ZN700 high speed issues

Thanks for all the suggestions.
Yes the caps are on the pilot jets. Yes, the o-rings are new and fit snugly. (And even if they weren't, wouldn't that cause the bike to run rich rather than lean?)

Sadly, the needles are not adjustable. At all. Can't even shim them because the screw that hold the needle in place does not allow enough room for a shim. If I were to try, the needle would be tight and have no movement at all.

Taking another wild guess. maybe non-stock needles are in there?

In regards to the vents mentioned by Nessism. see part #92005-1054 FITTING, AIR VENT (reference 92005B in the drawing)
www.partzilla.com/catalog/kawasaki/motor...uretor-assy-zn700-a1 click!
to boardbandit: not sure if you meant yours didn't have the vents or just not the tubing.

1984 ZN700 w black vent tees. EB item # 264196759126

Needles are stock as per partzilla. N10D
I meant they do not have the vents. Yes, I saw those carbs on Ebay. Immediately went back to mine because I did not recall seeing vents on mine. They have none. If you look at all the carbs on Ebay, you will see one set that does NOT have vents...at all.

It sounds like you're pretty knowledgeable. I've just been spitballing & don't know what to say about the vents, so bear with me about them. The diagrams at Partzilla show them. The ZN700 carbs I found either show the vents or the pics are bad so you can't tell. I guess a test you can try is to direct the float bowl tubing to a clear container & set the fuel petcock to PRI, then open the drain screws one at a time to see how the fuel flows. maybe crack open the fuel cap. I would think if the carburetors are venting, then the fuel flow would be steady. I guess this test will work.

Other stuff. Does the motorcycle have a good battery, fully charged & load tested? I've had a couple of times when my motorcycle started running poorly at high speeds because of the battery. one time the electrolyte had gotten low after a long hot summer & another time the battery wouldn't hold a charge. For any running problems the first thing I do is make darn sure the battery is good. plus this is easy. hee.

Yeah, thanks again...but I had already tried all those things.

Do you have an endoscope?
something like this
www.amazon.com/DEPSTECH-Waterproof-Inspe...tphone/dp/B01MYTHWK4

Just curious if you can look at the carburetor barrels from the air filter side while you run the throttle.

Are all of your plugs reading close to the same leanness?

Sorry but I do not have an endoscope. There have been several times when I thought one would come in handy though.
The only time the bike is not revving is when it is on the road. Tough to use an endoscope then. Sitting in the driveway, it will rev all day long so if I'm not mistaken, the slides are functioning properly. Besides, when I initially had this problem, if I gave it choke, it would take right off. If the slides were sticking/not working, it wouldn't run well with the choke on.
I am still working on this problem. It seems to be running fairly well with the mains that are way too big. something is still going on here but I'll be damned if I know what it is.
Yes, the plugs were lean across the board. All pretty equal. Which tells me it is not an individual carb thing but rather something affecting the entire circuit.

Finding those 112 mains are big red flag.
Obviously, the carbs have been messed with and they needed larger jets.
I'm wondering if a passage was drilled and those slides aren't going all the way up. It would be nice if you could get your hands on a single stock carb for that bike and compare passages.

Also check the spring pressure on the vacuum slide.

F64 wrote: Finding those 112 mains are big red flag.
Obviously, the carbs have been messed with and they needed larger jets.

Yup, agreed.
Don't know why I hadn't really paid attention to it but the ends of the mufflers have been drilled. I am thinking someone replaced the mains with the larger 112 jets to compensate for the lower backpressure. I will be plugging those holes this weekend and see what that does. I wouldn't think it would make that much difference but we shall see.

check to make sure there is nothing blocking the exhaust as well.

Can it be eletrical, since you have checked the mechnical side well?

What if you drive down a hill and brake with the engine and have a gear so the RPM is in the range where you have issue and then give some throotle to see how its perform?

Can that give some more ansaears?

Or can it be a solution to connect a osciliscope to the wiring to the coils while driving to see if you loose pulses?

I appreciate all your responses.
I had initially given some thought to it being electrical as well. I even went so far as to swap out the coils for another set I had here. I knew in my heart it wouldn't make a difference but I was grasping at straws. Let's think about it for a moment. If the bike runs well with the choke on, it's not going to be electrical. If it ran like crap with the choke on, however, one might argue that the spark isn't hot enough to ignite the extra fuel and therefore decide it's an electrical issue. But that's not what is happening.

When I'm going down the road and it won't accelerate, I close the choke and it takes off. It's a fuel/vacuum problem. I have checked the valves: spot on. I have checked the compression: also good. Air leak is another option: I have eliminated the possibility. The plugs are all equally lean therefore it is something affecting the entire fuel delivery system, not any individual carb,

It is indeed puzzling and I am still not convinced I will find the problem. I will, however, find a solution.