Shorter carb spring

[Jasonking]

Hi all, does anyone know what effect a shortened HS2 carb spring will have on general performance/running? The large damper spring inside the dashpot, just installed a new SU std red spring, and the old one is a good 30-35mm shorter?

[timmy201]

The important part is the spring tension rather than the length

For a given amount of air at a constant speed, a stiffer spring will make the SU piston ride lower. A lower piston means the air will speed up and draw more fuel

Stiffer spring = richer mixture

[Jasonking]

So a shorter spring should have more tension/stiffness? Therefore a ricer mixture?

[wantafaster1]

No.
You're old spring has been compressed inside a carb for however many years rather than resting at its natural free length.
The new one isn't faulty just because it's long.

[Peter Laidler]

Several years ago I remember making a weighted tester thinggy for measuring the spring tension of carb springs after I suddenly became the owner of about 25 assorted unknown coulour and value springs.

Someone on the forum had already made a much simpler gadget! But somewhere on the forum there'll be a simple method of testing the various springs by weight loading over the movement.

Tim is right. It is the tension hat governs the spring and not the length. What is important is to have two equal springs - if you've got two carbs of course!

[Jasonking]

Didn’t think the new red SU spring was faulty, just that the unknown coloured original is so much shorter even after 56 years in the single carburettor, also the needle jet doesn’t want to come out so presumably after releasing the grub screw it’s just being held in by carbon and gunge?

[timmy201]

You can easily test them with some scales, just compress to the right length and measure the load

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

[Festus]

I swapped to a shorter carb spring in my HS2 once and noticed a quicker throttle response, but it did mess with the tuning a bit. The shorter spring can affect how the piston lifts, which might cause the engine to run a bit richer or leaner depending on how it changes the airflow. I had to tweak the mixture and idle speed to get it back to a smooth running state. If you're getting it all dialed in, just keep an eye on how the engine behaves and adjust as needed.

[Cheeser]

Suggest you look at the new AC Dodd videos on YouTube explaining theory of the calibration of SU carbs, better than anything else I have seen.