kz750 wont reach top speed

Sounds like it real lean on the mixture and you have a clutch problem

Considering the air filter hasn't been changed in 15 years what's the chance the valves have been adjusted?

I'm not the mechanical guru nor well of knowledge like many of these guys, they have all bailed me out in the past with great information and sound advice.


For what it's worth...
One little trick I've picked up for helping to determine if a bike is running lean or rich (and to an extent how much so) is to use the choke as a diagnostic tool.
Get your bike warmed up and running at a steady speed down a straight road without traffic. Then without changing your throttle input start to apple the choke. Be prepared that the bike could sputter and stall, or pick up and run faster.
Putting the choke on enriches the fuel mixture so if your bike starts to run better you are probably on the lean side, and vice versa if adding choke bogs you down. It's also handy to find out how far off you are by seeing how much choke (1/4, 1/3/ 1/2, etc.) gives you the best results.
I usually use 2nd gear and not excessive speed to better be able to hear the motor over wind noise.

I reinstalled the old filter to see if replacing the inlet pipe did any difference but no, im back where i was at the start, topping out at 80mph.
Here you can see the new filter to the left and the old one on the right. The old one isnt made for this bike and the new one is a after market replacement based on the original part number.

Tried to pull the chock while going 70-75mph on 3rd gear and it did little to no effect. When the bike started to act weirdly when i pushed the bike faster, I then pulled the choke a tiny bit and it stabilized a small bit, but it didnt go any faster. Asking for more power or more choke made no difference, i just wont go faster.

I dont think the valves was ever adjusted, it had 5000km (~3100 mil) on it when i got it just before summer, for the last 15 years it has been driven 500km (300mil).

All bets are off when it comes to aftermarket parts used for the first time. If you have history with them and they are know to work then I’d say you would be ok. Not me, I try if at all possible to get OEM.

A previous post you mentioned the MPH stalled but the RPM continued to climb. Was that still the case on this last ride?

In all honesty without knowing all maintenance and periodic adjustments have been made, it’s hard to give you a specific item to look at that may be causing your problem .

I’d suggest you download a manual form the articles tab and start with all adjustments and maintenance as recommended in the manual. That may even expose your problem.

EDIT, using the choke to diaganose is useful but it’s more about throttle position then Speed. If you know at what throttle position the choke helps or not will tell you what circuit in the carb is being affected. I.e. pilot, needle, or main jet.

The tachometer instantly jumps from 4000 to 9000 rpm?
Is the tachometer mechanically driven or electrical?
If mechanical, I would think slipping clutch, but you didn't hear the engine racing.
If electrically driven, most likely a defective igniter module, as the electrical pulses to fire the ignition coils are also driving the tachometer.
Also, verify the ignition advance is working.

I am pretty sure the 750L has an electronic tachometer. I wonder if a bad coil or two could cause the same symptoms? The igniter seems a likely suspect.

Dont know the hisotry of the bike since the owner died and i bought it from the family fairly cheap.
And i believe the previous owner was a "if it aint broken, dont fix it" and when it is broken, a temporary solution becomes permanent.

Its a electronic tachometer, what i meant is that it was steady at 4000 and then when i twist the throttle its starts to "sputter" as someone called it, its lite it cant ignite the mix, like trying to light a wet lighter. the rpm then jumped as high as 8-9000 as max, it was mostly around 6000.

Are you saying the CDI is the issue?

The igniter puts out pulses that spark the coils. An electronic tachometer is driven off those same pulses. A misbehaving tach indicates the coils aren't getting pulsed as they should. Bad pickup/pulser coils could also cause this.
Ohm out the pulser coils and compare to the service manual specs. If they ohm out good, your problem is probably the igniter.
Also ,clean and use dielectric grease on all connections between the pulsers and the ignition coils.

Technically, it is not a CDI, it is a Kettering electronic ignition.
Cleaning the electrical connections is a good idea. Be sure to use a dedicated contact cleaner like Deoxit D5.
I am in the group that says to stay completely away from dielectric grease.

But why those the issue happen when the bike is in motion?
When its in neutral i can rev it all the way into the red without issues.
if something was wrong with the igniter, wouldn't it if be found that way?

Engine systems often perform differently under load. Revving the engine in neutral is not really a good test for most things.
A couple of posters have suggested that you get a factory service manual and perform all of the routine maintenance as if none of it was ever done. Your bike does not have many miles on it, but age also plays a factor. Fluids and soft parts deteriorate with age. Corrosion attacks metal parts. Critters can get into the deep recesses and cause damage. Making sure the maintenance items are up to snuff will eliminate many possible causes of problems and you can ride confident that those things will not let you down.
Once all of those items are up to snuff, it will be a bit easier to provide good advice about solving the problem. As it is, there are a bunch of possibilities, and it could be some combination of causes.
Be sure that you get the manual that covers your exact year and model, as there can be significant differences from year to year and model to model. There may be a PDF version that is downloadable from KZR. Check under the Articles tab. If there, it will be free.