AD016 blue su carb springs

[Robbie224]

Hello. I have a 71 reg 1275gt which has the ADO16 gear box with the sandwich plate 1300tg engine fitted, A 12g940 big value double springed head with twin hs2 carbs.
The twin carburettors are old 50+ years old and fitted with RED springs. Getting a complete brand new set of hs2s including all linkages being fitted next month. Question is should I replace red springs with RED or change to BLUE springs? 1300gt did come from factory with blue.
Thanks Rob

[gs.davies]

I’ve gone back and forth between blue and red springs on my car. Twin HS2, 1098, 12G295 and a 997 style cam.

The closest factory setting for this set up appears to be D6 needles and Red springs.

On advice I started with a pair of M needles and Blue Springs and that’s what it went to the rolling road with Fuelling was close to lambda at WoT but in the interests of sodding about I did try D6 and Red (flat as a pancake), M with red (did the Italy run on that combo and felt it was maybe a bit rich and I’m now back to Blue and M. I didn’t feel the car had much urgency to rev with the Red spring. Took forever to wind itself up.

The blue springs are lighter and will allow the piston to rise to a wide open position easier. This might seem a good thing but unless the needle is metering enough fuel for the airflow, it may actually be too lean when the throttle is snapped open quickly.

The heavier spring gives more of a throttle pump effect, richening the mixture by virtue of the increased gas speed over the bridge and I did find the M and Red combination to be best for day to day drive ability.
Now back on blue it can be a little bit flat at lower revs but is much better picking up once in the revs. Like opening the throttle at 60 to move out to overtake, it’s much more urgent.

Modern fuels also don’t behave like the fuels available when the factory specs were produced, so really it does need setting up properly in a rolling road with the engine under load.

PS, whilst my own rolling road readings were as they were at the time, it doesn’t in my opinion really indicate a real world drive situation as you spend most of the time at part throttle, where I think LFF is a little bit lean.

Not sure if that helps or not!

Gary

[Robbie224]

Thanks for reply Gary.
Very interesting reading you have tried blue and red on twin hs2. Mines always been on red springs and definitely runs rich, Spark plugs black and problems starting from cold after a period likely from over fueling?
Having work done fitting new carbs then rolling road. Mini innocenti 1300 was blue springs I think 1300gt definitely was blue very similar engine spec to innocenti.
Hoping blue makes better fueling with correct needles. Carbs are running with a MED filter with stub stacks.

[GraemeC]

My basic understanding is that the needle controls fuelling, the spring controls rate of change.

So if your car is running rich then its the needles that are wrong, not the springs. The springs will change the driveability.

[gs.davies]

A heavier spring will also cause a richer mixture by effectively choking the airflow across the bridge. Coupled with a bigger throttle opening this will also cause an increase in gas speed and more fuel to be sucked up out of the jet into the airstream.

I read somewhere (probably here..) that a heavier spring can compensate for a leaner needle, hence why my car also ran well on the D6/Red combination. It just never quite ‘came to the boil’ like it now does on a richer needle and a lighter spring combination.

Ideally the piston will rise to its maximum travel around the time you hit top speed/wide open throttle, this can be measured with a device which temporarily replaces the damper but again, need to actually see this with the car static on a rolling road.

That’s my understanding anyway.

[Robbie224]

Thanks folks.
Helpful reading. New carbs and linkages will be all brand new rather than ones fitted now that have to be 40+ years old. Why it has red springs don't know guess there a chance the carbs came off the last mark 3 cooper s? Anyway will be getting new carbs set with blue springs fingers crossed.

[Peter Laidler]

What Graeme says, above, is exactly what I was taught about SU carburation, except that......

Graeme says '.....the spring controls the chaNge.' I was taught that the springs control the chaRge. As in the rate or speed at which the fuel (in vapour form) flows.

Same meat, different gravy I expect. But well said Graeme.

Added later: As a bit of an aside to this I was given a box of (about 25 or so) assorted SU piston springs by a local forumer a few years ago. Some were clearly colour marked, others not, so I made a test jig from a modified piston and pot....., with small weights and...... you know the sort of tests and stuff you did at school. Really, to test the weight each spring compressed over a known distance with known/set weights

To be really honest, even the well marked red. blue and yellow springs hardly matched each other or barely equated with others to the extent that they could have fitted in the either/or camp! True, I only had a new set of reds to use as a datum and even they only just passed as a 'matched' set!

I had a bit of PM conversation with another forumer but don't recall what transpired

[minibitz]

I would say your current rich running is a symptom of the carbs being worn and not spring related as pointed out by others.

Even if still fitted with factory springs and needles the wear in the jets and needles will be sufficient to give rich fuel mixture.

[Robbie224]

Hope brand new carbs including all linkages will sort it, New has got to be better than 40+ years old.

[gs.davies]

Hope brand new carbs including all linkages will sort it, New has got to be better than 40+ years old.

That said, AC Dodd recently posted some of the process involved with preparing brand new carbs for use. Looked very detailed to me, including having to machine the pistons so they sat correctly. Dark arts I think!

[Peter Laidler]

If the jets were accurately positioned and locked into place, the needle would remain dead centre within the jet so there would not be any wear within either the jet or the needle.

Alas, it's not easy to do and best done on the bench with half an hour set aside.
Make sure hole in jet is perfectly round (I use a drill bit) Micrometer to check needle isn't eccentric

Personally, this is what I do. Seat needle in the piston with needle step level with the underside of the piston and lock up of course
Jet, screwed up to level with carb intake and test with a straight-edge. Brass clamp washer between jet and carb body greased with coppaslip - to make centralisation easier. Locknut finger tight. Now unscrew jet 3 flats - or half a turn.
Fit piston and bowl and move jet a bit at a time, left to right, fore and aft and so on until the piston and needle drops down with a distinct 'clunk' as the piston and tapered needle drops fully into the jet. Do it a few more times and then you know that it is perfect. Lock up the finger tight lock-nut. Then check again

This now done, as the needle lifts and drops a thousand times over the next few years, you KNOW that it is dead centre in the jet and there's no physical contact between the needle and the jet.

All easier written than done of course. Best taught and shown on a bench, in a classroom setting. But once these simple SU's are properly assembled and then set-up, you shouldn't need to be constantly fiddling with them. Except for the easily fixed butterfly shaft wear, metal to metal contact is minimal on these carbs

Added after a PM: The needle souuld not move from its now fixed position because the piston cannot rotate due to the anti-rotation stop pin and the fact that the piston is a perfect sliding fit in the chamber. The piston simply moves up and down together with the taperec needle in the accurately set jet.