No Accelerator Pump.

J

jeepguy1948

Well-known member
I’m not totally ignorant, I have some thoughts of my own but I want to hear what other (more experienced than me) owners have to share. Why does a Model A carburetor (Zenith, Tillotson, or Marvel) not have an accelerator pump? Was the technology not there yet or is it simply not needed? Would it benefit from one?
 
The Zenith carb provided with the 1932-34 Model B had a power jet tube that supplied more fuel under acceleration. Unlike the induced shot of fuel typical of conventional accelerator pump designs, the Model B carb relied upon a tube that supplied additional fuel via suction when the throttle butterfly opened up beyond a certain point.
 
The Zenith carburetor for the Model A also had a power tube that supplied more fuel during acceleration. The power tube is called a Secondary Well, see https://modela.org/secondary_well.html

An acceleration pump or other means to supply additional fuel during rapid opening of the throttle is required because part of the fuel supply travels down the inside wall of the intake manifold. This fuel is slower to arrive at the cylinders causing a lean mixture during acceleration. Under constant rpm the slower fuel at the wall of the intake manifold and the fuel suspended in the air make up the correct air/fuel ratio even though they arrive at different times.
 
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Updraft carbs don't have an accelerator pump because if you hit the pedal when the engine isn't running the gas would just run out on the ground.Some of the Tillotsons and a few other aftermarket carbs had an enrichening circuit.Really,most of us just call it an accelerator pump,but it's not.There are a few variations on how they go about it,but they all keep the gas in the carb when you hit the pedal.
 
The Marvel carb uses a pitot tube facing the incoming air. As airflow rises, pressure in the pitot tube rises with it and presses down on the fuel stored in the accelerating and idle wells, shoving extra fuel out of the nozzle. The pitot tube has no moving parts. Fuel flow is metered by mass airflow, like fuel injection, so there's nothing to wear out.
 
The Secondary Well is a reservoir for the Idle Jet. As the engine RPM increases beyond a fast idle, the Secondary Well does not refill fast enough to be of any meaningful assistance in accelerating the Model "A". Acceleration is a function of the Compensator & Cap Jets through mid-range RPM & the Main Jet for high RPM. The Cap & Main Jets both spray at high RPM, but only the Cap Jet is regulated by the Compensator Jet. The Main Jet gets its gasoline supply via a direct passage from the Float Bowl.
 
nkaminar said:
The Zenith carburetor for the Model A also had a power tube that supplied more fuel during acceleration. The power tube is called a Secondary Well, see https://modela.org/secondary_well.html

An acceleration pump or other means to supply additional fuel during rapid opening of the throttle is required because part of the fuel supply travels down the inside wall of the intake manifold. This fuel is slower to arrive at the cylinders causing a lean mixture during acceleration. Under constant rpm the slower fuel at the wall of the intake manifold and the fuel suspended in the air make up the correct air/fuel ratio even though they arrive at different times.
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It's more about keeping the air/fuel ratio correct at the instant a large volume of air is introduced. You open the throttle and the vacuum drops. So does the air speed that is critical to the fuel being sucked into the air stream. The pump supplies the fuel for that instant when a large volume of air is added.
The SU carb still does not have a pump it has a variable vacuum operated venturi to keep the A/F ratio in the right range. But if Henry had adopted the SU (invented in 1918) we'd be talking about what oil to use in it.......
 
The oil slows down the vacuum operated venturi so that the more fuel is supplied during acceleration. The viscosity of the oil determines the air/fuel ratio during acceleration. There is a piston in the oil chamber that operates like a shock absorber. It has a one-way valve so that when the throttle is closed the venturi quickly returns to the idle position.
 
nkaminar said:
The oil slows down the vacuum operated venturi so that the more fuel is supplied during acceleration.
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Yes increased vacuum sucks more fuel around the needle so you don't get a lean stumble. As the venturi lifts it also raises the tapered needle keeping the proper fuel ratio across the whole RPM range. It was pretty sophisticated for 1937 and still works well today.
 

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Bob Bidonde said:
The Secondary Well is a reservoir for the Idle Jet. As the engine RPM increases beyond a fast idle, the Secondary Well does not refill fast enough to be of any meaningful assistance in accelerating the Model "A". Acceleration is a function of the Compensator & Cap Jets through mid-range RPM & the Main Jet for high RPM. The Cap & Main Jets both spray at high RPM, but only the Cap Jet is regulated by the Compensator Jet. The Main Jet gets its gasoline supply via a direct passage from the Float Bowl.
Click to expand...

Here is how the "Accelerator Pump" function is handled in the Zenith Model A Carb:

Quote from www.Modela.org website:
https://modela.org/secondary_well.html


Secondary Well
The Secondary Well provides an extra shot of gasoline instantly when the throttle is opened rapidly.
The in flow of extra fuel at low speeds takes care of an otherwise lean mixture that would occur just off idle.
There were several variations of Secondary Well designs, with the undercut in different locations. In some combinations of castings and secondary wells, the compensator jet will come into contact with the brass secondary causing restricted flow. So, I recommend that 2 gasket washers be used with this jet to prevent that possibility.
With really dirty carburetors, the secondary well main bore and all of the small holes may need to be cleaned out.
The small hole in one side is a #70 drill size, but it's not critical. The hole is referred to as the "vacuum breaker" hole.
The small hole that goes through both sides is a #54 drill size, but it is not critical.
The secondary well threads are 3/8x24 UNF (SAE).
 
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