When choke is applied, what circuit flows fuel?

[ledslinger]

I rebuilt the Stromberg WW on my 1961 GMC V6 305 4WD Suburban and post rebuild, it would run fine, but I had to prime it to get it to start. I took it apart again, and found debris in the screw that holds down the accelerator pump nozzles. The hollow screw is also the fuel passage for the accelerator pump nozzles. I cleaned that out and problem solved, it will cold start on the choke.

I'd never pondered it before, but it seems that the circuit that mostly flows during choke application on that carb is the accelerator pump circuit. That appears to be a logical circuit to flow when pressure below the choke plate is reduced, because the fuel in that circuit is higher than bowl level, held above bowl level by a check ball. Of course fuel flows from elsewhere if the carb doesn't have an accelerator pump. On carbs with accelerator pumps, is that where fuel for enrichment comes from when the choke is applied?

 

[Rhymingmechanic]

My lifelong assumption, not informed by technical knowledge of any particular carb, is that the mixture gets richer because the amount of air is reduced. That is, when the choke is closed, enrichment happens because there's less air, not more fuel.

I'll watch this thread and will probably learn something.

 

[tom slick]

How a choke works

 

[JTK]

I don't think any of the fuel circuits are specifically cut off with the application of the choke. They'll still activate based on throttle position.

 

[KrisZ]

My carb experience is with motorcycles, so not sure how it translates to cars. However on motorcycles at least, there are two types, that I at least have experience with, of “chokes” one type is actually a choke, which increases the vacuum and Venturi effect on the idle circuit and draws extra fuel this way.

Another is actually a fuel enrichement circuit, but most still refer to it as a choke. Heck on my Kawasaki the actuating lever has “choke” stamped on it, but the carbs have an enrichment circuit.
That is a separate circuit that adds extra fuel when opened.

But in both cases the idle circuit is involved.

Here is how it looks like from the service manual

 

[barryh]

Looks to me like a conventional choke i.e. a butterfly that restricts or chokes the air flow and therefore enriches the mixture. There is a linkage which provides a fast idle at the same time when the choke is applied.

 

[tom slick]

The idle circuit is below the throttle plate so it isn’t affected by the choke since the throttle plate is already restricting airflow. When the choke plate is closed it restricts airflow above the throttle plate which lowers the pressure through the venturi, That pulls (technically pushes) fuel through the main circuit causing the mixture to be rich. The main circuit doesn’t normally flow fuel at low speed.

Pumping the throttle a few times uses the accelerator pump to pump extra fuel into the manifold