The fact that partial choke allows the engine to run normalish is indicative of a vacuum leak in my experience.
Partially closing the choke as the throttle plate is opened, causes an increase in vacuum applied to the carburetor compared to what the vacuum would be with the choke open. This increase in vacuum allows the carburetor to act closer to normal when there is a vacuum leak.
Have you done anything to the engine lately which could cause a vacuum leak?
Another possible cause could be a nonfunctioning heat riser. This problem really raises its ugly head during cold weather. Lack of heat applied to the intake manifold during cold weather will cause the fuel to fall out of suspension and puddle up on the floor of the intake runner. Puddled fuel will not burn. Incidentally, partially closing the choke can actually increase the speed of the volume of air/fuel through the manifold, forcing the puddled up fuel to move into the cylinders, clearing out the puddles.
It will be easiest to check the function of the heat riser. Do that first.
Use an UNLIT propane torch to check for vacuum leaks. It works better that anything I have ever tried.
BTW, Its never the carburetor.
Even if it is, its much easier to do an inspection of the heat riser and do a vacuum leak test than to open up the carburetor and stare at the guts and see nothing wrong even if there is something wrong. The other problem with tearing into the carburetor is that if you don't put it back together correctly, and it was never the problem to begin with , you will then have created the dreadful two-problems-at-the-same-time scenario without the ability to check for a vacuum leak because it won't run because now the carb is jacked up.
Last edited by 52Carl; 12/17/2023 6:15 PM.
