SU damper springs

[66S]

Hi guys,
I have works H4 carbs on my S and I am having trouble with a suspected leaness at about 60 to 65 MPH. I have #6 needles fitted but I can feel it tugging, even though the dyno says the air fuel is OK. I have been unable to identify the damper springs fitted, no colour visble but I was wondering if this could be possibly a spring problem. What effect would incorrect springs have on performance? It would be nice to know in practicle terms what the effect to springs that were too weak or too strong, or even none at all.

Regards
Al

[1071 S]

You want a spring that allows the piston to reach full travel at max power WOT. Too strong a spring will stop the piston rising all the way (thereby restricting air flow) and too soft will allow the piston to rise all the way before full power causing a leaning out at the top end (piston can't rise any further can't reveal thinner section of needle as air flow maxes out).

Cheers, Ian

[gr4h4m]

A stronger spring makes the mixture richer as you accelerate as it resists the lift and therefore the amount of air.

A weak spring can cause the car to lean out before returning to the correct afr as it allows the piston to raise too quickly.

[66S]

Thanks guys. What I seem to have is a leaness of an even (cruising) throttle around 4000 rpm. From what you are suggesting, the unidentified springs may be too weak ??

Regards
Al

[Chalkie]

http://www.minisport.com/mini-spare-par ... _Mini.html

they have plenty of springs i used Red in my mates and thats fine on twins but running 1380 and MJ

[66S]

Thanks, I am in Christchurch NZ and we have Larry @ Swift Automotive who hasd good stocks of SU and Mini parts.

Al

[mascher]

Springs can be checked or identified with a dial caliper or ruler and a postage scale. It's not uncommon to find that older used springs don't match which causes problems in dual setups.

The most common springs:
Red 4.5 oz 2.635" AUC 4387
Blue 2.5 oz 2.625" AUC 4587
Yellow 8 oz 2.75" AUC 1167

A friend had terrible problems tuning twin HS2s on an 1100. We eventually identified the springs in the carbs as Yellow. Likely that someone pulled the springs from a Volvo (common here in the U.S.) and thought they could get more power just by fitting stronger springs.

Kelley

[greyghost]

Just a quick question
At what length (mm) are you measuring the pressure of the springs?

[mascher]

Measure the load at the compressed length listed (inches). Free length of the springs is between 5 and 6 inches. Multiply inches by 25.4 to convert to millimeters.

Kelley