How SU carbs work - principle of piston movement

[RobWarren]

Yes there are dozens of threads on carbs but I have a specific question that I would like your help with.

I was reading an article explaining how they work that said the following:

When the throttle is is opened more air flows towards the engine because the intake becomes less restrictive. The air speed across the piston increases resulting in a larger pressure drop over the piston. The pressure above the piston decreases and the piston is being sucked upwards. The opening area increases and air speed and pressure drop across the piston decreases until there is a new balance between the underpressure above the piston and the weight of the piston and spring load. More flow means the piston moves higher into the carb. until the balance is restored. This is why the pressure across the piston (and the airspeed) is virtually constant...... and the carburettor is called a constant vacuum carburettor.

My question is as follows:

If the air flow across the piston increases in speed then the pressure drops in the venturi...would this not suck the piston down?

How does the air in the suction chamber decrease to a lower pressure enough to suck up the piston?

I've scoured the web for ages trying to find good articles and detailed diagrams but to no avail. If you have them, please post them!

Cheers

Bob

 

[john_j]

<span style="font-style: italic">If the air flow across the piston increases in speed then the pressure drops in the venturi...would this not suck the piston down? </span>

It's the vacuum above the piston that sucks the piston up.

(To be precise, the vacuum above allows the air pressure blow to push the piston up)

 

[GregW]

My question is as follows:
If the air flow across the piston increases in speed then the pressure drops in the venturi...would this not suck the piston down?

There is a hole(or 2) in the bottom of the piston that equalizes pressure between the vacuum chamber and the venturi.

How does the air in the suction chamber decrease to a lower pressure enough to suck up the piston?

The low pressure in the vacuum chamber is working in opposition to atmospheric pressure from a vent in the carb (the other side of the diaphragm), and the spring.

I've scoured the web for ages trying to find good articles and detailed diagrams but to no avail. If you have them, please post them!

There is a book called "How to Build & Power Tune SU Carburettors" that you may be interested in.

 

[Jerry]

Since we are talking about carbs, how about a question on the needle jet? HD8s have limited adjustment for mixture. I have a set that no longer has any adjustment left to lean out the engine. If I drop the needle a small amount, won't it shut off the jet and limit the gas flow into the intake manifold?

Jerry

 

[RobWarren]

Ok I'm starting to get my head around it. kind of.

I suppose my questions is how does a vacuum get created in the suction chamber in the first place?

If the suction chamber is vented to the outside and a vacuum is created in the chamber, surely the chamber would suck air in from the outside and simply equalise with the atmosphere?

Still a bit confused....mainly as I'm no physicist - but enjoying learning the principles.

 

[GregW]

I suppose my questions is how does a vacuum get created in the suction chamber in the first place? If the suction chamber is vented to the outside and a vacuum is created in the chamber, surely the chamber would suck air in from the outside and simply equalize with the atmosphere?

Hi Rob,
Here is a basic drawing which I hope will help. I've left some things out like the spring. When the engine is off, the piston rests on a little stop, so it doesn't close entirely. This allows air to enter the engine for starting. Once the engine fires, vacuum from the cylinders pulls air out of the vacuum chamber through the hole in the piston. See the red arrow in the diagram. Note that the piston hole is inboard of the piston stop. The vacuum chamber is sealed from the atmosphere by the diaphragm (thick black line on top of the piston in diagram). The vacuum has to be strong enough to overcome atmospheric pressure, gravity, the spring and the damper. Hope this helps.

 

[RobWarren]

Perfect.

Understood.

Thanks gents.....as always.

Bob

 

[GregW]

Since we are talking about carbs, how about a question on the needle jet? HD8s have limited adjustment for mixture. I have a set that no longer has any adjustment left to lean out the engine. If I drop the needle a small amount, won't it shut off the jet and limit the gas flow into the intake manifold?

Jerry

Hi Jerry,
If I was to venture a guess, I would think that the jet may be ovaled from mis-alignment. Probably best to replace them and the needles if they are worn too. Have you taken the jets out to inspect them?

 

[Jerry]

These are new carbs and needles. (one year old). I have not taken out the needles. I will have to try to see if they are positioned correctly.