TOPIC:

Is there a way to prime the carbs on a 76 Z650 B (fuel tap only has On, Off and Res). 

Not to get overly correct, but the first (K)Z650 B models were offered as a 1977 model.  It could be the date of manufacture was in 76, but it was sold as 77 model.

The petcock on that year and model is a gravity feed one, not a vacuum controlled one, so there is no prime.  It is the same type I have on my bike and I turn it to off when ever I get off of it and turn it to on when I get ready to start/ride.  Depending upon the level of fuel in the tank either on or res(erve) will sort of prime the bowls with fuel if needed prior to starting it. 

“On” should fill the bowls until the float closes the needle against the seat. When a petcock has a prime feature it’s used after the bike has stood a few days and float levels might be low due to evaporation. Don’t confuse prime with choke. They are different. Prime is to fill float bowls, choke is to add additional fuel to aid in a cold start.

The "PRI" prime position is only availiable on vaccum accutated pet cocks. "ON" and "RES" open the fuel flow only if the engine build up vaccum under e-start or runing condition.
"PRI" is the non vaccum/no e-start/no-runing position to fill the carb bowls with fresh fuel after a long period of no use.

Since we're on the topic. Does anybody know of a manual petcock that will direct bolt into a 2002 zr7S? I have rebuilt the vacuum petcock but I don't trust it and don't have room to install a manual shutoff inline (as I have done on other bikes).
I could measure but the bolt spacing is the smaller of the two common sizes and the fuel nipple comes out of the right side.

At the time (covid shut down) I did see a Yamaha unit online that looked like it would work but unless I can see the unit in hand I'm not buying it, as I also need the handle to be a certain amount below the lip of the tank.
Though I knew better, I bought a eBay Chinese tap. What a piece of crap. I threw it away. Serves me right.

Yamaha Raptor or Grizzly fuel cock. in various styles.  some fit snug under the tank, while others project downward more.  
Yamaha OEM. Made in Japan. And reasonably priced, if not cheap. I've seen them in the $20-30 range.
I'm pretty sure these have the 34mm bolt hole spacing (as opposed to the wider 44mm).  Just a couple of examples:
www.partzilla.com/product/yamaha/2HR-24500-02-00
www.partzilla.com/product/yamaha/5KM-24500-10-00   lever sold separately. never saw that before.
EB. yamaha fuel cock

ps:  I would not get a made in china version as these are reported to break or leak.
edit: didn't see your chinese comment, but I'll leave this post script here.

Martin, the Yamaha Grizzly and Raptor fuel taps are the exact ones I was looking at but there's no Yami dealers close by. I wanted to walk in and physically compare. For now I'm happy (not really) with the OEM rebuild but I'm always leary of the thing.
I wish I hadn't deleted the pictures of the Chinese unit I bought. I took it apart and swear I could see the thing leaking straight out of the box. It had a nice big nearly 3/8 inch brass outlet nipple yet the internal holes in the casting were about 1/8 inch encrusted with casting flash. I took a chance. Lesson learned, though I already know better. 

I took a screenshot from my other website of the photo I took comparing the OEM to the new Chinese petcock. Note the difference of the internal fuel passages and the dull rough finishes on the oring mating surfaces. 
 

As stated above, there is no prime setting on the 1977 KZ650 petcock.  Assuming there is sufficient fuel in the gas tank placing the petcock in the "ON" position is all that is needed.  Placing the petcock in that position will fill the carbs until the floats in the carbs shuts off the flow of fuel.  Placing the petcock in the "RES" position simply opens an orifice in the petcock that is lower than that of "ON".  Below is a picture of a stock 1977 KZ650 petcock and a picture of petcock instructions.   Ed

 

 

Thank you all!

I missed the fact that ON actual tops up the carbs if given some time. (I have had a hard time starting after long periods of stand still with (deliberately) drained carbs, and that led me to, wrongly, assume a PRI-mode was needed to help the start up).