Option (a) concerns me because that carb has been giving me so much trouble for many years. Even after the professional rebuild, I personally put a rebuild kit (gaskets, power valve, float needles etc) only to find it still behaved the same way). There must be something fundamentally messed up with that carb. I think I'd prefer to start with a different candidate, yet would like some input.
Finally, I still have NO CLUE what on earth goes wrong with a carb such that it causes an engine to idle, yet stalls when shifted into gear. I wish I could understand this.
Hi Angela. You seem smart and carbs are not that hard if you learn the basics. It's imperative that you are able to test the circuits of the carburetor or it's a guessing game. Warped parts are common because the screws do not need to be tight, just barely snug, and most people have over torqued the screws which causes internal and external air and fuel leaks.
The reason the car would stall in gear is that engine is loaded and needs more volume of the correct air fuel mixture to run, since the engine is asked to do more work. The problem with your existing carburetor could be the wrong base plate, bent idle screws, bent butterflies or the wrong idle screws we're used at some point and forced into the seat which damaged the port.
My guess is that there is debris in the bottom of the float bowl which is not allowing enough fuel to be drawn in to the carburetor at idle. When the engine was running in neutral I can guess it was running poorly and light popping could be heard which can indicate a lean burn misfire. Lean burn misfires can water the eyes where a rich condition smells like heavy exhaust.
When in neutral, the spark plugs we're just able to fire the lean mixture, when in drive the air passing the crack in the throttle plate is slowed since the engine drops in RPM, and when there was JUST enough pull on the fuel holes in neutral, there was NOT enough pull on the holes in drive so the engine just died, makes sense.
Here is how the idle circuit works. At idle, the engine is pulling air on the bottom of the carburetor. The primary throttle plates are barely cracked. Fuel is waiting at the little crack in the throttle plates in little round holes in the carburetor body. The little round holes are at the end of a idle tunnel which runs through the body of the carburetor and leads to the float bowl where the fuel sits. The idle screws are used to control the amount of fuel that flow through the idle tunnel, the screws intersect the idle tunnel. When you turn the screws out more fuel can flow, conversely when you screw the screws in, less fuel can flow. You have to follow the idle tunnel from the baseplate to the body of the carburetor and make sure it's free and clear with a air hose and a tiny air gun so you can hear the movement of air to the float bowl.
I just rebuilt a 1964 390 police interceptor 4100 and the thing had all sorts of problems from other incorrect repairs. It wasn't easy. The utube tutorials we're very helpful to me to sort it out. Now the car run runs awesome, I doubt a carburetor shop could have done the job right. It was not something that could be rushed. I had to stone several parts to flatten them out for proper sealing and to eliminate leaks, this alone took me a couple of hours. The diaphragm for the secondary venturi pull off that came in the kit was too thin and I was unable to get it to seal when I applied vacuum to it. So I purchased a good one for 25 dollars, the whole kit cost me 30 dollars! Remember these cars are 50 years old and one has to have a simple, but practical understanding of the carburetor to do the job right, and all carburetor parts are not created equal. Good luck!
