Constant Depression Carbs Theory of Operations

©By: Dan Masters,
        Alcoa, TN

Last update: June 12, 1999

This article is being reprinted with permission of the author.

This was originally written as an email post on the Triumphs and Spitfire mailing lists. It is posted here to help those not versed in the SU and Zenith/Stromberg carbs. (ZS) understand how they work. While there are a few differences between the SU and ZS carbs. the theory is the same.

One of the members (Patrick) of the mailing list asked:

>Next question, could someone explain to me what the difference is between
>a normal carb and a constant depression carb. And why do we put oil into it?

I'm an electrical rather than a mechanical, but I'd like to take a shot at your question if I may. Keep in mind, I don't understand things very well, so I have to break them down into bite sized pieces if I want to make any sense of it. That means this will be a long response. I'll describe the operation of the carburetor in layman's terms, because that's the only way I understand it, so my description may not be 100% technically accurate.

First of all, take a look into the throat of your carburetor, and look at the needle/jet assembly. You'll notice the needle moves in and out of the jet as the piston moves up and down. You'll also notice that the needle is tapered. As a result, when the piston is down, as in idle, the larger diameter of the needle almost completely fills the jet, leaving very little room for fuel flow. On the other hand, when the piston is up, as in high RPM operation, the needle is pulled out to where only the thin tip remains in the jet, allowing for a large opening in the jet for fuel flow. Basically, you have an infinitely variable jet, between the limits of the needle movement.

Next, look through the bore of the carburetor, from the inlet to the outlet. You'll notice that the inlet and outlet bores are about the same, but the piston creates a restriction to air flow through the carburetor. The restriction of the piston forms what is known as a venturi. A venturi has an interesting property, in that air flow through it creates a reduced pressure in the throat of the venturi. In America, we refer to a reduced pressure such as this as a vacuum, while our British cousins use the term "Depression." The magnitude of the depression depends on two things: the rate of air flow and the size of the restriction. For a given air flow, a smaller restriction gives a larger depression. For a given restriction, a larger air flow gives a larger depression.

As the needle/jet assembly is located in the throat of the venturi, the depression created by the flow of air "sucks" fuel out of the jet (I put the term "sucks" in parentheses because it is not technically correct, but for a layman's explanation, it'll do). For a given depression, the amount of fuel drawn out of the jet will depend on the position of the needle, and for a given needle position, the amount of fuel will depend on the depression. The rate of fuel flow will increase as the needle is withdrawn or as the depression increases.

Now, on to the meaning of the term "constant depression," or CD, as in "Zenith-Stromberg CD carburetors." Within the body of the carburetor are several internal passages, connecting various parts of the carburetor to various ports in the air flow path. Without going into a great deal of detail, the passages and ports are arranged to adjust the position of the piston to engine and air flow conditions such that the depression in the throat of the carburetor, and thus seen by the needle/jet assembly, remains constant under all operating conditions. At high engine speeds, and the corresponding high air flow, the piston is up in the carburetor, creating less of a restriction. At low engine speed, and the corresponding low air flow, the piston is lower, creating a larger restriction. The design of the carburetors is such that the depression produced at the low air flow/large restriction creates exactly the same depression as the higher air flow with less of a restriction.

As a result of the constant depression operation, the only factor determining the fuel flow is the position of the needle. The "suction" is the same at all engine speeds, but when the needle is out, the larger opening in the jet allows more fuel to be drawn.

The dashpots in a CD carburetor are probably the most misunderstood part of our Triumphs, even more so than the Lucas electricals. There is almost a mysticism attached to them, and to what kind of oil to use.

