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MG MGA - Carb damper oil use

Why would I have the problem of having to refill my carb dampers so often. Let say I fill them before I go out and take a 60 mile highway drive. By the time I come home the dampers are dry. The car runs great...lots of pickup and go. Is there a "norm" as to how often I should be filling my dampers.
Gordon Harrison

Interesting one. What grade oil do you use? I can only surmise that if a thick oil is used some of it will be pulled out by the damper piston and escape down the outer side of the damper tube when the carb piston lowers; whereas with thinner grade oil more of it will readily by-pass the damper piston?

I use a light machine oil that I have in copious quantities for my lathe.

It will be interesting to see what others say.

Steve Gyles

I had always been told to use 20wt hydraulic motorcycle fork oil.

No idea why any weight oil would be pulled from your dampers though. Almost seems there would have to be a leak somewhere.
Gene Gillam

I use light weight oil. Actually it is hydraulic jack oil. Maybe I should step up to something heavier.
Gordon Harrison

Maybe you are filling the dampers with too much oil?

The correct oil viscosity recommended by S.U. (Burlen) is SAE 20.

At one time I used to use 3 in 1 oil until I measured the viscosity and found it much too low (thin). I looked around for some thicker oil but then I thought why go to all the bother of trying something else and then wondering if the problem you have is due to the wrong damper oil when all you have to do is order the correct oil from Burlen and then forget about it. It's only 3 or 4 and you get about 100ml. The damper only needs 4ml so you've probably got a lifetime's supply..................................Mike
m.j. moore

I use engine oil, works a treat, ever goes away. If your damper oil is disappearing there must be a leak in the bottom of the bore hole. Some later SU HS-type carbs and Z-S carbs have a through drilled hole and require an O-ring for sealing the needle shank.
Barney Gaylord

The ideal type of oil is one that resists change of viscosity with temperature. Moss sells such an oil, which I have learned is simply a 20W vacuum pump oil.

Are you sure your pistons are completely dry? If you unscrew the damper and move it up and down the tube by hand, do you feel any resistance? If so then you don't need to put any more oil in. The level should be roughly 3/8" below the top of the tube but so long as there is resistance, you're fine. Filling to the top will only cause overflowing.
Steve S

I think the oil in the damper finds its own level. should be just over a teaspoon in there which of course you cant see so it appears empty, that's why the car runs fine. When you fill it up the excess is initially displaced by the damper and vents out through the small hole in the damper cap on the piston or down the sides of the guide. As an experiment fill the damper and feel the resistance using the lifting pin then go for a drive and test again, bet its the same although I haven't actually tried it.

AR Terry

I have always use 3 in 1 without any problems. It does sound as if it doesn't much matter what oil you use. I am with Alan's idea above.

Paul Dean

Sounds like the perfect test. I will try that.

Mr. Gaylord,
I will check my carbs for leaking oil.

I will check for resistance.

I normally fill the dampers to the first metal mark (line )below the top of the carb. About 1/2 inch below the top. I will alos switch to 20wt oil.

Gordon Harrison

Burlens, the owners of the SU brand, say 20SAE.

As Alan says, its just a small amount in there. I wonder if some people fill the chamber right up to the neck of the screw cap, instead of just the inner cylinder way down inside?

Steve Gyles

I have always used 3 in 1 oil. Never had any problems but like Gordon I do need to refill my carb dampers more often than I should have to. Could this be the reason that I get contamination on my suction chamber pistons?


R A Evans

Did your test and felt resistance. I do not see any oil in the top narrow portion of the carb, so I am thinking is is in the large bell area.

Mr. Gaylord,
I am going to buy some 20 wt oil this weekend. No leaks in the carb. PS: If you are in the vicinity on the weekend of Sept 19-20, why not drop by Stowe Vermont for the largest eastern coast British Car Event...THE BRITISH INVASION. Normally app. 700-800 British only cars. Well worth the detour.

Gordon Harrison

The oil is contained solely in the tube. There is none in the large aluminum area of the piston. The amount necessary is only enough to cover the brass valve on the end of the damper rods. So long as that is submerged while the piston is fully down, no additional oil will be necessary.

