Bike died dead battery,running off alt?

Prob a really dumb question but I took my 77 twin out this weekend for the first time the batt was dead but she kicked over and fired right up with kick start,I rode maybe a mile and bam died on me, pushed home and jumped with car (off of course) and started right up, my question I thought I could run off alt? Also checked the alternator output last fall and it was in spec per manual, I noticed after bike died and before jump start I had a weak spark,too weak to start bike I guess,did I wreck anything? Thanks I’m advance!

Probably did no damage, but as you discovered, a good battery is necessary.

I could be mistaken but I think that your bike has an excited field alternator and not a permanent magnet one. A battery is necessary to energize the field without it the alternator will not produce anything.

Last year this time there was a short in the battery. It got worse and worse until I got stranded 100 miles away. Managed to jump it and get home but it read less than 6V on the gauge. If I turned on the lights while riding the bike would want to die. Tense ride to say the least. Didn't seem to damage the charging system but it couldn't have been good for it. A good battery is a must for the bike to run as it should. Some bikes won't run at all with a bad battery.
Steve

If the charging system is good, and the battery is not shorted or open, you can usually run on the alternator if you can get the bike started.

A dead battery describes a wide range of possible battery issues. If it's merely discharged, and you can get it to start with kicking, the alternator should be able to recharge it while riding. But if the battery has intermittent shorting or open conditions, it could die while riding.

Sometimes a dead battery will have enough power to to run a bike without being able to electric-start the bike. Then a kick start might get you a few extra miles down the road. But if the battery was low because the charging system quit, then it will eventually die. A jump start will again restart the bike, but it will usually die pretty quickly after that. This sounds like what happened to you, but that indicates a charging system fault. A similar scenario could come about from an intermittent battery short or open.

Either way, a good battery is the first step, then a charging system check is next.

As Gd4now said, you have an excited field charging system which requires at least a small amount of battery life before it can start up. If the battery has enough power to start the bike with a kick, then it will also have enough power to energize the charging system.

Most Kz's have a permanent magnet which can start up a bike with a discharged battery, or even without a battery as long as a capacitor is put in place of the battery. But the excited-field charging system is much more efficient and can handle a wider range of loads when working properly.