouldn't start.  Now remember, this bike ran great last week; the only thing I've done was to try to tighten up the points gap and timing, which I did - the gap was right on, but the timing needed some adjustment, so I did that and now it hits perfectly at the "F" mark on all cylinders - but won't fire.  Based on this, I think it must be something I did, but I don't know what - all did was adjust the timing!  What do you think...? 

Kill switch?  We've all done it...
Pull plugs 1 and 2, rest them on the head with the plug wires connected, crank, watch for sparks.  If no spark from either, check for power to coil primaries when the points are closed for each.

If both spark, retrace your original steps beginning to end.

Good Ridin'
slmjim & Z1BEBE  

Have you disturbed the "insulator" on the points (Jim's spark test will rule that out if you have a spark)

Not as far as I know; I only hooked up an alligator clip to the teeny little nut on the end of the assembly and then clipped a ground to one of the cooling fins. I haven't neem out to take another look at it this morning, but will go over everything to make sure it's not something simple like that disconnected wire problem I had. 

Well - it seems to have been resolved! I put the freshly charged battery in it - Granted, I left the battery on the charger all night - but (again) I think this was just a case of 'operator error'. I checked for spark on 1 and 2, and I had it. There's maybe a quarter tank of gas in it, so I put it in Prime and left it there while I cranked the engine. I cranked it twice, while giving it a bit of throttle and it fired up, but it seemed to fire after I let up on the starter button - I think this is symptomatic of a ballast failure - or am I thinking about another bike, I'm not sure (I have seven different ones in the garage I've been getting going)...but it ran, and ran great again! And great throttle response too, so at least there's that; but if memory serves, I think that's the way I used to run it b/c the vacuum petcock wasn't feeding right in the 'On' position...a small thing, but I'll check that out too. But thanks guys for the guidance!

"Until Next Time...!" 

IIRC the starts-when-the-button-is-released phenomenon is indicative of a weak battery: even if charged it doesn't have enough juice to run the starter and to make fat sparks at the same time

Recommend getting a new high-quality battery and cleaning all of the contacts involved (solenoid, starter, etc.), but you can always test the theory by starting the bike while it's connected to a charger (if you have one that will do so safely)

What Nerdy said.

   Also, check and compare voltage at either of the coil primaries to that of the battery, measured directly between the (+) and (-) terminals  with the ignition sw. ON but the motor not running.

   Lets assume a battery voltage of 13.5v at the battery with the ignition sw ON., motor OFF.  If there's  more than, say...,  0.5v reduction (max) in voltage at the primaries, >13.0v in this example, vs 13.5v battery direct, that indicates voltage drop somewhere in the wiring between the battery and the coils.  The coil relay mod solves the voltage drop problem.

   Z1 ignition switches are fairly notorious for voltage drop in the WHITE circuit.  We've seen some of them run noticeably hot because of internal resistance and, one ignition sw  we've seen developed intermittent open behavior on the WHITE circuit due to heat-related contact deterioration.    The '78 KZ650 may use the same or similar switch design.

Good Ridin'
slmjim & Z1BEBE

I would just add, if you connect the red lead from your meter to battery positive and the black lead to the coil feed wire then the meter will read the difference between the two points (volt drop) this way, regardless of battery voltage you can see the drop off. As Jim said, 0.5v is the industry recognised acceptable maximum volt drop.