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I inherited my dads old hot street 900.  I have been doing some tuning on it because it was jetted super rich.  I have an o² sensor kit going on it in a few weeks.  Went down two jet sizes and up one click on the needle.  Its still a little rich.  From what I remember dad had a lot of head work done to it in a race shop back in the 90s and high compression pistons.  Its pretty bitchy below 3500 but about 9k it has a powerband like a two stroke and just takes off.  I talked to the builder since he is still in business and said to run it on race gas so I have been running vp purple. It sho does smell good!  As a previous RC-51 Owner, I'm impressed with this old dinosaur.  But stopping is another story ?

Sounds like you plan to keep your KZ900 - excellent choice !  (But we're kind of biased here).  A hot rodded Z1 or KZ900 in the late 70's was the king of the hill for performance, but they didn't stop any better than other contemporary bikes of that time.  An RC51 is almost on another planet for handling and stopping, but you've got to remember that 15 or twenty years of the four Jap bike brands competing with each other accelerated the development of their bikes very quickly.  I owned an 84  interceptor 750 and later a 93 interceptor 750 and the difference between them was noticeable just 9 years apart. (and your RC51 was another level above them!)  My humble advice is upgrade your front brakes with dual discs and master cyl (from a KZ1000 for example, an easy job) and enjoy riding one of the coolest old bikes around.         
* edit -  And don't forget braided steel brake lines, they really improve lever feel.

Awesome that you’re keeping the bike. I’m sure your father would be thrilled. It sounds great. Just FYI you have a gas tank from the pervious model Z1, 1973-1975. Personally I like the shape better than the Kz900 tank. Welcome to the forum. Don’t be afraid to ask questions and aslo if you can help please toss in your $.02. 

EDIT, watched the video again and you have also the Z1 tach, and rear duck tail. With the engine painted black I’m curious if you also have a Z1 engine. What’s the engine serial number?

Also Z1 fork steering stem, koni/icon shocks and a 16” rim laced to the stock rear drum. 

Really cool bike, she sounds angry 

I also have my dad's Z1, which he converted to a drag bike.  I'm not sure exactly what I'm going to do with it yet, but I feel lucky to have it.

In the spirit of looking out for each other, I was surprised to see the tach in your video swing up to 12k.  These old KZ engines are not high-revving like modern motorcycles.  Kawasaki even lowered the red line to 8500 for the 1000.  They just have too long of a crank stroke to rev higher. 

Of course, the first time I rode my dad's Z1-R I missed a 1-2 shift and zinged it pretty good.  With that in mind, when I got my Z1-R I added a Dynatek DRL-300 rev limiter.  They show up used on Ebay from time to time.  They are very easy to install and work with the stock ignition and the Dyna S type aftermarket units.

www.dynaonline.com/drl-300-inductive-kit/

How do you upload a photo? I haven't been on a forum in so long. 

See an older thread by Patton - "posting pics is easy" there are some good instructions. 

I was able to talk to the guy that built the engine and asked him why it makes a chirping sound when its cold.  He said when they did the beadwork for oversized valves, they put in titanium valves for the higher rpm.  No wonder mom was pissed when dad spend nearly $4k back in the 90s.  
youtube.com/shorts/tYYkjfDzas8?feature=shared
 

JMoney7269 wrote:

I was able to talk to the guy that built the engine and asked him why it makes a chirping sound when its cold.  He said when they did the beadwork for oversized valves, they put in titanium valves for the higher rpm.  No wonder mom was pissed when dad spend nearly $4k back in the 90s.  
youtube.com/shorts/tYYkjfDzas8?feature=shared

 

Sounds like a big gulp of air getting sucked in. I don’t hear anything mechanical amiss 

Edit: Quite the builder if he can remember specifics of a build 30 years ago.