KZ550 with GPZ engine strange sputtering...

Hello,
i own a KZ550 with GPZ engine.
The bike runs fine so far. Unfortunately it has some sputtering in certain situations.
When I drive in 6th gear constantly and then accelerate hard it sputters like fuel or ignition spark is missing. When I do that again it gets even worse. Its always only a split second then accelerates normal.
I synced the carbs adjusted fuel level and put new ignition cables on. The carb jetting should be correct. (95mains,32idle, needle second from top)
Haven't checked the valves yet,but don't think that is the problem.
Could this be the coils? Is there a way i could test them?
What I also recognized is that the cylinder base gasket is veeery slightly "sweating".
The gas tank cap should vent. It makes a sizzeling noise when i shake the tank. So air is getting out.
I am somehow clueless.

Somebody has an idea?

Regards David

The first thing I would check is spark (easy)
The second thing I would do is check the valve clearances.
The third thing I would do is check for intake leaks.
If everything checks out, it would be time to clean the carbs.

If not already done, would perform the clear tube test to assure that the fuel levels aren't too high. Because a too high fuel level can result in overly rich circuits, even in an otherwise perfect carb.

Am thinking of possibility that quickly twisting the throttle to wot (in a slightly over-rich main circuit due to a too high fuel level ) is causing the engine to momentarily "load up."

Good Fortune!

I had the same problem with my 550. I adjusted the float heights as Patton suggested and it seems to have fixed the problem

Hi,
Checked everything. Bike runs good as hell, fast acceleration. 
I can now describe the problem better.
When I go 60mi or so in 6th gear throttle approximately 1/3 open then close it a bit to keep speed at this level and now whack it open the sputtering occurs. 
When I roll the throttle slowly it does not sputter/cough

When I do this with choke partially applied,  it is worse.
So I assume it's too rich in this stage.

The y77 needle is in second clip from top, main jet is 95 keihin small head.
Carbs are synced, petcock is working good,  tank vent is free.

I had one carb with a bit high fuel level which I lowered and turned the mixture screws out 1 1/2 turns.
The sputtering got better almost gone.
The other carbs seem to have a correct fuel level regarding the clear tube method, but I am planning to lower them a bit too.

The plugs are more on.the lean side.

What do you think?

What is battery voltage at 4000 rpm?

Is full battery voltage present at the coil primary terminals?

Good Fortune!

Didn't measured it yet.
But in all other conditions the bike runs well and accelerates till red line flawlessly.

accubond wrote: ... 60mi or so in 6th gear throttle approximately 1/3 open then close it a bit to keep speed at this level and now whack it open the sputtering occurs. 
When I roll the throttle slowly it does not sputter/cough

When I do this with choke partially applied,  it is worse.
So I assume it's too rich in this stage.

The y77 needle is in second clip from top, main jet is 95 keihin small head.
Carbs are synced, petcock is working good,  tank vent is free.

I had one carb with a bit high fuel level which I lowered and turned the mixture screws out 1 1/2 turns.
The sputtering got better almost gone.
The other carbs seem to have a correct fuel level regarding the clear tube method, but I am planning to lower them a bit too.

The plugs are more on.the lean side.

What do you think?

The fuel level influences fuel mixture throughout all throttle positions.

At wide open throttle position, fuel mixture is being governed by the main jet, with size of pilot jet and clip position having virtually no influence on the fuel mixture.

Would suspect that a properly performed high speed throttle chop test would result in plug readings on the rich side, as such reading indicates influence of the main jet.

Sputtering when whacking the throttle open due to an overly rich (for whatever reason) condition would be inconsistent with lean plug reading under those circumstances.

Good Fortune!

Patton wrote: What is battery voltage at 4000 rpm?
Is full battery voltage present at the coil primary terminals?....

Sufficient battery voltage being supplied to the coils is a critical element toward proper combustion, especially when the throttle is whacked open.

If not already done, would assure air filter is clear and unobstructed. This would include the common dirty or over-oiled conditions, and also possible obstruction of the air intake under the seat, such as shop rag, gloves, whatever.

Good Fortune!

accubond wrote: ... put new ignition cables on....

If "ignition cables" means "spark plug wires," hopefully the new plug wires were solid copper core wires, and NOT the suppression type wires.

The spark plug caps, such as stock caps, already provide some resistance in the high voltage secondary circuit.
Adding additional resistance via suppression plug wires creates too much resistance in the high voltage secondary circuit.

Same applies to "R" type spark plugs -- don't use them unless also using solid copper core plug wires and non-resistant plug caps.

Good Fortune!

Are you running the stock airbox ?

Airbox is stock.

Battery has 13.7 -14.0 Volts at 4000rpm

I dont know if i mesarued right at the coils (between Green and yellow/red and Black and yellow/red cable) there are only 5-6 V!!

Do i need new coils?