Top end lower than expected - transmission?

My bike is not performing as expected. The engine is good, there is no stuttering or misfiring at any rpm ranges.

I've been having a look at a site called Gearing Commander and loaded in the specs for my 1980 KZ750. According to it, at 7200 rpm I should be doing 88 mp/h. I'm nowhere near that.

Now I was under the impression that the rear sprocket was wrongfully changed to a 40 tooth by a PO. Today I noticed that the sprockets and chain were converted to use 530 chain instead of the stock 630. Fair enough.

So currently it's got a 15 tooth front sprocket and a 40 tooth rear sprocket. With 530 chain, this should deliver results fairly similar to the stock setup. However at 7200 rpm I'm maybe doing 50+ mp/h? The rear wheel is a 120/90/16 so it's also slightly smaller than stock, but not by that margin.

Do I have some serious engine issues, or could I just downscale the rear sprocket to something like 34 or 32 teeth?

I must add that there is no problem with acceleration and/or pullaway. It does not feel like overall power is an issue, but the math suggests otherwise.

It sounds to me like your bike's clutch is slipping badly. In stock trim at 4500 rpm you should be traveling roughly 60 mph. In stock trim at 7200 rpm you should be traveling roughly 96 mph.

If you have access to a Kawasaki Service Manual I recommend you adjust the clutch. Also, What motor oil are you using? Ed

I do have the FSM, motor oil is Motul 10W40.

Thanks, I'll have a look at what needs to be done over the weekend.

I have a 530 chain, 16 tooth front sprocket and 41 rear and am roughly 4500 rpm @ 60mph (100kph). Your 15 front and 40 rear would not be noticeably different from mine.. Pretty much same engine.
I'm with Ed on this and also thinking clutch adjustment and/or if oil has friction modifiers which can cause wet clutches to slip

Just to be clear, are you guys talking about clutch cable adjustment? Because I did that whole locknut adjustment on the left side cover, granting the cable 2mm of play, etc.

It's not to say I didn't bork it.

Alright, so I adjusted the clutch release again by following the procedure of giving the cable slack, turning the adjustment screw out, back in until there's resistance and half a turn out again, followed by readjusting the cable. Went for a test drive, performance is still the same.

The engine oil is JASO M2 certified for wet clutch use.

Would the next step be to have a look at the clutch plates themselves?

When the bike is in 5th gear and you open the throttle wide open do the rpm rise at a rate that seems quicker than the increase in the bike's speed? That is a typical sign that the clutch is slipping. Ed

IOC750 wrote: Alright, so I adjusted the clutch release again by following the procedure of giving the cable slack, turning the adjustment screw out, back in until there's resistance and half a turn out again, followed by readjusting the cable. Went for a test drive, performance is still the same....

Some manuals are wrong or misleading as to which way the adjusting screw is supposed to be turned in order to first close and thereafter slightly open the gap between the actuator and push rod.

Would experiment by reversing the procedure.

Good Fortune!

650ed wrote: When the bike is in 5th gear and you open the throttle wide open do the rpm rise at a rate that seems quicker than the increase in the bike's speed? That is a typical sign that the clutch is slipping. Ed

That is a thing that I was wondering about today. Thing is, the performance is too consistent for that. I figured that if the clutch were slipping that there'd be "hiccups" or something of the like, but I always go through the gears smoothly and RPM rises consistently - even though I go through them rather quick.

patton wrote: Some manuals are wrong or misleading as to which way the adjusting screw is supposed to be turned in order to first close and thereafter slightly open the gap between the actuator and push rod.

Would experiment by reversing the procedure.

I could feel that it loosened going counter-clockwise and gained resistance going clockwise (like a regular screw) so I'm sure I got that part right.

Are you sure the speedometer is working ok? Maybe have someone in a car ride next to you to see if the car's speedometer and the bike's speedometer agree. Ed

650ed wrote: Are you sure the speedometer is working ok? Maybe have someone in a car ride next to you to see if the car's speedometer and the bike's speedometer agree. Ed

The speedo is accurate within 20km/h because of how it jumps around at high speed. Regardless, I can hardly overtake cars doing about 80km/h in the suburbs. If I took this bike on the highway, I would surely die.

Sometimes when at 6000rpm or higher, I try to gear over to 5th, only to realise that it is in 5th, When doing so, pulling in the clutch lever and releasing again immediately takes the engine back to the the previous rev count.

So again, if the clutch was slipping, it would not perform as consistently as it does. As the revs increase throughout all the gears, so does the speed, consistently. It just so happens to reach that top speed fairly quickly. It sounds like I'm riding the Isle of Man but it is a lot more sad than that.

I've decided to hold off on riding until I've sorted out this issue, but I'm stuck here. I'm wondering if the PO didn't tamper with the gearbox/gears themselves, though I don't know why anyone would do that.

Shifting through the gears doesn't take a long time at all, shift times are as though it's built for motocross.

I seriously doubt that gears in the transmission were changed. That would be a very expensive undertaking especially when the same thing could be accomplished by changing sprockets.

One easy way to test the overall gear ratios between the transmission and the rear sprocket is to put the bike on the center stand, put the bike in the gear to be tested (such as 5th gear), and remove the spark plugs and the ignition engine cover. With that done, you should be able to put a wrench on the crankshaft (the larger hex under the ignition engine cover, not the small one on the very end of the crank) and with the transmission in 5th gear turn the crankshaft exactly one revolution while watching the rear wheel to see how many degrees it turns. That would then give you a ratio between the crankshaft and the rear wheel that you could compare to another KZ750. Perhaps someone else on this site who owns a KZ750 would do the same thing and then you could compare your results to theirs to see if your bike’s overall gear ratio for 5th gear is similar. If the ratios between the two bikes are similar but the other bike runs at proper rpm for a given speed your clutch must be slipping.

Doing this test is really quite simple, so maybe someone with a KZ750 will be willing to help you?
Ed