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MG MGB Technical - SU Damper oil

Hi All,

I have a query regarding the oil in the SU carby damper.
In the dim dark past I had three British cars, all with SUs. The recommended oil was "light machine oil", ie sewing machine oil. This worked very well and once when I tried to use a heavier oil, ie engine oil, it killed the acceleration, slowly winding up to speed. Drainiing and refilling with the light oil cured this slowness.

Now on my B, the book says 20W 50, ie engine oil. Initially I used the light oil and the pistons moved freely with resistance upwards and dropped easily. Now, trying to solve my engine shaking problem, I replaced this oil with the recommended 20W 50, but now the pistons need a very hard finger push to move them up and they are sluggish to drop.
I wonder how others have experienced this.

H J Adler

I'm using a 0W40 synthetic which seems to work well, in the past I've tried 3 in 1 and 20W50..

The 0W40 works the best..
K Harris


3 in 1 or atf is way too thin. The oil reissts the upwards lift of the piston when the throttle is depressed. The slowness of the rise dictates the extra 'suck' on the main jet to increase fuelling during acceleration, this is the 'accelerator jet and pump'for an SU carb. We tend to use 20/50 oil. If the oil is too thin the car will hesitate under acceleration below around 3000rpm, above 3000 and the oil does not seem to matter.

peter burgess

I've been using engine oil i.e. 20W/50, 15W/50 and more recently 15W/40 in SU carbs in UK conditions for 40 years and never experienced sluggish acceleration. *Whatever* you use the piston should drop smartly, even when it has significant resistance to rising. If it doesn't drop smartly either the piston is binding, or you are using too thick an oil for your local climatic conditions. 20W/50 is frequently quoted as the oil to use for carb, engine and gearbox regardless, but this is only for 'temperate' conditions always above -10C/15F for which 10W/50 and 10W/40 are also suitable. For temps down to -20C/-5F 10W/50, 10W/40 and 10W/30 should be used. For temps consistently below -10C/15F 5W/20 and 5W/30 should be used.
P Hunt

Hi All,

Thanks for your comments, I guess I'll stick to 20W 50 for now.

By the way where I live it rarely gets down to 0 / 32 deg, but often up to 40 deg C.

H J Adler

Herb ,
Also living here in Australia I found light machine oil (Singer sewing machine oil as it happens) to be the oil of choice. It is still available too even these days.

It was also listed in the handbook for both my Mini and the MG when I owned British cars.
Also for my dad's Wolsley 6/80 all those years ago.

I also found ATF to be an excellent alternative and 20W/50 too heavy , even in Summer.

I never found Marvel Mystery Oil (MMO) here but I understand some of our US cousins like it and I never tried any other weight of engine oil either.

Cheers , Pete.
Peter Thomas

I like 20w or 10w-30 best. (It is not easy to find 20w).
M. H. Dabney


I've also used 20W/50 in my SU's, year round here in the Mid-Atlantic for the better part of fourty years with reason to change.

Best of luck for the New Year.


Larry C. '74 B/GT & '69 midget
Larry C '69 Midget

Did you say that the pistons are slow to drop? Because the poppet valve of the damper works in one direction only, the vacuum piston is allowed to drop quickly, preventing an overly rich fuel/air mixture whenever the throttle is closed rapidly. Should the poppet valve be rendered non-functional as a result of clogging, the resultant rich mixture when the throttle is closed rapidly will be revealed as a popping sound in the exhaust note should there be an air leak in the exhaust system. The poppet valve of the damper is accordingly an important feature in terms of throttle response, and as such should be kept clean so that it will function properly. Fortunately, this is easily accomplished by periodically spraying the damper with carburetor cleaner. In order to get the most out of the function of this design feature when driving hard, it is critical that the damper tube of the piston be kept filled with a 20W/50 oil that is relatively uneffected by temperature change, such as Mobil 1synthetic oil. Why an oil that is relatively uneffected by temperature change? Because an oil whose viscosity is strongly effected by temperature change would cause an inconsistent effect on the primary function of the damper mechanism. However, be aware that while the engine is warming up you will have to take care to open the throttle very gently. This will be due to the fact that when using a normal engine oil that thickens when it is cold, the viscosity of the oil in the damper mechanism is high and the degree of enrichment attained when opening the throttle is therefore greater than when the engine has reached its normal operating temperature. A more stable synthetic engine oil will not provide this cold-start benefit, and hence the engine will tend to stall more easily when cold. The other purpose of the damper feature is to prevent the pressure fluctuations occurring in the airflow of the incoming fuel / air charge from causing the vacuum piston to rapidly oscillate inside of the suction chamber (dashpot), playing havoc with accurate fuel metering.
Stephen Strange

Back in the day - the local factory authorized British Leyland dealer service dept. used red ATF (automatic transmission fluid)
in the SU dampers.
Daniel Wong

Hi Stephen,

Thanks for your reply.

