Sounds like it may be too lean.
If there was no economizer valve, there probably never was one, and the port was never drilled for it. I've never actually seen one with the economized valve, only pictures. I don't think it will hurt to leave it in, probably will make no difference.
First, be sure the distributor is in good working order, there should be minimal side play in the shaft bushing, and the centrifugal advance is free and working. Was the timing changed? Reasonably sure it is correct?
Is there a good, blue hot spark at the plugs? It should be able to jump 1/4" at each plug. If not, revisit the points, condenser, wires, and possibly coil.
How is the air cleaner? If that has an oil bath cleaner, there is a wire mesh element above the oil container. It is often overlooked, could be plugged.
Is there good fuel flow to the carb? There is a drain plug in the carb bowl, with the engine off, fuel valve on, remove the plug, catch the gas in a clean glass. The flow should start strong, then reduce as the bowl empties, but still continue to trickle. If it stopps or slows to a drip, there is a fuel delivery problem. Look at what was caught in the glass, if contaminated, the tank will need to be cleaned out. The contamination will also be in the carb. Even though it was cleaned, if the fuel supply is contaminated, it will need to come back apart and rinsed out.
Once everything is checked, the air cleaner is connected, the choke open, engine up to temperature, let's adjust the main jet.
Set the engine for full governed speed. If it won't take full speed, open the main jet until it will. Take hold of the governor linkage and push the throttle closed. With the engine held at idle, release the governor, the throttle should go wide open until the engine is up to speed. Listen to the engine while it is coming up to speed. If it hesitated or dies, open the main jet 1/4 turn, try again. Keep doing the test, opening the main jet 1/4 turn at a time until the engine will accelerate without hesitation. A single puff of black smoke is what you want to see.
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