It is a matter of fact that mechanics/technicians condem, adjust or replace the carb "TOO SOON". A carburetor can only function correctly (or be fine tuned) when the:
1. The engine mechanical condition is sound.(correct steady vacuum)
2. The ignition system is adjusted and operating correctly (synchronized with the mechanical timing components).
Always troubleshoot in the correct sequence.
Mechanical
Electrical
Fuel
With the choke partially engaged it indicates that the engine air fuel mix is "over lean" and the choke position is compensating by restricting air or enrichening bringing the mix closer to correct (15:1). The lean condition may be caused by:
Air being drawn into the cylinders at leaks anywhere from the intake manifold, gaskets,or carb, or fittings, hoses, accessories.
Lack of fuel being delivered by the carburetor idle/low speed circuit.
At idle the throttle valve is almost completely closed allowing fuel metering at the idle / low speed fuel ports. If these are clogged, closing the choke valve with cause fuel to flow throw the main jet at idle.
I would suggest:
Measure the ammount of vacuum at idle at the intake mainfold using a gauge. (18-21 HG) steady reading should be observed.
Unsteady wandering needle, or low pulsing indicates leaks.Check all gasket areas and fittings, hoses and any accessory that operates off of intake manifold vacuum.
Ensure the ignition system is tuned and timed.
(proper timing will result in a optimum vacuum reading).
Ensure carburetor passages and jets are clear and clean.(to be done right, dis-assemble and inspect)Carb cleaner followed by low pressure air usually clears anything.
