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What Oil Should I Use In My SU HS4 Carburettor Dashpots?

That is an interesting question and one which I have pondered over since my teenager years when I had my first car with an SU carburetor.

Back in my youth I was offered a set of twin SU carburetors and inlet manifold off a bugeye Austin Healey Sprite and after exchange of the agreed money I set to the installation to my Morrie.

However as a 2nd year Apprentice Fitter and Machinist I did not understand the process to tune the twin carburettors so after many frustrating weekends I finally followed my Father’s advice and paid an experienced Mechanic to tune them for me. Now after he had completed the tune up he handed me a bottle of light machine oil and told me how to maintain the oil level in the dashpots but stressed to me not to fiddle with the carburetors settings. Now that car on some colder days in Adelaide still had hesitation when I pressed the accelerator pedal but I continued to use the light machine oil that the mechanic gave me.

Now to the present time, I was using the 20W40 engine oil that I use in the engine of my MGB in the carburettor dashpots. This I accepted, as the Workshop Manual copy I have listed the same oil for the engine and for the dashpots. After a discussion with other club members I became confused as some used special SU dashpot oil and some sewing machine oil others light Machine Oil and yet after browsing on the Internet, many other oils have been recommended.

I have downloaded from the internet many articles about the SU HS4 carburettors and recently after sitting down and reading them again I discovered that in the design of the SU the damper in the dashpot delays the lifting of the dashpot piston and thus the needle out of the jet so as to enrichen the fuel mixture for acceleration.

But how does this happen as I thought the needle had to lift out of the jet to provide a richer fuel mixture? Well it seems the SU designers knew that on the opening of the throttle butterfly the incoming air speeds up over the bridge of the carburetor and thus increased vacuum entraining more fuel past the needle. This initial enrichment of the fuel air mixture assists with increasing the engine speed just as the accelerator pump does with most fixed throat carburetors. Thus a higher viscosity oil would assist with the dampening of the dashpot piston motion and prevent the hesitation that many SU carburetor equipped cars experience.

So what have I learnt about which oil to use in the dashpot?

If there is a hesitation when accelerating then a higher viscosity oil may solve the issue. Most carburettors will have wear in the dashpot piston bore by now which may decrease the oil damping so a slightly higher viscosity may compensate for this wear. However I have also identified that the genuine SU Dashpot oil is a SAE20 grade oil (based upon viscosity at a test temperature of 100C) and the viscosity would increase with lower temperatures. If this initial high viscosity at low temperature is known as well as the rate of viscosity reduction with temperature increase then that introduces a temperature compensation to the aforementioned throttle butterfly opening fuel enrichment description. Those SU engineers were a smart lot.

The SU Dashpot oil was designed for the UK temperature range which would not necessarily be applicable to most Australian temperature ranges.

Multigrade oils that we use today are formulated to reduce viscosity reduction with temperature increase so may not be suitable in the dashpot as the less change in viscosity would change the temperature compensation design. The higher viscosity oil at closer to usual engine operating temperatures could cause a richer fuel mixture than is desired when increasing throttle thus higher fuel consumption Thus I believe that I should use a straight viscosity oil or at least if a multi-grade oil then one that is closer to the 20 grade oil viscosity at Australian winter low temperature up to normal operating temperature of the carburetor. Or use a viscosity oil high enough to eliminate acceleration hesitation but not high enough to increase fuel consumption.

I somehow think I have convinced myself that there is no one solution especially if there have been changes to the initial factory engine operating pressures (camshaft timing and lift profiles as well as compression ratio), induction and exhaust system design.

Below is an example kinematic viscosity and temperature chart for SAE30 oil. I was unable to find a chart for the SAE20 oil so included the SAE30.

Engine Oil SAE 30 – kinematic viscosity and density over temperature

Graham Haywood

On The Marque April 2019

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