I'm aware of the functions of the damper.

Yes the drop speed is "slow" with the 20W 50, but the rise is very hard to achieve with it.The poppet valve works OK, its the oil. When using the light machine oil there is a resistance to lifting but not nearly as much as with the 20W 50. The drop is almost instantaneous.

Peter, in his post above, corroborates my recollection that light machine oil was specified, at least here in Oz. Unfortunately I have no idea what viscosity this light machine oil is, compared with other suggestions.

I have a reprint of the original Driver's Handbook, which says "In no circumstances should a heavy-bodied lubricant be used." See image. Unfortunately still a very subjective description of the grade of oil to use. Personally I would classify 20W 50 as getting to be a heavy-bodied lubricant.


P.S. A Happy New Year to All, hoping that all your MG woes will be solved and you can enjoy driving the great little cars.

H J Adler

Hi All

The resistance to the lifting of the piston when off the car is irrelevant and subjective, it is under driving conditions the piston must rise at the correct speed.Each person needs to experiment with oil to suit their local conditions, too thin and the engine will hesitate and miss under heavy acceleration below 3000rpm, too heavy and the engine will black smoke and miss under heavy acceleration below 3000 rpm, the right oil grade will be the one which doesn't run too weak or rich! Apart from early TF carbs which seem to prefer lighter atf type oil we tend to find the sus in the UK seem best on 30 grade or 20/50. In fact I thought the penrite SU carb specific oil is 30 weight.

peter burgess

'Poppet valve' is usually the name given to the valve in the throttle butterflies and is irrelevant in this context. The damper is what gives the damped rise and undamped fall of the piston.
P Hunt

I am aware that on this BBS when most people use the term "Poppet Valve", they are indeed refering to the poppet valve that is located on the throttle disc of the carburetor. However, the damping mechanism of the caburetor also incorporates a poppet valve to control the rate of rise of the piston within the vacuum chamber (i.e., the "dashpot"). In fact, the intake and exhaust valves of the cylinder head are also poppet valves!
Stephen Strange

Well, it's a new one on me, but then I've only ever claimed to be 'still learning'. In the manuals I have it is merely refered to as a 'damper'.
P Hunt

Yup, the manuals do have a distinct lack of detail on how this vital mechanism actually functions. The poppet valve feature of the damper is what allows the vacuum piston to rise and fall at two different rates, one for ascension and another for decension. That's why it's so important that it be inspected and cleaned rather than neglecting it and allowing it to become clogged.
Stephen Strange

ATF is usually ideal for a close to standard SU equ. MG
When dealing with modified engines and fuel systems its as Peter said, you may need to experiment. For those not involved with supercharging you may be interested to know that the SU oil Moss supply with their kit is SAE90. SAE90 works extremely well in this instance as being thicker when cold it tends to work like an auto choke thus preventing a nasty manifold backfire when the throttle is opened quickly. On the other hand gear oils are not as temperature stable as engine oil and thin out a lot more when hot, to even less then a 20/50. The heat soak from the engine, manifold and blower soon achieve this. This gives excellent throttle response right through the range.

I have been putting ATF any brand in customers SU's for 35 years and it is by far the best thing to use.
gl mr

Hi All,

Well I've certainly opened a can of worms with what I thought would be a simple question.

With the gamut of answers it seems that anything you care to put in is OK by someone's experience.

We've had the full range from sewing machine oil, ATF the whole range of multi grades up to 20W50 and even gear oil at SAE 90. The only things we haven't had are water or alcohol, in the carbys I mean. I guess they would evaporate too quickly in the heat.

My conclusion is that I will have to suck it and see.

Thanks everyone.

