POOR PERFORMANCE

The bike is my 83 KZ750 L3. Anyway heres what im seeing . If i get on it like im racing another bike it goes ike stink and i see it hitting 8000 plus rpm through 4th gear like its nothing. Shift into 5th and i can twist the throttle wide open on the freeway and not get more than 6300ish rpms. The speedo goes to 85 and itll be hard pressed to get to 80. Its as if theres a rev limiter coming into play when in 5th gear. Ive had many 750s over the years that would pull 105 mph like it childs play. So what gives with this bike? Its all stock jets, airbox and exhaust. I want some life outta this thing.........

Possibly restriction in the exhaust, seized ignition advance or possibly a restriction in the fuel supply 

Several things can cause those symptoms.

At the top of my list would be restricted fuel flow, ignition advancer not working properly or restricted air flow in or out of the engine.

Is this a new problem? If so, look carefully at the most recent repair or maintenance. Sometimes we create our own problems accidentally.

If it has been an ongoing problem, there are more possibilities.

Is the vent in the fuel tank cap clogged?
Is there an inline fuel filter? If so, are you sure it is designed for a gravity feed system?
Is the fuel level (not float height) correct in each carburetor?
Is there debris in the float bowls, indicating clogged carburetor circuits?
Is the fuel line kinked? Is it really old and full of barnacles?

Does the ignition advancer move freely and through its full arc? 
Are both springs in place and do they both look like factory springs?
Are the spark plug wires in good shape, or are they stiff and cracked?

Is there something like a rag or a mouse nest in the air filter box that is restricting flow.
Could there be a rodent  or large wasp nest in the exhaust system?

Are the throttle cables in good shape and properly lubricated, properly routed and properly adjusted?

Is the lash on all eight valves within factory specs?

Wookie58 wrote:

Possibly restriction in the exhaust, seized ignition advance or possibly a restriction in the fuel supply 

Ha! Great minds and all that.

If it pulls cleanly to the red line in all the gears except top it suggests you have too high final gear ratio. Bigger rear sprocket or smaller front will reduce the ratio. 
 

WARREN..
Yes it will pull to redline in first 4 gears then turn into a paperweight in 5th. Its not a new issue. Its the way its been since i got it like 6 or so years ago. Never did a lot of highway riding, just around town riding. Its been bugging me forever. I rebuilt the top end when i got it because it was raced and abused and the exhaust smoke never cleared up. Valve stem smoke when colds one thing but it could have fogged orange groves in florida with the thickness of the smoke it was spewing.
Valves were adjusted at that time. Oh the unnecessary joy of having to take cams out when they could have made it a shim over bucket engine. Starts easily when cold and warms up quick. I've used the colortunes in each cylinder and adjusted accordingly. Synced arms with the morgan carbtune pro several times. 
Guess ill have to find how many teeth are supposed to be on each sprocket and count them to see if they are stock or not. And ill recheck the advancer and put the timing light on it yet again.

So stock tooth counts are 13 and 33.

Despite your comments about it running great in the lower gears, it sounds like it's not firing on 4 cylinders.

All 4 are hot Ed. I was also thinking about maybe a clamp screw interfering with the throttle shaft. I gotta put an oil pan on a 04 Vulcan 2000 tomorrow then my lift will ne open again. Ill get it on the lift this week. Im also gonna do a compression check and use the colortune to see the burn condition in each jug.

All 4 are hot Ed. I was also thinking about maybe a clamp screw interfering with the throttle shaft. I gotta put an oil pan on a 04 Vulcan 2000 tomorrow then my lift will ne open again. Ill get it on the lift this week. Im also gonna do a compression check and use the colortune to see the burn condition in each jug.

The first thing I would check as you already know it's running on all four and other checks done would be to make sure there are no bad sealing around the airbox side, I had this on a 750 Zephyr where the box was not quite sealing correctly and under heavy load like in top would hit this wall, when corrected problem gone.

I have a Christmas present for you...

The 1983 Kz 750 L3 is "mostly" a 1982 GPz 750, and is easily capable of 110mph and 13 second quarter-mile sprints.

​​​​​Here is the answer to your frustration...

83 KZ750 L3 speed governer:

The version sold in the US has a speed governor...

Summary — 1983 Kawasaki KZ750-L3 (U.S.) speed governor

• Presence: U.S. market KZ750-L3 models were fitted with a factory speed-limiting device (governor) to meet local regulations/insurance practices of the era.
• Type & location: It's a mechanical/electrical limiter tied into the speedometer/speedometer drive and ignition system rather than an ECU map (bike is carbureted, no engine ECU). On many KZ-series bikes this was implemented as a small governor unit or an extra bridle in the speedometer drive assembly or a magnetic pickup interrupter mounted at the front wheel/speedo drive.
• How it works: The limiter senses road speed (via the speedometer drive/pickup) and interrupts or retards ignition timing or triggers a circuit that limits fuel/air (via a relay or tamper-proof plate) once a preset speed is reached, holding top speed below the intended limit. It’s not a gradual fuel-cut like modern ECUs — it’s a threshold cut/retard that makes further acceleration ineffective.
• Typical limit: Reported practical top speed with the limiter fitted is in the ~65–70 mph (105–113 km/h) range depending on condition; removing the limiter commonly restores the ~108–110 mph speeds seen on unrestricted machines.
• Factory appearance: Often a small stamped bracket, additional wiring and/or a modified speedometer drive gear (or extra slotted plate) visible near the front wheel speedo drive or under the instrument cluster. Parts lists for the 1983 KZ750L3 (U.S./Canada) in dealers’ parts catalogs show speedometer/sensor and related parts specific to those regional models.
• Removal/repair notes:
  • Removal usually involves replacing the restricted speedometer drive/pickup or bypassing the interrupting circuit and restoring standard ignition timing/fuel operation.
  • Common approach: fit a non-restricted speedo drive (OEM from non-US model or aftermarket), remove/repair the pickup interrupter, and restore original ignition advance curve if it was altered.
  • Safety/legal: disabling factory limiters may be illegal in some jurisdictions and can affect insurance/registration.

Happy Christmas!