kz750 wont reach top speed

I bought a Kz750-l4 (its the same as the L3 just minor electronic improvements ) as a project bike.
Had the carburetor at a shop were they changed cleaned it out, replaced the membranes and synchronized it. Changed the front and rear sprockets and chain and the spark plugs looks good, mostly 6 months old and clean.
Now everything runs nicely, all four cylinders fires och its a blast to ride on 1-3 gear up to 80mph, above that is where the problem is, there are no more power in the bike. I Twist the throttle fully opened and i hear and feel how the bike goes into "overdrive", its sounds like "brrrrrr" and there is a small shake/kick in the bike and then the engine sounds like "BRRRRRR", sometimes there are a small shake here and there, but there is no gain in speed. It goes back to "brrrrr" when i let go of the throttle.
Why is it like this and what could the issue be? Top speed should be 120mph-ish so im losing 40mph to something.
Also, why is there a small shake/kick when the throttle opens up?

Does the bike have the original air box & air filter and original exhaust system? Ed

What happens to the rpm at 80mph? Are they still climbing? Clutch slipping?

Throttle cable stuck or not properly operating?

Having carbs "in the shop" may have been a mistake and one that I have made in the past.

Are the plugs black after trying to get to top speed? If so, try a run without the air filter.
Will the bike go to red line in lower gears?

Check your ignition. Is the spark blue? Advancer, Primary and secondary resistance of the coils. spark plug caps and plugs them selves.


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Make sure the battery is good. I would charge the battery w a good motorcycle smart charger, then test it with a digital multimeter & do a load test. you never know sometimes and these checks are basic & easy.

If your first through third gears are ok, I would suspect transmission or aerodynamic issues(the wind resistance is causing problems)
See if you can skip over the problem gear. If it's fourth skip to 5th and see if the problem resides there as well.
Note rpm's or speed and gear where problem happens.

Yes i still have the original air box & air filter. The exhaust system is from a GPz and a 4-to-1 system, dont know why but it came with that when i bought it.

Forgot to check the rpm if it stilled climbed, if there still are some days left to ride i shall check it.
Throttle cable is not stuck, its as it should.

Haven't check if they are black, tried to reach top speed and when it stopped to climb i tried the choke to give it a little bit extra fuel but that made it worse.

Yes it will go into the red in lower gears, i noticed that the issue happens in the red on 3 gears as well.

I shall rewire the entire bike in winter so i can eliminate that issue.

I have thought that aerodynamic might be the issue, but its so quick to go up to 80mph but then it flat lines, Its like it start to couch and just cant give any more.
When i took it for a spin today i noticed that this happens in 3 gear as well. As soon as i hit 80mph its like a valve suddenly opens and the bikes starts to couch and giving it more fuel makes the engine loose power. Could there be a air leek causing the issue or the air filter clogged up? (haven't check it but probably never changed since 15 years ago)

r4ptor wrote: I have thought that aerodynamic might be the issue, but its so quick to go up to 80mph but then it flat lines, Its like it start to couch and just cant give any more.

I don't think aero has anything to do with it. I've got a 300 that will do 95 mph. If you r 750 can't overcome wind resistance at that speed then it has less power than it should.

Not aero in the sense that you are thinking of the bike being held back by the wind.
More like a wire that is being crushed or a loose kickstand switch that may be affected by the increased airflow.
How did the bike ride before the carbs were cleaned?

r4ptor wrote: or the air filter clogged up? (haven't check it but probably never changed since 15 years ago)

HOLY MACKEREL, SAPPHIRE!!!

Change that filter!

The sun showed up yesterday so i took the opportunity to change the filter.
Noticed the inlet pipe to 4th carb wasn't properly installed.
so i replaced all four inlets, replaced the filter and went for a test.
Now its even worse, it now im having issues going above 70mph compared to 80mph as before.
I then removed the filter completely from the air box and now the bike wont even reach 50mph.
I was in 3rd gear, going 45mph at 4000rpm, when I hit the throttle the tacho needle jumps like crazy going up to 8-9000rpm while im actully is around 4500rpm and the bikes started to shake.
unfortunately it started to rain so only had time to reinstall the old filter but never tested it.
Is it getting to little fuel or what could the issue be?