Carburettor dashpot oil

[wurlitzer]

I am trying to get my 1959 convertible ready for my daughters wedding at the end of the month by giving it a good service as I’ve got 150 mile drive.I need to top up the carburettor dashpot,what oil do I use,I’ve got 20/50 engine oil or should it be a thinner oil.The manuals just say thin oil but how thin?

[StillGotMy1stCar]

20W50 engine oil is perfect.
Regards John

[myoldjalopy]

If you want to be a perfectionist then SU make specific SAE 20 'suction chamber oil'. You can get it from Wares - https://www.morrisminor.org.uk/parts/12 ... gLTN_D_BwE - or, undoubtedly, from SU - http://sucarb.co.uk/dampers-caps-oil/da ... de-20.html

[wurlitzer]

Thanks,how much should I put in ,it seems a bit low and there’s no suction when I withdraw it

[philthehill]

The piston damper tube (not the cylinder) should be filled to the top.
There should be resistance when the damper is pushed down into the piston damper tube.
You will get less resistance when lifting the damper in the piston damper tube..
The oil level will find its own level so no need to worry about the exact oil level in the piston damper tube.
The correct SU damper oil is not always best - you can use 20/50 engine oil right through to EP 90 dependent upon what you want and how fast you want the piston to rise...……………..

[myoldjalopy]

"The correct SU damper oil is not always best - you can use 20/50 engine oil right through to EP 90 dependent upon what you want and how fast you want the piston to rise...…………….."
Phil, can you elaborate a little? How would you expect performance to differ between using 20/50 and EP90 in the dashpot?

[philthehill]

The thinner the oil the quicker the SU dashpot piston rises which may lead to a flat spot or hesitation in the increase of engine revs.
Having thicker oil in the dashpot will slow the rise of the dashpot piston so reducing the possibility of a flat spot or hesitation in the increase of engine revs.
With the dashpot piston rise rate slowed and the throttle opened the depression in the manifold increases and more fuel can be drawn out past the needle which also helps in reducing the possibility of a flat spot or hesitation.
I have tried different grades of oil in the dashpot and the final grade choice will depend on the engine build and SU carb needle(s) used.
Phil

[simmitc]

To second Phil's advice, for the first 20 years of Minor ownership I squirted "engine oil" (20/50) into the dashpot as per an old handbook, and the engines ran sweetly. Then I discovered the genuine SU oil and two out of three cars had flat spots. After returning to 20/50 the flat spots disappeared. If you overfill then you may find a small amount of oil coming out of the screw top where there is a small breather hole. No problem, just wipe clean and all will be fine.

[myoldjalopy]

Interesting..............there are, indeed, many ways to skin a cat. I have to say I have had no problems at all with SU SAE20 oil since using it - the car runs perfectly (it is a bog-standard 948cc engine). I also have to say it doesn't noticeably run any better than it did when I was using 3-in-1 oil........

[geoberni]

Interesting..............there are, indeed, many ways to skin a cat. I have to say I have had no problems at all with SU SAE20 oil since using it - the car runs perfectly (it is a bog-standard 948cc engine). I also have to say it doesn't noticeably run any better than it did when I was using 3-in-1 oil........

Must admit I've not had to top up Basil's oil in the short time we've had him, but back in 70s when I had a Viva with an SU Carb, I used 3-in-1 to top up and it was fine.

[ampwhu]

ive always used 3 in 1 thin oil in the dashpot. easily for the last 25-30 years and have never had a problem. most others I know have used the same.

we live in a strange world nowadays.

[palacebear]

Likewise 3 in 1 for years. Never given me any problems.

[philthehill]

Geoberni
The Vauxhall Viva was never fitted with a SU as standard.
What it was fitted with was a Stromberg CD variable choke carb https://www.ebay.co.uk/itm/VAUXHALL-VIV ... aoP63which was very similar to the SU but had a rubber diaphragm in the piston cylinder. The achilies heel of the Stromberg CD carb was its rubber diaphragm. They were very prone to failure which caused the engine to run badly or not at all.
https://www.ebay.co.uk/itm/Stromberg-CD ... SwoddaciUh
A popular conversion was to replace the Stromberg carb with either a single or twin SUs set up.
Fitting the Twin Stromberg CD 125 carb kit to the Mini and Minor was also a popular conversion.
Here is a nice Viva conversion kit:-
https://www.ebay.co.uk/itm/Ultra-Rare-V ... Sw8lhbXZOV
As regards the dashpot oil for SU and Stromberg carbs - put in whatever suits the engine best. There is no hard and fast rule as to what oil should be used - the recommendation is only an advisory.
Phil

[geoberni]

Geoberni
The Vauxhall Viva was never fitted with a SU as standard.
What it was fitted with was a Stromberg CD variable choke carb https://www.ebay.co.uk/itm/VAUXHALL-VIV ... aoP63which was very similar to the SU but had a rubber diaphragm in the piston cylinder. The achilies heel of the Stromberg CD carb was its rubber diaphragm. They were very prone to failure which caused the engine to run badly or not at all.
Phil

Now you've reminded me.... it was a long time ago, 1977 to around '84. But I do remember breaking down once because of the rubber diaphragm getting a hole in it. RAC guy did a temporary fix with electrical tape!!

[arjay]

The tuner who worked on my MGA recently swears by Carlube Jack Oil. Under £5 for 500 ml, SAE 10 but car is going like a dingbat (as much as such a car ever can). However, he did shave the new needles to ensure even performance right across the rev range, which most of us might not try.