1978 kz650 seems like it wont reach top speed

SO i just bought a 1978 kz650. It runs. but i feel like i really really have to push the rpms to get just to 60 mph. I'm mostly switching gears at 5000 rpm. the top speed is supposed to be 110mph. I can barely get it to 70mph. I know it will accelerate past 70 but the rpms will be pretty high. I don't wanna crank on the bike too hard cuz it's old. Assuming the tach and speedo are pretty accurate..even tho they bounce a little. It seems i have to push the bike hard to get it up to speed like i said. Any advice on how to make getting up to speed faster? i mostly cruise in 5th gear. Too bad it doesnt have 6 gears.

Your just getting to the power curve at 4000 RPM. The 650 is meant to be revved. If you need to move out you'll have to give it throttle otherwise putt putt. The engine really produces a horsepower boost at4000-9000 RPM. Good luck. R

Being old shouldn't be a problem --- the Kawasaki 650 engine is known to be "bullet proof". hopefully, it wasn't abused.
Make certain the battery is good --- test it, don't assume anything --- the bike needs a good, fully charged battery to run right as it provides the energy to fire the coils.

Possible causes of the lack of power: bad/weak battery. valve clearance check. ignition points check & adjustment. spark plugs & spark plug wires. ignition coils failing under load. fuel system/delivery. carburetors may need cleaning. Some of this stuff should be done anyway as regular maintenance & may be overdue.

Since the bike is new to you, I would change the oil & filter --- either use a motorcycle oil or a heavy duty oil marketed for diesel engine use, such as Shell Rotella or Chevron Delo. And check/replace the air filter & check/adjust/lubricate the chain.

PS: don't believe anything the po said. people forget, don't know, or flat out lie. sad but true .

Like martin said a good tune up would do wonders.
Good luck with the bike

Compare the gearing to stock.

yeah something is wrong, you may be running on three cylinders, look for a cold exhaust pipe when its running.

A 650 in stock fashion will easily do 100 ( and get there pretty quick). These engines are come alive at 6,000 if you put the bike in 2nd gear at 5 mph and give it full throttle all the way to 9,000 rpm you can feel the power band for yourself.

Does it had individual pod filters or a factory airbox.

Yeah very soon I need my step dad to help me check the timing, valves, and I wanna see if the cam chain is ok. I have no idea what maintenance was done on this except probably regular oil changes

It has stock air box. And I'm sure all cylinders are firing because all 4 tubes are hot

Explain. What rpm is the engine running at when you are in 5th gear moving 60 mph? It should be right around 4500 rpm. If it is higher either someone has monkeyed around and installed non-stock sprockets or your clutch is slipping. Ed

Does your step dad have your manual? :whistle:
Steve

I had to recently replace the clutch springs but didn't replace the clutch friction plates. Maybe I have to replace them. At 60-70 mph um running it between 5000-6000rpm

At 60 mph the KZ650 engine should be turning 4500 rpm. At 80 mph it should be turning 6000 rpm. You should take a ride and see if that's how yours is performing. If it is turning higher rpm at those speeds I suggest you carefully follow the instructions in the Kawasaki Service Manual and adjust the clutch. Keep in mind, there's more to it than just turning the adjuster near the clutch lever.

Also, what motor oil are you using? Using car oil can make the clutch slip. Ed