proper spark plugs?

Im New to this site and bike. Just got a 76 kz900.
Checked plugs and there are 3 ngk br8es, and 1 ngk br9es...all are black, but the # 3 cyl was blacker than black..bike wont rev past 5000 without backfiring. Other than that it idles fine, starts right up and no smoke out exhaust. Still has points, and only mod is exhaust 4 into 1

The plugs you listed are wrong. First - they should not be resistor type (there should be no "R" in the number). Second, one is the wrong heat range. They should be B8ES plugs. Ed

That bike should really haul above 5000 rpm. Several things could cause that problem. Job one with any old bike should be a thorough tune up and service according to the genuine Kawasaki factory service manual. The manual is often available on fleebay, and might be available for free i pdf form from somebody around here. Ther are a few specialty tools you will need, and the fsm may be the most important. Some JIS screwdrivers, valve adjusting tools, a torque wrench that is calibrated in inch-pounds and a Morgan Carbtune are a pretty good place to start.

There are a bunch of experienced, helpful folk around here who can walk you through any difficulties.

Oh, a decent volt/ampere/Ohm meter will likely come in handy, too.

B8ES plugs. Check your Valve adjustments. Yeah, I know it's a pain in the Arse and so many people don't do it because it's difficult and costs money. That's part of the tune up process.
Steve