To get a understanding of the dashpot function, pull the cap and plunger from one of your carburetor dahspots and examine it closely. You'll notice it has a slender rod, with a "gadget" on the end. You'll see that the gadget is a small cylinder, held in place between two washer-like items. You'll also notice that the cylinder diameter is only very slightly smaller than the internal diameter of the guide tube on the piston that it fits into. If you look real close, you'll notice that the cylinder, acting in conjunction with the two washer-like devices, forms a check valve. When the cylinder is down, oil flows freely throuigh the center of the cylinder. When the cylinder is pushed up, oil flow is blocked from the inside of the cylinder, and the only way for the dashpot oil to flow is in the very slight clearance between the cylinder and the guide tube walls. Oil, being a very viscous liquid, will not flow freely through the limited clearance around the cylinder, but will flow quickly through the cylinder.

You can demonstrate this to yourself by putting the plunger back, with the dashpot filled with oil, and make this simple test: With your finger, lift the piston in the carb, and notice how much force it takes to move it, and how slowly it rises. Now release the piston, and see how quickly it returns to the bottom of the carburetor. The design of the dashpot and the check valve is such that it is only effective in keeping the piston from rising quickly, and has no effect at all on the piston's falling.

By The Way (BTW), this tells you exactly how much oil to use, if not what kind -- As long as you have enough oil to cover the check valve when the piston is in the lowest postion, you have enough oil. Any oil that goes over the top of the guide tube has no effect on the operation of the dashpot, although you may want to add a little over the guide tube to make up for losses so you don't have to top off as often)

Now that we know HOW the dashpot works, the next question is WHY. Very basically, the dashpot serves EXACTLY the same function as the accerator pump in an American type carburetor, ie, it gives an extra squirt of gas when you stomp on the accelerator pedal. That is the ONLY purpose of the dashpot. During steady speed driving, or during gradual acceleration, the dashpot serves no function. Anytime the required rate of rise of the piston, from an increase in speed, is slower by nature than the limits imposed by the oil/check valve, the dashpot has no effect. During de-acceleration, when the piston is falling, the dashpot not only serves no function, it doesn't even work.

As the dashpot is only used during quick acceleration, we need to analyse what happens when we put our right foot down hard. When you stomp on the gas pedal, two things happen - the throttle plates snap open, and the engine requires a richer mixture. When the throttle plates snap open, the top of the piston is immediately exposed to the full vacumm of the engine intake, and it would immediately go to the top of its travel if the dashpot were not there. However, the engine speed cannot change instantaneously, so the air flow doesn't immediately increase sufficiently to offset the rise of the piston. We have the piston at its high speed position, but the airflow doesn't create the requisite depression, so we end up with a lean mixture, just when we need a rich mixture. By limiting the rate of rise of the piston, the needle/jet assembly still sees nearly the same depression as before, plus it is now exposed to the larger vacuum from the engine, so more fuel is drawn from the jet than before, giving the required richer mixture. As the air flow begins to catch up, the piston will have had time to rise to match. Now, then, what type of oil to use? For most of us, within reasonable limits I doubt that we would notice any "seat of the pants" difference at all. With no oil, or extremely low viscosity oil, the engine will stumble upon acceleration, very noticeably. With too high a viscosity, we would most likely notice sluggish acceleration, and would also probably notice a lot of black smoke from the exhaust from the rich mixture, although I'm not sure of this. Also, I wouldn't be surprised if you could get by with out any oil in a race car, as the throttle would be wide open, and the engine revs up, almost all the time, the only exception would be when braking or shifting gears. Even when braking, I would think the revs would stay up, as the engine is used for braking as well as the brakes. If your grandmother were to be driving your car, no oil at all would be required most likely, as she would probably never stomp hard on the gas.

If you want to be sure, the only way to test is to try different viscosities and see. Just remember, the ONLY time it matters is during hard acceleration -- there is no need to monitor ease of starting, smooth running, gas mileage, top speed, or any other performance aspect.

I may well be full of crap here, but at least that's the way I understand it. I know I'm not 100% correct, but I think I'm close enough for a layman's purpose. I stand ready to be corrected, and to be banished back to the world of Lucas!

Dan Masters,
Alcoa, TN

'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74

Return to the Index of Tech. articles

To email me with comments or questions.