Which dampening fluid for SU carbs?

Stromberg-Zenith 150s.  I've heard everything from ATF to 90 weight, any experience?

Thanks, Dan

I use ATF in my HIF44. I have never really done any testing trying to figure out if that is the best, it's just what I've used.

I've always used this, as it's what the manufacturer (or what has become of the manufacturer) sells:

SU & Zenith Stromberg Damper Oil

At different times I've used 20W50, airtool oil,  shock absorber oil, ps fluid, 3in1 oil & atf. In other words whatever's handy. Couldn't tell the difference in performance.

All the Datsun roadster guys state 20W oil.  

Ask me how I know.  I know cause I had one and all the books called out 20w. 

Thanks Folks!

I have used 20W50 since that is the oil the engine and gearbox use. Couldn't tell any difference either.

Dad always used straight 10W motor oil in the Jag's carbs.  That is what the guy who serviced it and the dealership told him to do.  Unscrew the little plastic know, put it in the top and be VERY gentle putting it back together

Marvel Mystery oil__easily smells the best__since viscosity doesn't seem to be a factor.

RoddyMac17 said:

I've always used this, as it's what the manufacturer (or what has become of the manufacturer) sells:

SU & Zenith Stromberg Damper Oil

 

Yes the oil does matter first it has to be a straight viscosity because the last thing you want is the speed of the damping piston to change as the oil heats up.  

Next you want it to rise at the speed extra fuel is required.  Rises too fast and the engine goes too rich,  rises too slow and the engine starves. 

Finally you want to know when attention is required.

It’s the reward for doing things right.   

In reply to frenchyd :

Just a note, the viscosity of all oils changes with temperature, even straight oils. That's not at all what multi viscosity oils do.

I've always used hydraulic jack oil.  Won't degrade the seals.  Its a good viscosity for the dampers.

tuna55 said:

In reply to frenchyd :

Just a note, the viscosity of all oils changes with temperature, even straight oils. That's not at all what multi viscosity oils do.

Use multi viscosity oils if you want.  I don’t because I learned from the SU importer ( name escapes me at the moment) what to use and why.  I’ve worked on enough engines on the dyno to understand what happens when things other than the proper oil is used.  

In short you start to lose power.  Not great big feel it in your numb butt when you aren’t paying real attention but enough to be measured. 

My MGTD makes 54 horsepower and losing one or two is enough on a long race track like Elkhart Lake to get past a competitor.  

I once heard sewing machine oil, but were it me, I'd go with RoddyMac's suggestion.

Somehow I ended up with a container of sewing machine oil and used that.  Then I used 3 in 1 oil before moving on to ATF.  Cant say I noticed any difference.  

The damping action delays the rise of the piston when you crack the throttle open, which temporarily enriches the mixture just like an accelerator pump.

Thinner oil yields less enrichment, thicker oil yields more enrichment.

So it depends on what works best for your setup.

On our SCCA G Prod Spitfire, we ran them damp, just a splash of 3-1 oil to wet the parts.

ISTR that Kastner recommended this in the Competition Manual.

On my street cars, I usually fill with 3-1, but as said, just about anything works.

In reply to clshore :

Exactly. The needles look tapered but are actually little steps  that correspond to certain richness at certain vacuum.  Changing needles changes that relationship as does changing oils.   

 I spent a lot of time learning exactly what changes and why.  Changing to something else because it’s handy,  or you have some, or whatever reason is ignoring all the development work that has gone by before.  

No racer ever leaves the engine stock but only recently have computers allowed us to get fuel mixture exactly correct. Vintage car racing and vintage cars don’t allow that. So if you want anything to work properly you have to start from some well established base and develop from there.  

frenchyd said:

In reply to clshore :

. The needles look tapered but are actually little steps  that correspond to certain richness at certain vacuum.  

No, they are not little steps. The SU needle charts just show what the diameters are a set distance (1/8" increments)  apart. The needles are a continuous taper, they are not notched..

In reply to wspohn :

You are correct but I’m trying to explain that if the needle jumps up 7 steps on acceleration with one oil and 3 steps with another lubricant the mixture will change.   That change will affect the rate of acceleration. 

That’s why the oil you use does make difference.  Maybe not so much on a daily driver or commuter but definitely whenever real performance is called for.  

It’s true that you can use anything from baby oil to virgin olive oil  or even nothing and the car will still run.  To some that’s good enough. 

We always used an had great running with airplane red brake fluid.  Its a mineral based oil with a high boiling point. used in go kart brakes aswell.

I know the theory but I'm a little mixed up.  When the piston rises, that pulls the needle out of the jet and that would richen the mixture.  So how does thicker oil, which slows the rise of the piston, create a momentary richer mixture as an accelerator pump?    

In reply to wannacruise :

You are correct that the needle taper enriches the mixture.

But when the piston rise is delayed, throat opening remains smaller, while more air is being ingested by the motor.

That creates a temporary increase in the airflow velocity that creates more vacuum (Bernoulli effect) and draws more fuel, enriching the mixture.

Once the piston rises and reaches equilibrium, the needle taper determines the mixture again.

My experience is with Hitachi SU style carbs; I experimented with different oils and Frecnhyd is correct that different oils can have a very noticeable effect. I mostly used ATF and also 10wt fork oil. The ATF worked best for my car.

It also dampens fluttering. (I heard).

clshore said:

In reply to wannacruise :

You are correct that the needle taper enriches the mixture.

But when the piston rise is delayed, throat opening remains smaller, while more air is being ingested by the motor.

That creates a temporary increase in the airflow velocity that creates more vacuum (Bernoulli effect) and draws more fuel, enriching the mixture.

Once the piston rises and reaches equilibrium, the needle taper determines the mixture again.

Well said,  let me try another way using extremes.  Assume two SU carbs.  One using 90 weight  gear oil. One using kerosene. 

One will suck a lot of fuel and one will suck a lot of air.  

Why and which one?  

Let me try another approach. A straw sticking out of a bottle of pop.  

First you open your  mouth as wide as you can and suck in.  

Next wrap your lips around the straw and suck in.  

One will get a lot of pop in, the other not much.