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Classic Engines, Modern Fuel

Comments on Topic: SU and Bing carburetor

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Submitted by Anonymous


Su and Bing (for bikes) are quite similar. The mixture is made constant through a taper needle that lifts into a jet. The needle is fixed to a piston that chokes more or less the fresh air flow.

But the difference is in the command.

For SU, the command acts on a butterfly and through a vaccum sensing circuit, the piston and needle are lifted.

For Bing, there is no butterfly, the command directly lifts the piston and needle. Simpler.

Why SU choose this more complicated system ?



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Submitted by The Author


The difference between the two carburettors is that in the SU, the piston MEASURES the air flow through the carburettor and lifts the needle out of the jet to provide the correct mixture. The volume of air flowing into the carburettor is controlled by the butterfly.

In the BING, the volume of air flowing into the engine is CONTROLLED by the piston, in the same way as the butterfly on the SU. There is no means of MEASURING the actual air flow.

When the engine is running at a steady RPM at a constant load, there is no difference between the two carburettors. The difference arises when the throttle is opened or closed. The easiest way to appreciate this is to imagine the following.

When the engine is running at low RPM, low load, there is a small volume of air flowing into the engine, on both the SU and BING carburettors the piston is virtually closed.

When the throttle is fully opened.

On the SU, the piston rises slowly in response to the increasing RPM and volume of air flowing into the engine, maintaining the correct mixture. Indeed, the damper slows the rise of the piston to enrichen the mixture to improve acceleration.

On the BING, the piston is fully withdrawn. As there is only a small volume of air flowing into the engine, the pressure in the choke rises, reducing the flow of petrol. The engine runs weak until the RPM and air flow rises to the point where the mixture is restored.

Bikes usually have a higher power to weight ratios than cars. This means the engine revs can increase more quickly and the problems caused by the BING is less severe.

Hope this adequately explains the difference.


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Submitted by Anonymous

Thanks Paul this is clear. SUs are more sophisticated.

My Bike (Bings) at cold start is ok at standby but the egine immediately stalls when I turn the throttle. All becomes right after a few minutes. This is probably the reason. I know others have this phenomenom with those bikes.

The drawback of Bings increased when cold because the engine needs to run richer at cold start.


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