1981 KZ750 E2 will not accelerate past 70 mph

I am a new member to the KZR site with an old problem. I purchased a 1981 KZ 750 E2 three years ago and have been fighting with it continously since then. It will not accelerate beyond 70 mph, indicated RPM will not exceed 6000. The following parts have been inspected or changed:

Carbs cleaned
New idle jets
Carbs Synched
New Carb Holders
New Airbox Boots
New Air Filter
Rebuilt Fuel Valve
New Fuel Lines
New Vacuum Lines
In-line fuel filter
Dyno-Jet Stage 1 Carb Kit Installed
Dyna-Tec 3.0 Ohm Coils
New Spark Plugs, 7mm non-supression, copper core
New Spark Plug Wires
New 5000 Ohm Spark Plug Caps
Cleaned and Lubed Ignition Advancer
New Battery
Dyna S Pick-up Coils
Compression is 130 psi common to all four cylinders
Valves clearances are with-in specs
Cam chain tensioner cleaned and inspected
Disconnected every wire, applied di-electric grease, re-connected
Removed, cleaned, inspected exhaust system, no obstructions or corrosion
Cleaned, inspected fuel tank vent system
Checked resistance of all wire bundles
Verified 12 volts is available at required locations:
Battery
Coils
Starter Switch
Kill Switch
20 Amp Fuse

Bike starts easily and will settle into a smooth 1000 rpm idle. At rest the bike will reach 9000 rpm instantly when throttle is snapped open.

The bike will pull red-line in the first three gears, however, it does not do so quickly. Once 4th gear is selected RPM's will not go beyond 6000 and the top speed stalls at 70 mph. At 70 mph if the clutch is pull in the engine will accelerate to 9000 easily when the throttle is snapped open. The problem I am having only shows up when a load is felt by the engine; the weight of the bike, rider weight, wind resistance, friction, etc......

Have review hundreds of on-line forums, at this point I am out of ideas.

Bike is in excellent condition and I would like for it to run as good as it looks.

Thanks for any input that may help me fix this bike

Is your fuel filter rated for a motorcycle? Some of the "Car" filters are designed to be used with a fuel pump. Tried going past 70 mph with the tank gas cap not locked down, to see if fuel starvation is the problem? How's the sparkplugs look? White is too lean,Black too rich.

Something to do with the brakes maybe? Looks like you've been focused on the engine but maybe start looking at the driveline for a problem?

You have a Jet Kit installed, do you have a Non- Stock Exhaust? Wondering why the Kit?
Also since you have been in the carbs, have you set the fuel level with a Clear Tube as a sight gauge? This can effect the signal to the different carb circuits. Best to be sure of the Fuel level.
Then the exhaust, since an engine is first a Air Pump, is there a restriction in the exhaust? Say maybe a aftermarket system, with a heavily packed baffle? Just a thought, because if the exhaust has excessive restriction, it won't make good power. This and the jetting is what it sounds like. Either the jet kit is to far off from target, or the exhaust is blocked.
Have you done some plug readings to see what they look like?
Then you also state you have a Dyna S, have you set the timing with a timing light, both initial and full advance should be checked. if the ignition is retarded or even to far advanced it will effect power.

Chain too tight or dry? Rear wheel recently worked on? It's easy to get the bearings too tight against the spacer. Is it aligned properly? Does the bike make any squealing or scraping sounds when you roll it without the engine running? That could be wheel bearings or brakes dragging.
Divide and conquer, make sure the running gear is okay, then go to the fuel system, then the ignition, then the transmission, etc.

Has anyone in the past changed the sprockets?

Original sprockets, front and rear, are installed on the bike.

Spark Plugs were inspected, they appear normal, no soot, oil, fouling.
EBC Organic brake pads were installed front and rear this year,
Front and Rear Brake Calipers were overhauled at that time.
Front and Rear Master Cylinders and Pumps were also overhauled last year.
Front and Rear wheels rotate easily, no binding or noises when spun, bearings were clean and well lubricated.
When bike is run with gas cap opened to atmosphere no changes to how bike runs.
Stock exhaust is currently installed on the bike with no modifications done to it.
Static timing to Dyna S system checks good at idle and full advance.
Dynamic timing with strobe also gives the same results.

Borrowed compression tester form local auto parts store, 30 psi indicated on all four cylinders, either it is a leaking gauge or I have a big problem. I will try another gauge.

Thanks to all who responded, I will continue to update as I learn more.

30psi in all 4! That can't be right, it wouldn't run at all with that amount.
Make sure the shradder valve in the end of the gauge hose is good, tight etc. Also the O-ring seal where it goes into the plug hole.
But it is possible to have low compression, it is a very good idea to check it. Also consider doing a Leak Down Test, to find where any loss of compression is leaking at.

If you bring the bike up to about 50 mph and then pull in the clutch, put the tranny in neutral, and let the bike coast does the bike seem to slow down too quickly or does it coast freely for a long distance? Ed

Going to recheck compression this week, agree, 30 psi sounds like a faulty gauge.
When going 50 if the clutch is pulled in the bike will coast freely for a long way, the brakes are not dragging. The chain is set to 1 1/2" of slack, the sprockets are aligned correctly and have no excessive wear. The bike has 22500 miles on it. I have ridden it 5000 miles since buying it three years ago. I am determined to fix this problem! Thanks to everyone for sharing some insight.

I hate to be the one to say it, but it sounds like the carbs may need to be cleaned again. Ed