Hard to Start 1980 KZ650E1 when cold

Ok here is the deal with this. I rebuilt this bike for guy in my area . He called me the other day after picking his bike up from me and stated that it is very difficult to get started when the engine is at ambient air temprature. 60 to 85 degrees average. I bench syncronized and vaccum syncronized the carbs. Adjusted the valves to OEM spec. Now this bike started for me no issues for three days in a row including the day he picked it up. Also he stated to me that the battery dies after few attempts to start the bike.
Now assuming that he is not holding the starter on for more than 10 to 15 seconds max. I would say that the battery maybe a potential cause. However he then he stated the bike starts fine when hot/ warm. Now this leads me to believe that it maybe a mechanical issue such as worn rings, valve seals or guides or even a burnt valve. He has stated to me though it idles smooth and revs smooth as well both loaded and free reving. Compression was a 125lbs give or take a pound or difference per cylinder.

Anyone have any ideas?

I'd check the spark output of the coils. Are they oem stock coils? They will age and breakdown with heat/stress and produce weak spark, I'd do the "Wired George" coil mod to make the bike a better starter and more dependable. His website is www.wgcarbs.com with a list of parts and where to buy.

If the compression is o.k. and air and fuel flow are again o.k. then it's got to be spark or lack of.

That is what I have been thinking the whole time so when he brings it Friday I will verify. It is the only thing that I can say has great possibilty for failure at this point. Exspecially since the bike was sitting for five years in a barn before it was brought to me.

Perhaps the difficult cold starting is because
of improperly touching the throttle while
attempting to crank a cold engine.

As known, the carb enrichener circuit (choke)
is designed to function with the throttle slides
in the fully closed (all the way down) position.
The enrichener circuit doesn't function
when the throttle slides are raised.

The short answer -- advise him to keep his
hand off the throttle during initial
cold cranking with the choke on.

Good Luck!