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SU Carb Questions

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KVH

KVH

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From another thread below:

DrEntropy said:
SU's are dead simple once you grasp what's happening in there. About as complex as a bathroom faucet.
Click to expand...

Here are some questions I've always had:

1) The SU needle rises when you hit the accelerator. Is that because the butterfly valves open and let in

<span style="font-size: 14pt">MORE AIR OR MORE FUEL ??</span>

Which is it? Chicken or the egg?

3) And how does the fuel then get evenly spread to all 4 cylinders?

2) Also, how does the extra fuel get into the cylinder to explode in the first place? Does it just run sloppily down the intake manifold when the butterfly opens? That seems too primitive. If the gas is being sucked into a vapor after running up the jet needle path, that causes me to ask the chicken or the egg question again.

<span style="font-size: 14pt">How is the engine drawing in greater air to lift the needle before the extra gas got there in the first place??</span>
 
To perhaps over-simplify things...

The engine is an air pump. When you open the throttle butterfly you are allowing it to draw in more air. That air flows through the carb throat creating a bit of low pressure. It also exposes the BACK side of the SU's piston to engine vacuum. (Look on the back of the piston near its bottom edge and you'll find a hole or two). Those holes in the piston allow "vacuum" to be exposed to the inside of the vacuum chamber and therefore... the top of the carb piston. This in turn causes the piston to move up against the spring, drawing the needle out of the jet.

The air flowing over the jet is at lower than atmospheric pressure and this allows the system to draw fuel out of the jet. This is somewhat similar to how a single-action paint air-brush works. Some of the fuel will be atomized as it is drawn up out of the jet. Some will "dribble" down the walls of the intake manifold.

As far as which comes first, air comes first. As I mentioned at the start, the engine is an air pump and its ability to draw in air (even at low speed while cranking on the starter) will draw in atomized fuel allowing it to start. As for starting, when you apply the choke, the SU jet is lowered which simply increases the area for fuel to flow out of the jet.
 
Kentvillehound said:
Here are some questions I've always had:

1) The SU needle rises when you hit the accelerator. Is that because the butterfly valves open and let in

<span style="font-size: 14pt">MORE AIR OR MORE FUEL ??</span>

Which is it? Chicken or the egg?

3) And how does the fuel then get evenly spread to all 4 cylinders?

2) Also, how does the extra fuel get into the cylinder to explode in the first place? Does it just run sloppily down the intake manifold when the butterfly opens? That seems too primitive. If the gas is being sucked into a vapor after running up the jet needle path, that causes me to ask the chicken or the egg question again.

<span style="font-size: 14pt">How is the engine drawing in greater air to lift the needle before the extra gas got there in the first place??</span>
Click to expand...

I'll try to answer. Others will no doubt correct or add to my answers.

1. The butterfly valve opens to increase the flow of air through the carburetors, manifold, and into the cylinders. We want to keep the mixture constant with changes in flow, so a fuel metering system, in this case an SU, must then increase the flow of fuel.

2. The air flowing through the reduced cross-sectional area above the jet has a lower pressure than the air above the fuel in the float bowl due to Bernoulli's Principle. Bernoulli said, "In a venturi, velocity increases and pressure decreases" in Italian, I guess. This difference in pressure pushes the fuel from the float bowl and out the jet. Or you can think of the fuel being sucked out the jet if this makes you happy. The low pressure causes the fuel to atomize and mix with the air on its way to the cylinders.

3. Because the manifold splits it up evenly, more or less.

Unnumbered large print question. Because the opened butterfly valves allow more air to flow into the cylinders.
 
Looks like John & Doug have pretty well nailed it. To restate slightly, the cylinders are always trying to suck a full load of fuel/air mixture; the throttle is what keeps them from being filled completely. So when you mash the loud pedal, it lets more fuel/air into the cylinders, which increases the flow through the carbs. The additional flow through the venturi (the restricted area under the carb piston) causes the vacuum (or depression) to increase, which in turn lifts the pistons until it is roughly the same as before. That's why these are sometimes called "Constant Depression" carbs.
 
John Twist of University Motors did a 2 part series on Youtube.com about the basics of SU Carburetors. Explained it really well for the layperson to understand. Go to Youtube and do a search for "SU Carburetors" and I'm sure you'll see it. Thanks!
 
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