high speed sputter

[van rock]

Hey ya'll hope this finds everyone well!!....quick issue for some feedback.......
not that it happens that often but when rolling on from say 55 bike pulls pretty good up to around 80-100 then acts like its starving for fuel or ignition limits....no excessive rpm here were talking 6th gear from around 4k once around 6 or 7k it stops..cannot make bike go over 80-100...

nothing to to with rpm it seems as from standing or low speed start it pulls like a frieght train up to redline through several gears...havn't had enough room to redline all 6....

i still have stock petcock...stock ignition...stock breather box....

any ideas??...i sure appreciate....

 

[golddrakul]

Possible plugged venting?? From standing/low speed wouldn't pull as much fuel from bowl, running at speed and going up would probably require more fuel in the bowls. Also check float levels.

 

[van rock]

venting???...........please advise?

 

[golddrakul]

Gas tank venting, as gas goes out of the tank the same volume of air has to be able to replace it or you end up with a vacuum [lower pressure in tank than outside tank] situation; this will prevent fuel from flowing to the carbs. Think of sticking a straw in a glass of water, if you put your finger over the end of the straw and lift it from the glass, the water stays in the straw. Take your finger off the end of the straw [venting it] and the water will go out the lower end. Same applies to your gas tank. Easiest way to check is a CAREFUL ride with the gas cap open. preferably with about a half tank of gas so it doesn't slosh out. If the problem goes away, you will need to check your tank vents. On the 900, the vent is through the little rubber nipple in the hinge area just behind the gas filler with the gas cap/doghouse open. Air should flow freely from there to the metal pipe coming out the bottom of the tank at the rear. The metal tube inside the tank is prone to plugging, particularly if the bike hasn't been regularly maintained.

Also, just as a throw in, there are two fuel bowl vents on the carbs, they are between 1-2 and 3-4. This are fairly large and rarely give a problem, however, if there are hoses off them to take any fumes under the bike it is possible to kink one and cause a similar issue.

When checking float levels, do it with a clear piece of tubing attached to the fuel drain at the bottom of the fuel bowl; when the drain screw is loosened the level of the fuel should come up to the seam between the fuel bowl and the carb body +/- .5mm. Have the bike sitting level when doing this.

 

[entoria]

My zl900 does exactly the same thing at the same rev range. I have checked and replaced everything except the pick-up coils and CDI. The only thing I haven't changed is the pilot jets in the carbs. Now people will say the pilots do not play a part in this rev range but tonnes of research and hours and hours of mucking about and then talking to carb specialists, I found out that they actually still play a part. So the next step to try is replace the pilots on all 4 carbs. My bike had sat for 7 years before I got it. Carbs and all jets were blocked to the max. Now got a FP kit in it but this stutter has not changed. Will order new pilots jets are report back.

 

[van rock]

Let me know what ya find out I just pulled my carbs off again and noticed someone had crushed the little washer that sets under the pilot screw on the #4 carb......so you couldn't set the pilot mixture correctly....also the float levels were way off......i've got a new set of plug wires coming tommorow so i'll let ya know how it goes......I have an incliing the float levels had alot to do with my issues......
do make sure the gas cap is functioning properly when I first got my bike this was an issue.