11 replies to this topic

[087dave]

Hi all, just 2 questions on a carb

 

1. Does the spring in your carb really matter in your carb if so which one works best with a hs4

 

2.I have a 75 998 auto with a stage 2 head and stage 1 kit HS4 carb, I have a ABD needle just

wondering should I change to a ABY I do not have access to a RR

 

Thanks

[Dusky]

The spring does matter for accelerayion I believe..

[Guess-Works.com]

Yes the spring matters, to a lesser extent in road cars....

 

It's there to make the range of the needle fit the range of the engine...

[Dan]

The spring isn't just matched to the carb like that, an HS type spring works best with an HS4 but the exact one of the 3 or 4 available is part of the tuning of the thing. For most Minis a red spring is the right one, but a Midget with the same carbs would normally use a light blue.

[087dave]

Which one would u guys think

998 auto stage 2 head stage 1 kit
Tks

[Guess-Works.com]

The standard one will be fine

[Dusky]

Winsu also gives what spring to use ( I need yellow)

[carbon]

087Dave - the carb spring you should use is the red one.

 

In my 998 auto I found it pulled much better when I fitted a replacement red sping from Minispares about a year back. Although the previous spring was also red there are differences, and the more recent spring was stronger and eliminated a weak spot when pulling away.

[Ethel]

The spring is a critical component. The speed of the air going across the "bridge" creates the depression that lifts the piston in the dashpot to raise the needle & meter the fuel. Apart from the piston's weight, it's the spring that determines how high the piston lifts relative to the depression, and thus the amount of fuel drawn by the engine. Too weak a spring and the piston will reach is maximum opening early, losing the ability to control the air fuel ratio; too strong and the carb can restrict the air flow.

[Dusky]

So stronger is better?

[carbon]

Stronger is not better... it just needs to be strong enough

 

The spring needs to be the right strength for the application, for a starting point check out the 'Tuning SU Carburetters' book which gives a great range of examples. Almost without exception a correctly sized single or twin 1.25 or 1.5 inch SU will be blue or red.

 

Yellow spring is very rarely used on the 1.25 or 1.5 inch SUs. When it is used it is probably because the carb is seriously undersized for the application or there is sometheing else unusual.

[Dusky]

They adviced me a yellow one, fitting 288 deg cam..