Your engine may be backfiring through the carburetor.
Most likely this is due to too much air, or not enough fuel. Look for a vacuum leak. Check all connections that might have vacuum. Also check any vacuum hoses. You can use a spray carburetor cleaner and spray around the connections. If the idle smooths out, then you probably found the problem.
Next look for reasons the carburetor isn’t getting enough fuel.
Check fuel pump pressure (compare the number with your motors manual).
Check the float level.
The needle may be sticking in the seat, not letting enough fuel in.
You could have plugged passages. Ethanol will leave deposits behind that cleaners and compressed air will not remove. Use a thin wire to run down all passages.
Are the idle mixture screws way out of adjustment?
If the carburetor hasn’t been rebuilt, then do that if you suspect dirt is your problem.Check the idle mixture transition slot. As the throttle is opened more of this slot is exposed allowing additional fuel/air to be introduced. This gets the engine over the space between idle and the accelerator pump. Follow this slot all the way up to the venturi to made sure the passage is open.Make sure all the small passages in the venturi are clear. They are small and easily plugged.Make sure the gasket under the venturi is flat. They sometimes need to be trimmed.
More from my friend Jim:
My first thought is a “BUNA” black rubber accelerator pump cup that has enlarged due to alcohol contamination. With little or no accelerator pump function there would be a lean pop or backfire or sever stumble. When there is a vacuum leak, unwanted air enters the fuel mix stream and causes either high idle or rough idle and stalling. Closing the choke butterfly eliminates some of the normal air flow and the fuel mixture becomes more normal. I would remove the air horn to inspect the accelerator pump cup. I would also to see if an inlet pump check is required and/or present.(some “Mercarb” carbs don’t require a check ball and refill from a slot inside the main body). I would remove the Venturi cluster and blow the cluster and the passages in the main body.
UPDATE: Customer found the holes leading from the carburetor throat and idle mixture screw was plugged. This would mean little to no fuel from the idle circuit.