It is true that any fluid will work. But it does make a difference what you use. A lighter oil will reduce piston damping and heavier oil will increase. This has an effect on air/fuel mixture when you accelerate and decelerate. A car with a standard setup typically wants a 20W oil. Some racers are set up with a lighter oil. As the oil heats up it will thin, which also affect performance. Using an oil that resists viscosity change will improve overall performance. Motor oil is a decent product and can be used when a better type of oil is not available.
Steve S

When you overfill the dash pots, as has to be the case here, the excess oil will be sucked into the dash pot chamber. The dash pot and its piston should be dry. You can check this out.
If the piston and dash pot are wet with oil, any dust/grit which gets past the air filter will tend to stick in the oil film and can cause the piston to hang up in its bore and give a very lean mixture and a dreadful misfire. I couldn't even run at 40mph! Yes I've been there.
I simply cleaned up the oily dash pot and the problem went away. Since then I hardly ever do any topping up. More recently I've also replaced the original oil soaked wire air filter with a felt type, it has be better. I'm another user of 20SAE damper oil - that's what was always recommended for an "A".

I have had a similar problem. Losing oil from the damper tube. When it happens the car loses power. Stopping and filling up the damper cures the problem, and runs fine again. This seems to occur whatever viscosity of oil is used.
I have checked that there is no leak at the bottom of the damper tube.
A drop test has revealed a low figure in the back carb of 3 secs.
I now suspect that due to excessive wear between the damper tube and guide the damper oil is being sucked out under engine vacuum into the suction chamber and engine.
Conclusion need a new suction chamber assembly?

I have been using automatic transmission fluid for over 20 years. I fill each chamber with only enough fluid, that will allow a very small lift of the piston before feeling the resistance. It is important that under acceleration, both pistons react the same.

George Raham [TD4224]

I use Penrite SU damper oil
I am sure all the other alternatives offered above work fine - the advantage of the Penrite stuff is that the nozzle and squirt bottle allows you to put just the right amount in without overfilling (I have use 3/4 of the bottle in 9 years - drive my car about 30 days a year so is not a major expense).
Gordon - are you filling the damper tube correctly, a pencil torch will let you see that the level is correct - about 3/8" down from the top of the tube when full - if you have resistance before you screw the damper in you will have enough for the day's drive.
Mike Ellsmore

As an example of fill level, the HS4 dashpot from a B manual.

R Taylor

Note that the piston is raised about 3/4" in that picture, so the oil would be a further 3/4" down from the cap normally.

I check my dashpot oil every six months or so or when I do a service. I rarely add any oil. My daily driver is a Morris Minor and I don't think I have needed to add any oil in that in the last couple of years. (As oppose to the engine, which needs oil adding every week)

Where is this oil going, or are people just over filling and letting it spray out of the vent hole? It sounds a bit like radiator topping up after every trip...

Neil MG

Neil, by my reasoning the oil level in the tube does not change as the piston goes up and down!
Mike Ellsmore

I agree with Mike. Putting my lateral thinking cap on I wonder if the issues suffered by some owners is lack of air balancing in damper area. i.e. some of the caps that should have the hole drilled don't have it drilled (requirement varies on carb modification state), whilst others may have the air balancing holes to the piston (not the damper) topside blocked by an incorrectly fitted rear filter box. This latter issue is more common than many realise.

Just a thought.
Steve Gyles

Hi Steve,
Your comments made me strip the carbs to inspect.
I have the correct vent hole in the damper cap, along with an undrilled external web on the suction chamber top. Its either one or the other I believe, ie drilled cap or web.
The path for atmospheric air to the underside of the suction disc, through the air filter, is not blocked.
There is a clear path for vacuum, from underneath the piston to the top of the suction disc.

The manual refers to a close fitting piston. What should the clearance be between the piston and main body of the carb. I can rock my piston around a bit.


Neil, by my reasoning the oil level in the tube does not change as the piston goes up and down!

Mike, note that the tube is part of the piston.

The level in the tube stays the same (relative to the tube), but the tube moves up and down so the level moves up and down (with the tube) relative to the top of the carb. If the above section drawing is to scale then the oil level would be about 2 1/4" from the top of the carb. with the piston in its static position.

H, the piston should be a good fit in the carb body. With piston and cover removed, hold upside down with the piston placed inside the cover. You should be able to control the rate at which the piston falls by blocking the oil filler hole with your finger. with the hole blocked the piston will resist all but the slowest movement in either direction.
Neil MG

Neil, I think we mean the same thing but something maybe lost in translation - I think we both agree that you shouldn 't loose oil when the piston goes up and down
Mike Ellsmore

Hi Neal,
I think what you are describing is very similar to the drop test in the SU Workshop Manual.
The time for the piston to travel the full length of the chamber is given as 5-7 seconds for a good piston/chamber assembly.
In my case one carb, the rear, gives a figure of 3 secs.
For this reason I was suggesting there was excessive wear, which was allowing the damper oil to be sucked out of the tube into the area above the suction disk?

This thread was discussed between 03/09/2014 and 16/09/2014

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