The right oil for the dashpot?

[jagnut66]

Hi,
Until recently I have used the proper SU dashpot oil for the dashpots of my carburettors, without issue.
However, upon recent (insistant) advice, I have swapped this to engine oil.
I suspect there may many varied opinions on this but I'm going to risk asking the question anyway, as I'd like to know, so:
What are the pros and cons in the case of 'Engine Oil vs Dashpot Oil in an SU dashpot'?
I look forward to the answers, as it may decide whether I swap back or not.
Best wishes,
Mike.

[Bill_qaz]

At the risk of being shot down
The heavier the oil more damping so sluggish throttle reponse.

Genuine SU/Burlen dashpot oil is a straight SAE 20 mineral oil.
Personally I've always used multigrade engine oil in my minor and previous various SU equipped cars in std form and never had an issue.

[philthehill]

"The heavier the oil the more damping so sluggish throttle response"

Not necessarily!
I have tried all grades of oil (from 3 in 1 oil to proper SU damper oil and on through to EP90) in the SU dashpot and in some cases the heavier oil has improved the Venturi effect and the engine has picked up quicker.
https://en.wikipedia.org/wiki/Venturi_effect
It is a question of horses for courses and some heavier oils in some engines improve the performance. All you can do is try the different oils and see what happens.

When I worked in the BMC garage we just used to put engine oil 20/50 in the dash pot with out any ill effect and never had any complaints.

[Bill_qaz]

Shot down

Yes when I worked on bmc's in the fleet, always used std 20/50 and as I said always done same on my own cars.
Seem to remember trying 3 in 1 on a tuned 1100 allegro. Can't remember if it made a difference but probably convinced myself at the time

[myoldjalopy]

TBH I have tried engine oil, 3-in-1 oil and SU carb oil over the years, depending on what was at hand, and not really noticed any difference....not that I was attempting any comparisons.

[jaekl]

The function of the oil is to delay the rise of the piston when opening the throttle as more engine vacuum starts to pull the piston up, thus increasing the air speed through the venturi which will enrichen the mixture aiding acceleration. Thinner oil would allow the piston to rise faster. Yes, the needle would be allowing more fuel but this would not enrichen the mixture. No oil would be the extreme and if you ever had this situation, the engine would stumble as you try to accelerate.

[59Elva]

So 20w-50 would result in a richer mixture in cold conditions and leaner when hot?

[philthehill]

It could do - hypothetical for example - the thicker oil restricts the piston rise so increasing the vacuum in the venturi therefore pulling more fuel through the main jet. But that could be offset by the additional air being taken into the engine as the throttle is opened. I have to say that I have never experienced any problem with 20/50 engine oil in the dash pot with the choke still being required to be pulled out to compensate in cold weather.

[geoberni]

I just use engine oil.
The Worksop Manual specifies it on the Lubrication Charts....
The 'MM' Lube Chart says engine oil, which was 20W/30.
With the SII/1000 it changed to 20W/50.

Seems simple enough...
If people want to experiment with thinner or thicker oil, that's their choice

[bobtail4x4]

I have used redex occasionally before now,

20w50 normally,

[myoldjalopy]

The function of the oil is to delay the rise of the piston when opening the throttle as more engine vacuum starts to pull the piston up, thus increasing the air speed through the venturi which will enrichen the mixture aiding acceleration. Thinner oil would allow the piston to rise faster. Yes, the needle would be allowing more fuel but this would not enrichen the mixture. No oil would be the extreme and if you ever had this situation, the engine would stumble as you try to accelerate.

I had exactly this once, a long time ago, but the engine would barely idle, let alone accelerate. You couldn't drive it. So I do regular checks on the dashpot oil now, just in case.