So this "extra set of SUs" What car did they come from? What Springs are in your current Carbs?

I have 6 springs in my Carb Parts Stash, and just measured them as best I could and put the best matched pair into my HIF6's

If you have spoken to Joe C you probably have the best available information,

But to be honest, the viscosity of the dash pot oil and vacuum spring rating are all parts of the Black arts of SU, and honestly from my experience a good pair of matched springs of any flavour will suffice, then try some different oils. It seems VP auto parts only sells one spring type. Call them and ask what "color" they are

https://vp-autoparts.com/en/artiklar/spring-vaccum-chamber-su.html

Have you looked at the SU reference documentation? I seem to remember seeing a full list of various Carb/vehicle model/spring types somewhere on the SU UK site, but cannot find it now, but it's out there, it was like this following Jet /Needle chart

Jet Needle Recommendations (1933–1957) | SU Carburetter Guide | SU Carburetters

Discover jet needle recommendations for SU carburetters from 1933 to 1957. Find the ideal needle profiles for various vehicles and applications to optimize engine performance.

sucarb.co.uk

 

BFF;

Red marked Dashpot Springs are indeed what is required for the Volvo application...you could measure and compare the free-length against those presently on your car (laying on a surface is simplest)...if they are the same length, and otherwise unmolested they wouldn't need replacing...why do you feel you need to change them?...they don't go bad, unless mucked with by some "tuner"...

See also: SW-EM SU Carburettors

Good Hunting!

 

scaramoucheii- I know that VP sells (one) size spring... But maybe you and I both know that these cars (maybe excluded to just the 1960's) had at least three different size/model SU carbs. One model car might have the same carburetor as another model, if it was the same year or era. (This spare set I have I think were originally on a late 60's P1800). Year, over model, typically seemed to determine which carb (I think). What may be the only factor for a 60's Volvo owner, with a pair of SUs, is that one pair of carbs has a larger piston than the other (?), and most likely (therefore) a different diameter spring, and spring weight resistance.

I have two sets of carbs, and this is the spare set that is almost ready for use. (What is on the car presently is pretty good). But I found a catch, or slight resistance in one piston/carb (the piston not dropping as fast as the other). When I pulled the springs, I noticed one spring stood much taller than the other, and therefore it must have a different resistance. Given these are about "$8" a piece (said Joe), I'll take the chance of having one less variable in my stereo, of being out of balance.

• I'm using the same weight of oil in each. (-not an issue)

Ron, I am unsure of their history. I know that they once sat for a long time, but have also since had an ultrasonic cleaning. As noted, I pulled them from an old P1800. I removed the springs and I think, though I am not sure, that the binding or resistance, where one drops slower than the other, could be a little bit of something on the cylinder wall (?). Maybe it's not the spring (?), but I do want this eliminated as a variable. The tension measurement of those springs is usually a factor of X grams of weight (yes?- 20 something grams comes to mind?). I noted this to Joe, and he said something about using "scotch-brite" on the cylinder wall to get them even, or cleaner for an even drop.... (Yikes). I know that Joe is the man, but if I did that (use scotch-brite), I would look for some pretty soft scotch-brite for hand cleaning those alloy cylinder walls.

I think my question is answered- "Red Spring". And for the cost of a pair, I have reduced one variable and can move on. The sticky resistance is minor (slower). I'll look further to work that out. New springs will eliminate that one other potential, or variable. I just was unsure on which spring belongs on the 3 bolt carb.

Thank you both.

 

BFF;

Dashpot Spring is a function of displacement of the total number of cylinder fed by a given carb...in order to keep the Dashpot rise (and therefore exposed Metering Needle) in a usable range. ALL Volvo engines from B18 to B20 therefore used the Red Springs. This info is confirmed by my SU configurations list from the book: Tuning S.U. Carburetters by Speedsport Motorboks 1975 ISBN85113-072-0 It shows ALL HS6 carbed Volvo cars B18/1780cc calling for Red Springs and most of the HIF6 B20/1990cc calling for Red Springs (only a couple fitted with HIFs call for Green Springs), so that includes 2bolt or 3bolt carbs. If one of your springs is longer than the other, I agree at least one has probably been mucked with, so both should at least be measured for free-length against a non-mucked ones...

Rate of Dashpot drop is more a function of the Damping Valve, and the rate at which damping oil moves through it when open...rate of (dampened) rise is determined by rate at which the oil is allowed to move through the closed valve, past the clearance of the Dashpot tube, and against the counter force of the Dashpot Spring. The Dome wall finish and condition has ZERO to do with this, as there is NO CONTACT with Dome Wall(!), and Dashpots and Dome are precision fit and were selected and matched at time of manufacture. You can do the 5-7 Second drop test to see how evenly the two carbs are matched, but my advice is not to change anything without a very good reason and an equally good understanding of what results such a change are intended to bring. Finally, and in no uncertain terms: NO ABRASIVE OF ANY KIND including "Scotch-brite" should be used on the Dome inner wall...EVER(!)...they may, and should only be cleaned with a soft rag and carb cleaner!! I question the expertice and understanding of SU function of any individual who recommends this!

See also: SW-EM SU Carburettors

Cheers

 

Might be teaching my grandma to suck eggs but have you properly centred the needle and jets, if off by a fraction it shows up as a slow to drop piston close to the end of the travel ( closed)