cds150 damper spring rates

[Iain T]

I've been twiddling' in the sun today and took the car out to check if the new leccy fan works (it does) but I noticed the AFR meter showed it was running rich. The engine has been 'got at' with a ported and gas flowed head, increased CR so the standard 6AC needles were way too lean at higher revs and acceleration. I have 6J needles installed (each one the same height and the jet depth is calibrated) which are more tapered and are much better but not quite right. The carbs are also synchronised. It seems as though I need to screw the jets up to weaken the mixture (it's running at around 12.5 at 2000rpm) but if I do it gets too weak accelerating from about 3000rpm up. I have a set of 7B needles which are slightly thinner over the middle 2/3rds range but this would result in even richer running. I have 20/50 oil in the dash pots.

My question is does the damper spring rating effect the mixture on acceleration or is it there purely as a return spring on de-acceleration? Would a stronger spring and raising the jet work?

I can try the 7B needles in tomorrow and wind the jet up to get a good reading at tickover but am curious as to the function(s) of the damper spring.

Anyone's thought please?

Iain

[johny]

I think the spring strength determines the piston position for a given vacuum and the oil thickness changes how quickly the piston gets there...

[Iain T]

8 minutes ago, johny said:

I think the spring strength determines the piston position for a given vacuum and the oil thickness changes how quickly the piston gets there...

Thanks, so a weak spring allows the damper to rise more and the mixture would be weaker? I have no idea the current spring rate as it not colour coded. But then with a weaker spring would that also weaken the mixture on acceleration, something I do not want. 

Mmmm dunno....... 

[johny]

to make the mixture richer on acceleration you could try a thicker oil... 

[Iain T]

12 minutes ago, johny said:

make the mixture richer on acceleration you could try a thicker oil... 

I need to wind up the jet and use treacle in the dash! 

[johny]

yes a bit complicated as it needs to be matched to the right needle profile as well.

The weight of the piston should have the same effect as the spring so it might be possible to remove your spring and try different weights to adjust the mixture...   

[Iain T]

I need to have a shot of rum and think this over! Too many permutations. 

Iain 

[johny]

yes its interesting. I wonder if you can estimate the length of the spring when its installed and then take it out to put weights on it. When its compressed to the same length thats the equivalent weight youd have to start with on the piston when driving without a spring.... 

[Pete Lewis]

yes the spring richens

yes a heavier weight diaphragm ring richens

all by dropping the piston and increasing the air velocity over the jet 

there are 8 springs http://zenithcarb.co.uk/cd-piston-springs.html

some where there is a compressed length/ load table but cant find that either 

eg the 1600 had no spring but a heavy ring ,   cant find a link to them 

and if youre on pancake/KN filters the best richen route is to fit std air filters ...does what it says on the tin 

Pete

[johny]

Think Iain wants weaker on constant running and richer on acceleration so need to work out which is the standard spring and find a weaker one?

[Peter Truman]

Iain this is what I've gleened, I wanted it for twin 150CD's I'm fitting to a Spit 1500 engine,

I made a test rigup to check the springs I had, one light and one heavy as per below info, plus a shorter length spring that approximated to light, I stretched that one to the same length and it recorded 75gms at 2.09in compressed length.

[Peter Truman]

Here's a pic of the Damper Spring Test Bed, with weights, rule marked at 2.09in

[Iain T]

10 hours ago, Peter Truman said:

Iain this is what I've gleened, I wanted it for twin 150CD's I'm fitting to a Spit 1500 engine,

Peter,

I like the test rig simple and effective and shows a huge jump from blue to red springs. 

I'm going to lean the mixture today as its still a bit rich on tickover and buy two blue and two red springs. 

When driving at 50-60+ and applying medium acceleration the mixture gets lean which indicates the springs are too weak, as I said I have no idea of the age or state of the current springs. But on seeing your test rig I might check them. 

I think the 6J needles are probably ok with 50.0 thou at bottom as opposed to 56.1 thou for the standard 6AC. The 7Bs are 50.6 thou and slightly narrower than 6Js through most of the range but the final stage. 

Iain 

[Colin Lindsay]

14 hours ago, Peter Truman said:

Here's a pic of the Damper Spring Test Bed, with weights, rule marked at 2.09in

How do you stick them all to the wall like that?

[Iain T]

I checked the spring rates and the rear carb spring was about 100g but the front was a good 120g. So they are the correct medium springs for a Mk2 Vitesse. I'm not sure if 20g makes much difference but I've ordered two new medium springs from Burlen. The difference to the heavy spring seems too much, but then I don't really know! I will check the new ones before I fit! 

Taking a look at the plugs the front 3 are black and the back 3 the correct colour. Again not sure if the slightly stronger front spring is causing or contributing this difference. I also measured the jet to bridge and were 1.16 and 1.17mm so very equal. After several tries last year the needle heights are similarly equal. Therefore they are not contributing to any imbalance. 

I'll report back when the springs are fitted. 

Colin, just seen your post. Peter I think is from down under where the water swirls round the plug in a different direction, it's a funny world down there! 

Iain 

[Nick Jones]

4 hours ago, Iain T said:

checked the spring rates and the rear carb spring was about 100g but the front was a good 120g. So they are the correct medium springs for a Mk2 Vitesse. I'm not sure if 20g makes much difference

20%? I’d say that’ll make a noticeable difference and the stronger spring will give a richer mix, explaining your plug colour imbalance, though the jet heights are the same.

Nick

[Pete Lewis]

with the air filters off look at the relative piston heights from the bridge  and you should see the imbalance by the differing  springs 

well theres not much on tv  you need a little job 

Pete

[Iain T]

13 minutes ago, Pete Lewis said:

well theres not much on tv  you need a little job 

Very true, but I have to digest my banana, ice cream and caramel dessert 😋

Iain 

[Pete Lewis]

so you like to be spoilt   ha 

Pete

[Iain T]

4 minutes ago, Pete Lewis said:

so you like to be spoilt

Again very true! 

I'm stoking up on energy for our first trip to Oxford Street tomorrow......I'll do the test when I get back as I'm curious if the stronger front spring actually does push the piston down further relative to the rear. 

[johny]

I think you'll be lucky to see much difference unless the cars on a rolling road and really doing some work so the butterflies are well open...

[Peter Truman]

Colin re joke about my inability to post photo correctly do remember I got it halfway right so as not to offend anyone as living in Aus I could have posted upside down!!

[Peter Truman]

Iain were both springs the same length when unloaded, could one have been inadvertantly stretched a little increasing the load resistance.

[Iain T]

10 hours ago, Peter Truman said:

Iain were both springs the same length when unloaded, could one have been inadvertantly stretched a little increasing the load resistance

Peter, the springs were the same length so there's something wrong with the front spring. Unless it's been replaced who knows what's gone on in over 50 years. 

I will be checking the new ones thoroughly before installing. 

Having an afr meter with the sensor in the exhaust is a big help. 

Iain 

 

[Iain T]

The medium springs finally arrived from Burlen on Saturday, they are the same length and both measured about 120g to compress to 2.09" (approx 53mm). They should be 99g but then I sag when used...... 

So after an abortive fitting of a new fan belt, both bought and stamped the same but the new one is 20mm shorter and just too small, I leaned the mixture and went for a spin. At tick over its about 13.5. All is fine until I went up to 60+ or at higher revs when it crept up to over 16+ on the afr. So it's out with the 6J needles and in with the 7B's which are narrower mainly in the mid range up but still taper to 50 thou the same as the 6J. The flowed head must be working well! 

More later, I'll get it right sooooon! 

Iain