Now, as far as race condition, the engine speed will always, always, always be determined by the volume of air, and fuel being introduced. I'm sure you know this. Look for areas where air may be getting in that is not part of the metered means through the venturi of the carb. #1. Throttle shaft loose in the bore. It is always downstream from the venturi, and causes many vacuum leaks, which are compensated for by making the fuel mixture richer. #2. Intake manifold flange, and gaskets. I'm sure you are using new gaskets, but make sure the flanges are clean and flat. A warped flange will let in a lot of air. #3 Cracked intake manifold. Hard to see, not common but possible.
Read all your plugs. If they look light brown, then the mixture is close, but you are compensating with added fuel for a vacuum leak. ALL the air for the engine must come through the venturi. It is the only way to properly meter the mixture. I would get it started again, then back off the low RPM and the main fuel jets (A and B nuts in the diagram) so that the engine is running as lean as possible. Then, once it is running very lean, use a can or carb cleaner spray on the various parts of the carb and intake to identify vacuum leaks. When you spray on a leak, the rpm will go up quickly as added fuel reaches the engine.
There is also something called a "Gunson ColorTune" that can be very helpful in old engines for setting the right mixture. Can be bought on Amazon or Ebay. Gives a very clear visual indication of correct mixture settings. I use it on old Ferraris to tune each of 12 carb bores, for each cylinder. (and I charge a lot of money too!). Lucky, you have only one carb to adjust.
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