[SemiFast]

Hi, I'm late to the party, sorry about that.

I would think the fuel levels would have more effect on slow speed running instead of making it balk at the throttle.
If it has the stock air box and exhaust system... a jet kit will only help to confuse things even more.

Does it have the stock air box and exhaust?
The answer to that question depends on where we go from here....

 

[SemiFast]

...... making sure everything is in its place where and how it is supposed to be. Open your manual or rebuild sheet start at the first step and go one step at a time making sure everything is to spec exactly, .......

This ............. ^

 

[SemiFast]

With the main jet and main jet holder (with emulsion tube) removed have you sprayed carb chem thru all passages of the carb body itself to be sure all are clear?
There are air jets just inside the back (rear) of the carbs venturi. Most have two, one for mid range / main jet and one for slow speed / pilot jet running.
The smaller of the two is for mid range / high speed / main jet running. When you spray carb chem thru this air jet you should see a good flow of chem coming out where the main jet holder has been removed.
Have you done this? Is this area of the carb body itself clear?
Are you sure there are no missing "O" rings, washers / parts or whatever because it sounds like this area could be where your problem is.

Jet needle and needle jet: (My terminology).

What I call the 'jet needle' is the long needle that is attached to the bottom of the slide diaphragm and is not adjustable like it is with a jet kit.
The 'Jet needle' works with the 'needle jet'.
The needle jet protrudes up into the venturi a little and is held in place by the emulsion tube/main jet holder assembly.
All of that just so I can ask... The needle jets are not missing are they and are they installed correctly? (Meaning not upside down).

One other possibility. >Probably should have started with this.< Lol
Are you sure the jet needles are installed correctly so they stay put and can not move independently of the slide diaphragm.
If they are somehow "floating",so to speak, that could cause the problem you describe.
Wow, that was long...:surprise:

 

[SemiFast]

With the pilot jet, main jet and main jet holder (with emulsion tube) removed have you sprayed carb chem thru all passages of the carb body itself to be sure all are clear?

I ask because many times people don't understand the passages (ports) inside the carb body are very hard to clean without soaking the carburetors and spraying carb chem thru the ports until a healthy flow of chem can be seen coming out of every port outlet. (All jets removed).

When you spray chem thru the pilot circuit (pilot jet removed) you should be able to see a good flow of chem coming out of the port where the fuel mixture screw outlets into the venturi.
This slow speed outlet port (for fuel mixture screw) is located in the top of the venturi and in front (engine side) of the throttle plate.

All of what you describe sounds like you still have plugged ports inside the carb body itself.

See my post number 13 about mid range / high speed port checking and cleaning.

Note:
I looked up a 1985 year model GPZ to determine where the fuel mixture screw is located. This one shows to be in the top of the venturi, most (or many) are in the bottom.

 

[SemiFast]

Possibly oil fouled spark plugs? Maybe rusted cylinder walls allowing oil by? Possible weak ignition system?
I'm all out of ideas.