I wanted to relay an experience this week, after remembering we discussed SU carb damper oil a while ago.

I was asked to tune a V12 E Type that has the conversion to SU HIF44 carbs from the original strombergs. This car has been to the local Jaguar specialist in town (who was a dealer trained mechanic) several times and he had been unable to get the car running well. It was grossly rich, and I went through the carbs and set everything as per the manual (the float height was way out) and then connected my air-fuel ratio meter.

As I'm at about 3500 ft elevation, the carbs needed leaning out from the base setting. On a test drive I had the carbs set nicely for cruise, but under brisk acceleration the car hesitated, and occasionally popped and spit back through the carbs and went to an AFR of over 18:1. I had put ATF in the carb dampers on the initial setup. As I knew that the mixture was correct under cruise, the problem was going lean and needing more acceleration enrichment. The weight of the damper oil controls this, so I dumped out the ATF and put in a straight SAE 30 motor oil. Took it for another test drive and the hesitation was gone and the car accelerated strongly and the AFR didn't go lean anymore.

So the take away is if the car runs well otherwise but hesitates under acceleration, don't touch the mixture; but instead change the oil in the carbs to something heavier than whatever is in it when tested. I was surprised at how much difference the oil made, but the both the air-fuel ratio meter and the driving experience don't lie.

 

That makes no sense to me, a heavier weight oil will keep the pistons from rising faster, thus leaning it out further _ it should have made it worse.
This assumes that the viscosity of the ATF is heavier then the SAE 30, then I can see an improvement.
But as you said, the SAE is heavier then the ATF, this makes no sense to me at all.

 

You're half right. The heavier oil does keep the pistons from rising faster, which increases the velocity of the air across the bridge of the carb. The increase in velocity drops the air pressure further in that area, which draws out more fuel, thus enriching the mixture. It's the venturi effect.

ATF's viscosity is roughly equal to a 20W engine oil, so straight 30 is heavier.

 

Jagboi64 is absolutely correct
I have used this heavier oil for years when dashpot systems get worn.
Lighter oils allow a rapid dashpot rise when there is a need for a slower rise to achieve a richer mixture under acceleration.

 

An SAE 20 synthetic is the right oil with new dampers. Synthetic due to high VI and minimal change in viscosity with temperature. The Dampers run at very close to 0 deg C. (freezing). Fiddling with oil viscosity is not advised. SU can sell you the right oil. (now synthetic & a stupid price)

An ATF is really a 10W (everybody blends at the bottom of the viscosity range for energy conservation). New dampers have just arrived for my car. Although I'm not experiencing any issues. She accelerates perfectly both at Altitude and coast ~ Johannesburg ~ 1,753 metres (5,751 ft). Why Boeing and Airbus still test their Aircraft here.

 

Glyn
Absolutely correct but living on the edge of civilisation in OZ we sometimes have to make our own "fixes" .
I first had this lean cough problem in a MK7 in 1967 while living in Cunnamulla in outback Queensland ie over 400 miles west of the Brisbane capital of the state.
In winter it was OK but in summer typically around 40C+ ( I kid thee not) it would cough/backfire when accelerating.
I was the local charter pilot. Our aircraft engineer was an expat New Zealander and he suggested heavier oil in the dashpots.
We experimented with grades 30, 40, 50,and 60SAE. (We needed 60 for the radial engines. I think it was Mobil green band)
Eventually settled on SAE 40 and had no re-occurence of the lean/cough on acceleration.
It seems that the high temps. were thinning the damper oil out and along with wear and tear the dashpots were no longer functioning as advertised
The Mk7 had over 230,00 miles on it when I sold it so I suppose it was getting worn out.

 

Understood Bill. We have a saying in South Africa "Boer Maak 'n Plan". Direct translation from Afrikaans to English ~ Farmer makes a plan. Most of them are miles from nowhere. Very much like Aus.

 

For SU damper oil, I'd err to the more viscous. So long as the driver doesn't pump the throttle, it has insignificant comsequence for economy, but will ensure the mixture doesn't go lean on acceleration. If the mixture goes a bit to the rich side during acceleration, neither engine nor driver will notice or suffer. Certainly, it's better than the car suffering a cough/hesitation when you move out to overtake.

 

You will just see a little puff of brown smoke from the exhausts under acceleration. I don't like messing with viscosity if all is well. People in SA do some stupid things with damper oil. SU did not just dream up an ideal viscosity for a broad range of ambient conditions & consider how cold it runs.

 

The dampers did have some of the SU oil in them when I got the car, and I topped it up with ATF, as I didn't have any of the SU oil. It was also a 30°C day, and that V12 engine does put out a lot of heat which may contribute. I suspect 4 carbs is over carbureatted the engine, so there is less cooling in each carb than in a twin setup on the 6 cylinder engines.

After going for a drive, I pulled into the garage shut the engine off and opened the bonnet. In that time the dashpots were almost too hot to touch. The previous mechanic has wired the electric fans to run continuously, so there is lots of hot air being blown over the carbs.

 

Yes. Most people think that the dampers run at approx underbonnet temperature. They don't. They run damn cold which is why the Mini suffered the carb freezing/icing issue. Venturi effect. And so came the electric tea cozy to keep the carbs hot enough.


SU heater ring. (tea cosy)

 

"Venturi effect" + Joule-Thomson throttling + latent heat to evaporate fuel (to be an anorak). In fact, a carburettor is rather a nice little refrigerator. Ice cream one side of the engine and hot dogs on the other.

 

Indeed ~ LOL! Tea Cozy above. I did not think I would find a Pic. There were many iterations of how people mounted them. Some rather amusing.

 

I have one of those "tea cosy" rings, came in a box of misc SU carbs I got at a garage sale. I wondered what the original application was. It also had an Otter switch with a heating element around it, not sure what that was for.

 

On our old Mini, the there was a simple flexible fuel line that went directly from the bottom of the float bowel and was directly attached to the bottom of the jet.
This was a point of freezing in the winter time, the tea cozy ring went there.
If I remember correctly, there was a spring on the fuel line to keep in from kinking.

It was a neat set up as noting was needed to seal the jet to the carb housing.
The jet was lowered and raised as normal for cold starting with the usual large nut to adjust the mixture.

I suspect the Otter switch with the heating element was meant for hot climates where you didn't need the AED to stay on so long.

 

That's the HS series of carbs.

 

The carb I had with the "tea cosy" and spacer plate with Otter switch. I'd never seen one before.

 

I too have never seen before in the flesh. Only pictures & I did not know what it was. (Otter switch & gubbins)

 

These heaters were used in the late 50's, early 60's as diesel engine fuel heaters.

In respect of S,U damper oil, most manufactures used the same oil as that specified for the crankcase, except most specified SAE 10 for Canadian winter conditions.

 

I would be reluctant to put a 20W-50 in the dampers (which on the face of things would seem a good idea at operating temperature that low). I'm not sure milled polymer in that volume is a good idea. I would stick to monogrades. Ideally a 20W synthetic. People must do as they wish. Reality is they are going to anyway. I know many favour a 10W ATF. It's really too thin. Replacement of dampers from time to time is a good idea. They wear.

If your car runs well with no flat spots & doesn't belch unburnt gasoline (brown smoke turning to black if very rich) from the exhausts every time you accelerate ~ be happy.

Ambient conditions vary vastly. Canada is not the only cold country globally.