Carburettor air valve spring variation - significance

On checking over my Stromberg CD-175 carbs I found that two of the springs had a free length of 146mm and two had a free length of 150mm !

I rebuilt the carbs way back in 2004 with a new rebuild kit so I was quite surprised to find this variation and I wondered if anyone out there on the list could offer an opinion as to the significance of this. The spring rates of all the springs appear the same based on the compressed length under a constant load.

The car has run fine through the summer and it’s now hibernating so I am thinking should I replace the springs or is this an insignificant issue ???

Best regards,
(1973 DDS Series 1)

That’s less than 3% difference. Do you suppose the springiness of the rubber diaphragms in the four carburettors are matched equally well or whether there is exactly the same amount of fluid in the dashpots?

You may be surprised to know that compression tests on cylinders are not that evenly matched and neither are the individual cylinder volumes.

The spark advance controlled by a distributor is even less well controlled.

I’d hazard to say the spring length variation isn’t going to stand out as particularly worrisome.

kind regards


Thanks Marek, … a good point well made… a guy at a carb specialist tells me the new springs are 155mm long so maybe they all set a bit with age.

(1973 DDS Series 1)

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