TOPIC:

To begin: I am working on a 1984 Kawasaki ZN700 (an offshoot of the KZ series) and am having trouble with anything above 4,000rpm and/or more than 1/4ish throttle. It balks, sputters and generally does not want to rev. It might get up there, but grudgingly. Sounds simple, right? Let me clarify: the airbox is sealed with a new, paper filter. No air leaks around the intakes as far as I can detect. Valves are spot on. Petcock is clean and even on prime, it does not help. Tried running it with the gas cap open too. Compression is good. Floats are at spec. Carbs have been cleaned (Yes I know how) Plugs are new and after a ride, insulators are white. Seem lean, right? I bought new main jets (It came from the factory with #92 mains so I upped them to #95's) When I pulled the old jets, I actually looked at them this time (I hadn't when I cleaned them, assuming them to be stock) and they were 112's! So I installed the new #95 jets, went for a ride and to no-one's surprise, it ran like poop. Sigh.
So to further explain, (are you still with me?) the carbs are Keihin and have both a main jet, which passes through a nozzle straight up to the throat of the carb and a secondary main which is under the jet needle and needle jet. So also a straight shot to the throat. The bike starts right up with a little choke (and it IS a choke, not a fuel enrichment system. This will become important later) and idles perfect. Runs great at 30-40 mph. But if you give it gas, it sputters like crazy. Put the choke on, and she's off! All this tells me it is still very lean. Even if there is an air leak, it shouldn't affect it so much at speed and not at idle, right? And would choking it also increase the vacuum, helping draw fuel into the engine? So is it a vacuum problem and not a fuel problem? Argh!
Please? Any help here?

How are the diaphragms?
Steve

Diaphragms are perfect.

Well, it might not be the best solution....but I have a solution.
I had some jets kicking around from a Honda Sabre 1100cc. They were 120's and 128's. Stock for the Kawasaki is a 92. I thought, what the hell. At least I would know if larger is better. It is. It runs fine. I put the larger jets in the two center carbs and the 120's in the outer ones. And the plugs are still white/tan ish. I don't know how this thing isn't running incredibly rich but it isn't. I was going to swap out one of the sets but after a 40 mile ride today, it seems fine so I might just leave them. I don't know what's going on but at least she's rideable now.

Something is wrong. No way you should be using jets that large. Did you fully break down the carbs and remove all the parts like the main nozzles? They have a tiny orifice thourgh them and need to come out to make sure they are open. It sounds like one of the main jet circuits isn't working at all.

Yup, I agree; shouldn't even be working at all. But it is.

Did I break down the carbs? Yes I did. I get it. You don't know me, you don't know my background. I'd question me too. But I have been working on bikes for 30 years. I have seen nothing like this and it makes no sense whatsoever. The carbs were completely disassembled and all parts removed, soaked in parts cleaner and put through the ultrasonic cleaner. All passages are clear as are the small holes in the main nozzles. The jet needles are stock and as far as I know, so are the needle jets. Everything tells me that this bike should run fine with the stock jets. But it ran like crap even with the oversized 112's that were in it when I bought it. Put in some 120's and it's good. Shouldn't be but it is. There is even still some minor roughness when the main hands off to the secondary main. I am betting it would do better if I bumped up the mains from a #68 to a #70.

If the engine is requiring more fuel to run correctly, (assuming that it is getting more fuel), why would that be? You mentioned secondary main jets. Can they effect or limit the fuel delivery of the main- main jet ? If not, then one of the 4 strokes must be using the fuel. Just my 2 cents after 3 beers.... Good luck.

The main jet and secondary main work in tandem. One doesn't steal fuel from the other. Even if that were true, the fuel would still be getting through the carbs to the cylinder.
Yes, I have asked myself why it needs more fuel. I don't think it does. It needs the same amount of fuel it did when new. It simply needs a larger orifice to deliver it. An engine needs the correct ratio of fuel to air. To the best of my knowledge, there has been no increase of air. The airbox is stock, there are no leaks and it has a standard paper filter.
The question in my mind currently is whether there is sufficient vacuum to draw fuel up through the carbs. But vacuum is directly related to compression and the compression is fine.
Not quite sure what you meant by "one of the strokes must be using the fuel". Maybe it was the beer talking? Beer has some strange logic sometimes.

Are the bowl vent tubes that come off the T between 1-2 and 3-4 open to the air? Wondering if the float bowl is somehow under vacuum and restricting fuel flow?

This particular model does not have the vent tubes. Not sure why they produced two different sets of carbs: one with and one without, but they did.

Increasing the main jet size usually provides less impact than expected because the main jets don't control fuel volume until you reach 3/4 throttle. The main control is the combination of needles and needle jets. I don't know those carbs at all but are the needles adjustable? If so I would try raising them.

Are the polyester caps fitted in place over the pilot jets? and with new o-rings?