Herb Adler

Would advise against that, oil can be toxic.

But if one has been putting the same thing in one's carbs for 35 (or 40) years one isn't in a position to say "it is by far the best thing to use" as one hasn't anything to compare it with. The best one can say is that it hasn't caused any problems, which presumably all of us say with whatever we use.

Coincidentally an article on a supercharger in Enjoying MG this month and a gallon can of Marvel Mystery oil which is used to (total loss) lubricate it. That is said to be 10wt, and as such is probably the thinnest of the substances mentioned with the exception of water and alcohol
P Hunt

I note the OE workshop manual says to fill the HIF4 carbs 'with new engine oil(preferably SAE 20)'Section D.6 11 page 115 D.14 MGB Issue 5 86842.

peter burgess

The SU damper is not a poppet valve. A poppet valve is one where there is a hole and a 'poppet' to seal it.

A damper, in the mechanical engineering sense, is a device which exerts a force which varies with velocity (as opposed to a spring, which exerts a force which varies with distance). A dashpot is a kind of damper, using a fluid, whereby the motion of a body through a fluid or the resistance of a body to the movement of fluid, provides a force.

In an SU, the force required for the piston to lift is caused by engine vacuum. It is resisted by the piston spring (in terms of position) and the damper (in terms of speed of rise). The force required for the piston to drop is caused by gravity and the spring, and resisted by the damper. At piston lift, or any other position for that matter, the damper does nothing.

So in a steady state, eg idling, the damper does nothing. It is only when the piston tries to move that the damper functions.

If the oil is too thin, the piston rises too quickly - allowing more air into the engine quicker than the fuel can respond. And hence you get a 'lag', a momentary weak mixture and a hesitation. Vice versa with oil that is too thin.



"The force required for the piston to drop is caused by gravity and the spring, and resisted by the damper. At piston lift ... the damper does nothing."

It's the other way round (and contradicts other parts of your post), the damper resists the *rise* of the piston to cause mixture enrichment in the initial part of acceleration, and has no effect when the piston drops i.e. it should drop smartly. As such the damper includes a one-way valve. Most definitions of a poppet valve describe a hole and a plug i.e. as in a cylinder head. One hydraulics terms document describes it "A valve design in which the seating element pops open to obtain free flow in one direction and immediately reseats when flow reverses" which *does* represent what happens in the carb damper and the throttle butterfly, but not a cylinder head. Another says the term comes from 'puppet' i.e. as on a string and 'remote motion transmitted linearly', which *is* like a cylinder head valve, but not the carb damper or butterfly.
P Hunt

Sorry, not clear. Meant to say 'at FULL piston lift', or any other position, the damper does nothing. Meaning that the damper has no effect on mixture at steady state - only when the piston is rising or falling.


It has none, or virtually so, when falling ...
P Hunt

I have a 78 MGB with a Zenith-Stromberg. Car ran ok with 20W-50 in the damper but it was hard to start after it had been sitting 15 to 20 minutes. I saw a thread from Tony Barnhill the Autoist on another forum where he recommended automatic transmission fluid for the damper. So I dumped the 20W-50 and tried Mercon V ATF. It cured the hard starting problem and still accelerates smoothly.
G.E. Bulwinkle

I think the important thing is that both pistons react the same when the engine is up to temperature. In this condition when you hold both pistons at full lift, they should fall as a matched pair from the top of the dashpot, and hit the bridge at the same time with an audible clonk. Sometimes you have to tweak the viscosities a little to achieve this, but your 20/50 should be fine as a starting point.
F Pollock

I don't think falling is the important thing as the throttle is closed or closing when that happens and the engine is on the overrun, the damper is 'out of circuit' on a falling piston hence the grade of oil will have no effect (unless treacle compared with watch oil). It is piston rising which ideally should be balanced i.e. when the throttle is opened, when the damper comes into play and hence the oil. But as the pistons are at or very near the bottom when starting (unless perhaps you crank at full throttle) I can't see the grade of oil having that much effect anyway.

The bottom line is, with all the millions of cars where 20W/50 is the recommended oil for the damper (in the appropriate climatic conditions of course) if any one of us finds it cause a problem that simply points to the real problem being somewhere else.
P Hunt

This thread was discussed between 27/12/2009 and 25/01/2010

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