My G3 filter was never full and I never had a problem with the float levels or lack of fuel. I gave up wondering why, years ago.

 One can pull the chambers and pistons off of the carbs and check to see that the fuel is 1/16" below the top of the fuel nozzles. Easier than pulling the float chamber tops.

Any chance the exhaust system is involved?  

Also, just to distinguish between an RPM limit and a power limit, could you try up-shifting early?  AKA short-shifting.  See if you can go faster even though you can't get the RPM up.

I highly doubt it's exhaust. 

No matter what gear I am in I seem to be limited to the same top speed. It's not rpm capped per say, but definitely speed. Even on a downhill I can barely get going much faster than that. 

My fuel bowl level was quite low based on what I've read. With that and tightening up on the fuel hose clamps I hope to solve the issue. 

With the fuel bowl lid off, the fuel level on the front carb was roughly 35 mm from the top of the rim. Rear carb slightly higher. Fuel level at jets seemed well below the top, so I will sort that out.

I think my logic on the pistons might have been off in my other post, but it really seems like those pistons are stuck closed or the throttle blades aren't opening.  I think that if you were lean you'd get some popping or bucking.  But you're just describing nothing happening.

Maybe your oil leaked out or viscosity changed.

 

I set mine at 23mm below the lids from everything I read. Pulls like a tractor now even going up a mountain.

Hopefully you are "there". 8^)

The carb pistons and throttle valves are buttery smooth. 

I've read that 23 mm number before. I take it that is with the lid off and measured down with say a caliper from the rim? If so, I can do that easily and precisely... And is definitely way off where I am now.

Will be funny if I end up with significantly more power than I had previously. 

I measured from the bottom side of the lid, where the fuel would splash to.  The top of that square boss on the outside of the float chamber magically lines up at exactly 23mm.  Thanks @zKars for pointing that out.

Google "240z float level classiczcars.com" then click IMAGES and you'll see it.

Sounds like it could be linkage slipped. Did you have someone push pedal to floor while you lift carb piston and see if the butterflys are fully open?

Did you replace all your rubber fuel line hoses?

Wanted to double back on this thread and say it's solved for now.

Either by tightening the fuel line at the inlet to the mechanical fuel pump, or properly setting the float bowl levels the car sorted itself out.  I also mashed the gas pretty good when it seemed to stumble a bit in my first test drives after those fixes and it seemed to clear its throat.  So i'm not sure what did it, but for the moment I'm happy it seems to be remedied... and I got a more accurately adjusted float level out of it as well.

Thanks for the help and suggestions everyone. Strange indeed.

So, from what I have read online for the past 20 years or so and from personal experience, here's a list of things I have seen that can cause the fuel starvation problems similar to what you are experiencing.

1. Rust/debris in the fuel tank.
2. Cracked hard/soft fuel lines that allow air to be pulled into the fuel lines. (I suppose loose connections could fall into this category.)
3. Kinked fuel lines or some other blockage. (Personal experience there from when I replaced old, rotten stock rubber hose with standard fuel hose and didn't watch the bend radii.)
4. Clogged fuel filter.
5. Bad fuel pump aka leaking diaphragm. (Make sure your oil isn't smelling like gas.)
6. Bad floats.
7. Improperly set floats.
8. Blockage at the SU filters.

Being an electrical engineer, I look to divide the circuit (system) to assist in diagnostics. Check the fuel pressure downstream of the mechanical pump, aka between the pump and the carburetors. If it is good, you can focus on 6-8. If it is low, focus on 1-5.