83 KZ440LTD Starving for fuel at top end

I'm THINKING that it's a fuel-delivery problem...
Once it warms up,it idles great,but out on the road,it stumbles when I feed it fuel.It did this from the get go,when we first bought it.I pulled and rebuilt the carbs,both diaphragms were in great condition,no cracks,not dry-rotted,still very pliable. It's still stumbling at highway speeds.
Petcock was cleaned out,new fuel-line and filter.
I noticed,while it's sitting idling,the fuel-filter will NOT fill,and in fact,it slowly drains,and if I rev it up,I can clearly see more fuel entering the filter...but still will not fill it up.Is that normal for these carbs?
Is it possible that I have too long of a fuel-line,and the filter is mounted too high above the petcock?
I have no experience with these vacuum carbs,I've only ever worked on gravity fed carbs.
I can run it around at city speeds with no issues,but as soon as I get to highway speeds,and really push it,that's when it stumbles.
Anyone?
I'm desperate,as this bike was just sold,and I need to get it running better than it is.
Thanks!

sounds to me like it maybe a clogged or partially clogged jet.. Do you have aftermarket pods or anything? As far as the gas line goes as long as it doesnt drop below the carbs I would think they would be ok. I'd check the petcock first. I did away with mine and ran a inline filter.

100% stock,no aftermarket parts on it at all.Factory airbox,and exhaust.
I will pull the bowls off again,and check jets and float levels.
It just seems odd to me,that it ran exactly like this BEFORE I rebuilt the carbs(same symptoms).
It DOES seem to be running better after the carbs were done,but still starving at around 5-6k
I will also pull the timing advance mechanism,and make sure it's cleaned and lubed...

Double check the fuel line any place it bends. Sometimes what seems like a gentle bend can be enough to pinch down the inside of the line and reduce the fuel flow rate. Ed

I checked the routing of the fuel-line a few times,and there are no serious bends in it at all...thanks.
It was doing this before AND after replacing the line,and the old one was not pinched or bent anywhere.

Check your cap vent is not restricted, and the internal tank screen on the petcock. Check the flow through your line and filter into a catch can.
Also look at your carb schematic, check for a filter above the float valve, which would be a small screen filter.

When I turn the petcock to prime,with the tank-cap closed,the fuel flows nice and solid.(I have the tank off)

Make sure your Fuel level is et right. Check it with a clear piece of tubing, hooked to the bowl drains, held up along the side of the carb. You should have Fuel right about level with the bowl gasket to carb body. Usually about 2-3mm below where the o-ring seats. If the fuel is to low, it will run lean, high to rich. This is very important before changing any carb settings.

Me and my coffee just got to thinking...and I'm going to look into this,after work later...
If I remember correctly,it seemed to act up when I got to a certain speed on the highway,from stop to about 30 miles per hour,it seemed to run fine.
I changed the plugs the other day,and the ones that were in it were really rich burning,very dark and sooty.Thinking they were just old,I didn't pay much attention.
So here's what just popped into my head...the air box,there's 2 pieces,and when I went to put the secondary piece back in place,I noticed a ring around the opening where it connects to the primary section,a ring of foam residue where there was a foam seal at some point.
So,I'm thinking that at higher speeds,that section might be sucking in air there,and not allowing the 2 factory ports on the secondary part of the box to pull air in,and properly distribute it to the carbs?
Judging by the plugs I pulled out,could it be starving for AIR,not fuel? I HATE these CV carbs...yuck!
I'm going to seal that back up with some foam tape and try again later.
Does anyone have that style airbox,that could have a look and see if the 2 pieces are sealed?
Thanks again!

Just went for a ride,it's still acting up.Came home,pulled the plugs...they are BLACK.

Hi Sandy
You might find some useful info at kz400.com. I just took a quick look and and there's lots of tech stuff including technical bulletins for the 440. One of these addresses hesitation under acceleration and while the diagnosis is "lean" rather than sooty plugs it does explain how the airbox should go together......... might help with your earlier question regarding sealing around the box.
www.kz400.com/WorkshopManFrameset-5.html and scroll down until you see 440A1, D1 carbs and air box

Good luck

JR,great info,thank you very much!
I've only ever worked on my KZ1000,so this is all pretty new to me.
I pulled the carbs again,and found the main jets to be #85,where it should be #62-65,so that is really looking like it could be the culprit,at this point.
I sealed up the airbox,but that made no difference.
I DID learn from your link,that it is really easy to install that secondary section of the airbox wrong though!
Thanks again,JR...really good info there to help me with this baby Kawi.