1978 kz750 b3 engine bog at full throttle

[jon707]

Hello, my 78 kz750 b3 runs strong and well through all gears and rpms until I am in 5th gear on the highway. It runs fine until I (giving full throttle) hit just about 70 to 
75 mph near 6500 rpms at which time the engine bogs out completely as if I ran out of fuel. If I roll the throttle back about a quarter of the way after around 1 second 

the engine cuts back on and I can maintain around 60 - 65 mph @ 6000 rpm as long as I want if I do not give any more throttle than 1/2 to 3/4 throttle.  If I do go full  

throttle the exact same thing happens. This prevents me from going any faster than 65 mph which is not sufficient on a 70 mph road. 
 
This did not start occurring until I had the carbs cleaned and resealed. After which I rode the bike a few months without knowing the problem as I did not at that time 

attempt any top speed riding. So if the mechanic was responsible it was too late to claim any warranty work.   
 
 If you can tell me what wrong I would greatly appreciate it.

[Andreas E.]

Hi jon?!
It seems as if the gasoline filter in the tank is clogged. This can lead to the fact that not enough gasoline reaches the carburettors at full load.
By the way, a short presentation of you and your bike in the " you and your bike" section would be kind and very interesting for us. We are always curious
Greetings
Andreas

[jon707]

Thanks for your reply Andreas, I appreciate you taking the time. I went and checked the filters and they are clean. But your reply made me think that if the bike is starving for fuel at high demand if the float bowls are emptying out because of some adjustment?
What do you think? and yes I will do the "you and your bike"! Thank you agian

[Bruder Lustich]

Hi jon,

you wrote filters?
There are more than one in the tank?
You need only this one in the tank. Throw the others filters away. Looking clean, but they aren´t.

The Motor is normally good for 105 mph Topspeed . So i´m very astonished. How many miles run the Motor?

Greetings

Gerd


@ all: Weis jemand, ob und wie die Maschinen event. für die USA gedrosselt wurden??

[Andreas E.]

I argree to Gerd,
The reason for running out of fuel at high load is often a clogged filter.
There is nothing to adjust at your carburettors at the moment. I´m sure your carbs are ok. 
Please check and clean the filter inside the Tank, throw away the filter between petrol tap and carburettors.
Use new NBR nitrile petrol hose. Others may go hard and cracked after a while.
To clean the filter you must remove the petrol tap at the fuel tank.
The Filter is inside of the tank.
Greetings
Andreas

 @all: Die B´s haben doch keinen Unterdruckanschluss am Hahn oder?
Also dürfte es nicht an der Membran liegen...

[Michael]

Hi Jon,
welcome !

Hi guys,
there is nothing that reduces the power of our Twins intentionally, when exported to the US (keine Drossel auf 27 PS oder ähnliches).
All Bs have a manuel pet cock, no vacuum membrane is in use.

Jon,
remove the full/refueled fuel tank from your bike and place it on a "horse" or trestle.
You may place it on a table as well.

Fit a long new fuel line into a canister, so you can reuse the fuel.
Open the pet cock to "ON" position. A constant stream should come out.
Let at least 1 liter (1/3 of a gallon) run out.
Is it a constant stream or does it slow down over time ?

Turn to "RES" position and try again.
Same result, constant flow should be achieved.

If that isn't the case, open the tank cap, which enables air to enter the fuel tank.
That is a function within the tank cap as well.
Did you make any modification to it like powder coating, painting, foliation, etc. ?

Maybe you have to clean the tank ventilation within the tank cap.

Cheers, Michael

[jon707]

Hi jon,

you wrote filters?
There are more than one in the tank?
You need only this one in the tank. Throw the others filters away. Looking clean, but they aren´t.

The Motor is normally good for 105 mph Topspeed . So i´m very astonished. How many miles run the Motor?

Greetings

Gerd


@ all: Weis jemand, ob und wie die Maschinen event. für die USA gedrosselt wurden??

Hello Gerd, Thank you for your reply. Yes I have one inline filter for each carb. I removed them after reading your reply and ran the bike with the same results. 

I really appreciate your advice. I am wondering about the main jet being too small after installing open slip ons.

I began to be suspicious after watching this video:  https://www.youtube.com/watch?v=y836ZPjvoMI 

This guy seems to be logical and makes sense.  When I get the chance I take a look at the carb jets. I am most certain they are the stock jets. When I have a day off I will check them and let you know.

