9 replies to this topic

[gaspen]

Hi !

 

My 998 is feeded by an AUD679 HS4 carb. The specs says it has a red spring which gives a load of 4,5 oz. at 2,635" length

 

I measure the spring on a small scale and it shows ~4 - 4.1 oz.

 

I see no color on the spring. Another springs have pretty different values so I guess this is (was) a red spring.

 

Does this deviation make any difference in the engine's operation ? 

 

https://sucarb.co.uk...on-springs.html

Edited by gaspen, 05 November 2024 - 04:56 PM.

[timmy850]

Changing the spring to a different rated spring (like yellow or blue) would make a measurable difference when tuning the carb.

The small amount yours is off the quoted spec may make a tiny difference. Comparing a new 4.5oz spring to a lighter 4oz spring - the lighter spring will give a slightly leaner mixture

If you have a single carb you probably won’t notice it, but if you had twin carbs and they were mismatched it might cause an issue

Best thing to do is measure the fuel mixture and see if it needs to be replaced

Edited by timmy850, 06 November 2024 - 09:00 AM.

[Rubbershorts]

Changing the spring to a different rated spring (like yellow or blue) would make a measurable difference when tuning the carb.

The small amount yours is off the quoted spec may make a tiny difference. Comparing a new 4.5oz spring to a lighter 4oz spring - the lighter spring will give a slightly leaner mixture

If you have a single carb you probably won’t notice it, but if you had twin carbs and they were mismatched it might cause an issue

Best thing to do is measure the fuel mixture and see if it needs to be replaced

 

Hi Timmy, could you explain that to someone who doesn't fully understand the workings of a carb yet please? I thought the stronger spring would keep the mix lean, by exerting more force and pushing the needle back down into the jet. Have I got this wrong?

 

Cheers, Daz.

[gaspen]

Changing the spring to a different rated spring (like yellow or blue) would make a measurable difference when tuning the carb.

The small amount yours is off the quoted spec may make a tiny difference. Comparing a new 4.5oz spring to a lighter 4oz spring - the lighter spring will give a slightly leaner mixture

If you have a single carb you probably won’t notice it, but if you had twin carbs and they were mismatched it might cause an issue

Best thing to do is measure the fuel mixture and see if it needs to be replaced

 

Thank you! Actually the car works fine, I was curious about this thing

 

I remembered that I have a Mini 850 in pieces with a HS2, with the same spring. I will test it also :) 

[Lplus]

 

Changing the spring to a different rated spring (like yellow or blue) would make a measurable difference when tuning the carb.

The small amount yours is off the quoted spec may make a tiny difference. Comparing a new 4.5oz spring to a lighter 4oz spring - the lighter spring will give a slightly leaner mixture

If you have a single carb you probably won’t notice it, but if you had twin carbs and they were mismatched it might cause an issue

Best thing to do is measure the fuel mixture and see if it needs to be replaced

 

Hi Timmy, could you explain that to someone who doesn't fully understand the workings of a carb yet please? I thought the stronger spring would keep the mix lean, by exerting more force and pushing the needle back down into the jet. Have I got this wrong?

 

Cheers, Daz.

 

Increasing the spring force not only keeps the needle further down which weakens the mixture, but it also increases the pressure drop across the jet bridge which richens the mixture because of greater suction.  I think the spring is intended to ensure the carb remains linear throughout the power range by ensuring the piston doesn't quite reach full travel at full throttle/revs - at which point fuelling would depend purely on pressure drop.  Others may have more information.

[timmy850]

For a given amount of air passing through the carb (say cruising at a constant speed), a lower piston will give a richer mixture. The same amount of air passing through a smaller area means the airspeed will be faster and will draw more fuel from the jet

Remember that the jet doesn’t “pump” fuel into the carb based on the height of the piston. The fuel is drawn out of the jet based on the speed of the air passing over the jet, and the size of the opening in the jet

The spring needs to be selected to match the engine spec. Too weak and the piston will fly fully open at too low of a load, and then it is unable to use the needle profile to add fuel as load/rpm increases. If the spring is too heavy it won’t open up fully and will reduce airflow potential

This effect is also noticeable when the incorrect damper oil is used and you press down quick on the pedal. Too thin means the damper lets the piston rise quick and you get a lean bogging condition (as the piston is too high and the airspeed drops). Too thick oil means the piston is too low and that causes an overly rich condition. If the oil is just right it’ll provide the correct enrichment (aka accelerator pump effect) under throttle

Edited by timmy850, 06 November 2024 - 07:46 PM.

[Spider]

The descriptions given in many places as to how Constant Depression carbs work that I have read, including in the SU manuals, is a bit misleading I feel.

Before we go too far, just be sure you have your head around flow (rates) and pressure. Keep in mind too, that in reality, there is no such thing as vacuum,though we often use the term, particularly when talking about manifolds etc.Flow and Pressure are 2 quite different things, however, in various publications, they are often intertwined and can mislead the reader.

 

SU Carbs work, as Timmy mentioned, on Air Flow (not Vacuum as the books suggest). The Rate of flow through the Carb is what lifts the Carb Piston. The Piston balances the quantity of flow in 2 ways, by the ratio of the area above the Carb Piston to that below it and by the spring rate, which us used for fine tuning of this balance. It measures the flow rate by the lift of the piston, which in turn, lifts the needle to meter fuel in direct proportion to the air flow rate. It's this operational aspect of the SU that in my view, stands them head and shoulders about all other types of carbs. To get anything better, you need to go to fuel injection.

[gaspen]

The descriptions given in many places as to how Constant Depression carbs work that I have read, including in the SU manuals, is a bit misleading I feel.

Before we go too far, just be sure you have your head around flow (rates) and pressure. Keep in mind too, that in reality, there is no such thing as vacuum,though we often use the term, particularly when talking about manifolds etc.Flow and Pressure are 2 quite different things, however, in various publications, they are often intertwined and can mislead the reader.

 

SU Carbs work, as Timmy mentioned, on Air Flow (not Vacuum as the books suggest). The Rate of flow through the Carb is what lifts the Carb Piston. The Piston balances the quantity of flow in 2 ways, by the ratio of the area above the Carb Piston to that below it and by the spring rate, which us used for fine tuning of this balance. It measures the flow rate by the lift of the piston, which in turn, lifts the needle to meter fuel in direct proportion to the air flow rate. It's this operational aspect of the SU that in my view, stands them head and shoulders about all other types of carbs. To get anything better, you need to go to fuel injection.

 

Connected to the original question : could you explain what is the purpose (pros-cons)  of the swinging needle ? I learnt that it is self-centering, but the needle guide (or holder) holds it in an angle.

Edited by gaspen, 08 November 2024 - 11:44 AM.

[Spider]

Connected to the original question : could you explain what is the purpose (pros-cons)  of the swinging needle ? I learnt that it is self-centering, but the needle guide (or holder) holds it in an angle.

 

To maintain fuel metering accuracy.
 

[timmy850]

It is “bias sprung” which means it always is biased to the same side