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Updraft Stromberg SF-3 or 4 Carburetor wanted


alsfarms
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Hello Jon,  Do both carbs. have  the provision for the accelerator pump?  I had mis-understood information on these Stromberg carbs.  I had thought that each series would have a variety of throat sizes.  I am now guessing that the variable aspect of the SF-3 is in the jet sizes.  The problem I have with the SF-4 is it is a larger and different design than the SF-3.

Al

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Alan - this link will explain the bore or "throat" size, and also the internal venturi sizes:

 

http://www.thecarburetorshop.com/Stromberg_SF_carburetors.htm

 

Note that some SF-4's do have accelerator pumps, others do not.

 

The numbers from your tag don't seem to show up in the Stromberg literature.

 

The best way to get a feeling for if a specific carburetor has or doesn't have an accelerator pump, is to look for the presence of the pump adjustment screw. Go back to the link, and scroll down to the pictured SF-4 carburetor. In the upper left corner of the picture is a long vertical 10x24 fillister headed screw. This screw is the pump adjustment screw. By varying the depth (adjustment) on this screw, one can vary the length of the pump stroke, thus saving gasoline?????:P

 

The carbs without an accelerator pump will not have the screw, rather just a short headless brass plug in the machined hole.

 

And the variable aspect, using your words, is in the internal venturi size. The physical size (SF-4) is the external size. The venturi size determines airflow. And the jetting is selected to correspond to the airflow.

 

Jon.

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I have an SF-4 Stromberg for sale in excellent shape. Doesn't look as if ever installed. Screws look as untouched. I will post some Pic later. I have had it for sale a couple years ago on this site.

Harold

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  • 2 years later...
  • 1 year later...
On 5/29/2018 at 9:22 AM, carbking said:

Alan - this link will explain the bore or "throat" size, and also the internal venturi sizes:

 

http://www.thecarburetorshop.com/Stromberg_SF_carburetors.htm

 

Note that some SF-4's do have accelerator pumps, others do not.

 

The numbers from your tag don't seem to show up in the Stromberg literature.

 

The best way to get a feeling for if a specific carburetor has or doesn't have an accelerator pump, is to look for the presence of the pump adjustment screw. Go back to the link, and scroll down to the pictured SF-4 carburetor. In the upper left corner of the picture is a long vertical 10x24 fillister headed screw. This screw is the pump adjustment screw. By varying the depth (adjustment) on this screw, one can vary the length of the pump stroke, thus saving gasoline?????:P

 

The carbs without an accelerator pump will not have the screw, rather just a short headless brass plug in the machined hole.

 

And the variable aspect, using your words, is in the internal venturi size. The physical size (SF-4) is the external size. The venturi size determines airflow. And the jetting is selected to correspond to the airflow.

 

Jon.

Jon, do you by chance have instruction on how to adjust that pump adjustment screw? Thanks

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The adjustment is really quite simple.

 

The suggested INITIAL setting is the top of the screw should be even with the face of the flange.

 

To understand adjusting, one must understand the working of the vacuum accelerator pump.

 

The pump ONLY functions AFTER the engine has started, NEVER before.

 

The accelerator pump is composed of an upper brass vacuum piston, a lower piston with a lower cup, and a spring-loaded connecting rod.

 

Once the engine is started, vacuum pulls the upper piston up into the vacuum chamber, charging the spring.

 

When the engine is accelerated, vacuum goes to zero, and the charged spring now drives the lower piston with the leather cup downward in the pump cylinder, releasing the fuel in the cylinder BELOW the pump.

 

The function of the adjustment screw is to allow the mechanic to limit the height the upper piston can rise into the vacuum chamber, which in turn limits the fuel in the pump cylinder below the pump.

 

So, with fewer words, the pump adjustment screw can adjust the volume of the pump shot.

 

For the maximum pump shot, simply remove the screw, and replace it with a 10 x 24 headless plug.

 

Jon

Edited by carbking (see edit history)
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