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: