Hello guys, hope you doing fine!

I have a 1997 FZ 400 Fazer (4YR), it's basically an FZS 600 frame with a 400cc engine, similar to the FZR400.
I recently adjusted the valves and something weird is happening.The bike starts really smooth and I have plenty of power up to 7k rpm, above that, its struggling no matter how much I twist the throttle..If I give if full throttle right after I pull from a stop it will rev up to the limiter, a bit slow, but with no struggling at all..If I close even 1/6 of the throttle the bike will just fall back to 6-7k rpm..
I've cleaned the carbs and replaced the air filter, checked the fuel lines and all seem to be working fine..
The carbs are 4 Keihin CVK 30s
Here are some details about the bike
https://bikeswiki.com/Yamaha_FZ400_Fazer

Any thoughts on this one??Thanks in advance and sorry for my bad English
Cheers!

One more thing: how's the fuel level on the carbs? That can affect the main fuel system (main jet, needle jet, jet needle). Can also have to do something with fuel delivery (not getting enough fuel, CV carbs tend to be milder on these situations)

It sounds strange that happens on higher revs...how about spark plugs? Plug caps and wires? Any misfire?

The carb diaphragms and the covers, those must be really checked with a discriminating eye. A single unnoticeable pinhole or crack can cause problems.

Might be unusual but are the valve clearances correctly set? You need to recheck that after running the bike. Especially if that started happening AFTER the adjustment was done!

First thing I'd check are the carbs - your looking at the jets - make sure you haven't swapped the main for an idle jet. I did this once and it would just stop at 8,000, where the mains come on.

Secondly - make sure the cams are timed properly it's possible to think one is ok but after you do the cam tension up it can slip.

Fuel levels and valve clearances are all within spec..
I was thinking of fuel delivery as well, given the fact that this model uses a vacuum fuel cock, I'll be checking that too!
Diaphragms are like new and I've double checked them
The spark plugs are new, the caps are good, and I hear no misfire
I will try to record a video and post it here so you can see what I'm talking about!

Thanks for all the replies!I will return with a video!

Hello again guys, hope you doing fine
So, here is the video.Unfortunately the quality is not that good.
On the first section I'm doing 60mph (100kph) on fourth gear and 3rd gear and opening the throttle all the way, as you can see the revs are on 8k and 9k, while the tachometer needle is not moving at all (actually its falling a bit ).
On the second section I'm accelerating all the way to the red line with WOT so you can see how slowly it actually speeds up..

https://www.youtube.com/watch?v=SrXITDd ... e=youtu.be


Hope this helps

Seen the video.

Makes me feel that the engine does not have the guts to spin faster and harder, even at WOT and a considerably long straight...

Other suspects that I can say of:

a. Air filter/Airbox - how's the filter looking? Is it dirtied and cause the motor to choke? Sounds a bit off-the-wall but can be, with someone came up to me why their 125cc pit bike isn't giving that blast(turned out their filter wasn't replaced for three years...look at that)
b. Carb manifold rubbers - are they seating correctly? A small-ish leak cause issues, particularly at low RPM performance and makes the air/fuel mix too rich at high RPM due to extra air getting into the engine without passing the carburetor first.
c. Carburetor choke not closing properly/rubber cracks on plunger - Highly unlikely but it is good to ensure they're good, most do carb rebuilds and forget replacing choke plunger seals and such
d. Compression - Sounds silly, but, just to be sure. A lack of power, especially the 'tug' of the bike, says something about compression. Check after doing all the above. Re-check the valve clearances if they are still within spec, and drop some oil in the cylinders that seems to be low on compression to distinguish valve issues or piston/cylinder/piston ring problems.

To be a bit offset, the fuel pump and the fuel filter also would have to be checked...it is better to do simple things first before the not-so-fun stuff.

Again I'd check the carbs jets are put back in the right place
Really though I think you should take to a dyno and have it checked over

Hello all, sorry for the huge waiting time, but we faced some health issues..Anyways, here's what I found out:
One of the diaphragms was not seated correctly on the groove!Also the 2 inner springs where stiffer than the outer 2..!
Good carb diaphragm springs are hard to come by here, looks like the "mechanic" who serviced the bike before I buy it, was looking for 2 nice keihin springs..Any thoughts on that one?As far as I am concerned zxr 250s are using cvk 30s and the zxr400s are using 32s..will a set of springs from any of these bikes fit mine?
Thanks again!

You could try putting the stronger springs on either the outer or inner carbs and see how that goes ???

How did you clean carbs?
Did you remove all jets and scrub fuel-circuits behind them all way to carb-venturi?
Poke out all lateral bleed-holes in pilot and emulsion tube with matching-size copper wire?
Ultrasonic soak for days?
Micro soda-blast all fuel-circuits?
Separate carbs and replace all O-rings?

If not, I'd say carbs still needs further disassembly and restoration.