1983 KZ440 Top Speed Issue

So I rode it in to work today and found that in 6th gear it tops out at 65-70 MPH/5500 RPM at WOT. I have been up shifting at 4000 RPM. My question is should I be in a lower gear and higher RPM's or is this just the speed this bike does? Not looking to go 90 MPH but I would like to have some more throttle to get out of the way if needed.

All the periodic maintenance done except fork oil. Carbs thoroughly gone through, synced and idle screws adjusted to 2 5/8" out. Air filter cleaned. Valves set to 0.20mm. Exhaust leaks some at the crossover.

Yes something is wrong. My 81 KZ305 does 84mph. completely stock. My son's 82 yamaha 400 Maxim does 92mph. What mods are on your bike? Most performance mods improve one RPM range, but can hurt another. How do the plugs look after a few minutes of WOT riding?

No mods on the bike. It is all stock. I am going to check the plugs tonight on the way home.

Check the ignition advance. If it is not going to full advance, that would cause reduced top speed. I would expect top speed on that bike to be over 90 mph.

Sitting here thinking about it the last 7 hours, I came to that thought as well. Thank you.
I was also thinking that maybe a hole in a diaphragm might be the issue as well?
I will look into both.

The stock Kz400 can get near 100 mph, so the 440 should be able to do at least the same.

I didn't do a plug chop last night as it was pouring rain when I got home.
I did check the diaphragms this morning and sure as heck, there was a 2-3mm tear in one. I just ordered a new set from JBM industries.
More money. More shop time. Less riding again. Why does this bike hate me? :S

You should be able to get the diaphragm out without removing the carburetors from the bike. If it were me, I would but a piece of tape over the holes and ride like that while waiting for the new diaphragm.

Getting the diaphragms out is no big deal. Just pull off the tank and and remove the screws. 2 minute job. I am going to wait for the new ones. They should be here in a few days. Looks like rain most of the week anyway..I don't want to risk anymore issues arising from this one.

The bike doesn't hate you. It's those diaphragms. I have yet to find a 400/440 with diaphragms that aren't torn or on the brink of tearing. I usually convert them to piston-slide carbs, but those are getting harder to find in decent condition. I've now gone to mechanical-slide carbs.

Oh I think it does. Every time I get it running and think I can finally ride, it finds something new for me to fix.The list of things done to it has gotten quite long. I really thought I had it this time. Oh well. I have learned so much over the last 17 months through reading the manual and the knowledge here that it has all been worth it. Keeps me out of trouble. :laugh:

Since I feel a thread should have the solution posted; I am going to swallow my pride and do so. While waiting for the diaphragms to come in and not really knowing where WOT really was I rolled the throttle open while watching the linkage. I found that the fuel filter setup I had routed around the front of the throttle cable had vibrated down and was blocking the linkage at 1/2 throttle. I re-routed it behind the cable and all was good. Sigh.
Now as far as the diaphragm goes, the JBM Diaphragms are awesome. Super easy to install, fit perfect and a great price. And after re-syncing the carbs and checking the air/fuel mixture, the response of the bike is back to where it was awhile ago.
Thanks for the help on this issue.