If you watch the video please let me know what you think.

Thank you again Gerd

[jon707]

I argree to Gerd,
The reason for running out of fuel at high load is often a clogged filter.
There is nothing to adjust at your carburettors at the moment. I´m sure your carbs are ok. 
Please check and clean the filter inside the Tank, throw away the filter between petrol tap and carburettors.
Use new NBR nitrile petrol hose. Others may go hard and cracked after a while.
To clean the filter you must remove the petrol tap at the fuel tank.
The Filter is inside of the tank.
Greetings
Andreas

 @all: Die B´s haben doch keinen Unterdruckanschluss am Hahn oder?
Also dürfte es nicht an der Membran liegen...

Hello Andreas and thank you for your input. I really like you guys on the forum. 

The first time the problem happened for me I was given the same advice you gave me about the tank and inner filter. They all checked ok. Clean and flowing well.
Ok so I replied to Gerd's post and gave him a link to a video about changing pipes to wide open and adjusting the carburetor jets to match the change in engine airflow.  Here is the link please let me know what you think.

https://www.youtube.com/watch?v=y836ZPjvoMI

Thank you again for your time

[jon707]

Hi Jon,
welcome !

Hi guys,
there is nothing that reduces the power of our Twins intentionally, when exported to the US (keine Drossel auf 27 PS oder ähnliches).
All Bs have a manuel pet cock, no vacuum membrane is in use.

Jon,
remove the full/refueled fuel tank from your bike and place it on a "horse" or trestle.
You may place it on a table as well.

Fit a long new fuel line into a canister, so you can reuse the fuel.
Open the pet cock to "ON" position. A constant stream should come out.
Let at least 1 liter (1/3 of a gallon) run out.
Is it a constant stream or does it slow down over time ?

Turn to "RES" position and try again.
Same result, constant flow should be achieved.

If that isn't the case, open the tank cap, which enables air to enter the fuel tank.
That is a function within the tank cap as well.
Did you make any modification to it like powder coating, painting, foliation, etc. ?

Maybe you have to clean the tank ventilation within the tank cap.

Cheers, Michael

Hi Michael and thank you for the warm welcome. It is great to be among like minded kz twin enthusiasts! 
  I took your advice and checked the flow from the tank. Everything seems to be flowing well in "on"  and "res"
I am not sure how to check the ventilation of the tank cap though. One thing comes to mind about that idea is how well the bike runs in any other scenario other than 5th gear full throttle reaching between 70 and 75 mph
I wanted to know what you thought about the video link I left the other gentlemen who were kind enough to respond to my post. I will leave the link with you also.   https://www.youtube.com/watch?v=y836ZPjvoMI 
I am willing to try the theory of an increase in main jet size because of the open slip on pipes I put on. 

I just realized I did not tell you guys about the muffler change. I apologize that I did not think about those pipes giving the 

problem as the slip ons I replaced I thought were completely open. As I checked this I realized that there was a small (very small) baffle within those slip ons.   I wonder if that was enough to cause a need for a carb jet adjustment.

Thanks for your reply and if you get time let me know what you think.

[jon707]

I argree to Gerd,
The reason for running out of fuel at high load is often a clogged filter.
There is nothing to adjust at your carburettors at the moment. I´m sure your carbs are ok. 
Please check and clean the filter inside the Tank, throw away the filter between petrol tap and carburettors.
Use new NBR nitrile petrol hose. Others may go hard and cracked after a while.
To clean the filter you must remove the petrol tap at the fuel tank.
The Filter is inside of the tank.
Greetings
Andreas

 @all: Die B´s haben doch keinen Unterdruckanschluss am Hahn oder?
Also dürfte es nicht an der Membran liegen...

Hello Andreas and thank you for your input. I really like you guys on the forum. 

The first time the problem happened for me I was given the same advice you gave me about the tank and inner filter. They all checked ok. Clean and flowing well.
Ok so I replied to Gerd's post and gave him a link to a video about changing pipes to wide open and adjusting the carburetor jets to match the change in engine airflow.  Here is the link please let me know what you think.

https://www.youtube.com/watch?v=y836ZPjvoMI

Thank you again for your time

Hi Again Andreas,
I realized as I was responding to Michael that I did not mention the open slip ons I installed. I feel stupid for not mentioning that fact but as I told Michael I thought the ones I replaced were open but found out they had a small baffle within.
I think I will check out the situation with the jet and let you all know
thanks again