Have you verified your TDC mark yet, and checked the timing?
Most Engines that I have worked on, will pick up extra rpm with extra timing, up to somewhere around 20/25 degrees, or sometimes even more. If your Vcan is plumbed to the intake, those have a capacity to go to 22/24 degrees with mods; altho most slanty cans run around 9>11, sometimes down to just 7*.
If she were mine, and;
If you have cranked up the speed screw in an effort to keep a shaky engine running;
1) Check the firing order first, then the inside of the cap. if good then;
2) do a cylinder balance test. if bad, then first check the wires. If OK,
3) then do a compression test with the engine fully warmed up.
4) but if if test 2 is good, then go back to the carb and check the WET fuel level. Or you can just do this first.
If the fuel level is good then;
I'd plumb the Vcan to ported spark, and reset the Base timing to not more that 5* advance, and with the mixture screw(s) set to 2 turns out max. Then with the speed screw reset to put the underside of the throttle valve at about 1/16 inch from the bottom of the Transfer slot,
That's where I'd start
If the rpm is still outta sight, she's sucking air somewhere.
The usual culprits in order of most likely, and as may be applicable, are;
at the carb base gasket,
in the PCV system,
at the gasket between the intake and the head,
the booster,
the purge hose or valve,
at the floor of the carb heating chamber, between the two manifolds,
or at the tailpipe. yeah you heard that right. If one (or more) intake valves are not sealing on the intake stroke, the engine will suck from the exhaust rather than from the plenum, under the closed throttle. Time to check the valve lash.
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