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MG MGB Technical - Where Does Carb Damper Oil Go?

Where does the oil go that is supposed to be in the carb dampers?

Car was running rough on way home yesterday, I checked lots of things and went to the dampers. It took a bit of damper oil (8-10 drops each) to get the little plungers to momentarily stay up or so before gravity slides them the rest of the way down into carb. Id swear I had these topped off a few months ago. I'd also swear there used to be more residence to the plungers going down to the base.

Just how "momentarily" is the resistence to the plungers sinking into the carb supposed to be?

Maybe I still don't enough oil in the dampers?

See my next question on Emissions, just in case there is a relationship in problem.

R.W Anderson

Some useful info on carb dampers and damper oil at Paul Hunt's site
Brian Shaw

I'll look tonight to see if oil is up that high in the reservoir.
R.W Anderson

In my experience the pistons won't sink into the oil on their own, but need quite a firm push to get them down through the oil. So I guess there isn't enough oil in there.
Mike Howlett

Well, now I'm thinking I should go out at lunch time to see how high the oil is in the reservoir.

The pistons will sink quite quickly to point of screwing caps in, with only a slight momentary hesitation to sinking.

I am using Moss Damper oil instead of my old 20w-50 motor oil.

More later.
R.W Anderson

I went out side during lunch, dug out a flash light and looked down into the opening. Oil level was still way down below top of reservoir area. It took many more drops to bring it up to top, then I slowly took oil out drop by drop until it was 1/4" below inner reservoir top. Plunger stays up 1/2" plus and takes some pressure to push it down, matched level in both and tightened caps.

I'll see how it runs after work, in a few hours.

However, my question remains - where is the oil going. I hear some say they top theirs off in the spring and check it again next season. I'm in mine every 500 miles.
R.W Anderson

You said "dampers" so I assume you have SUs, not a ZS. On the ZS carbs there is an o-ring at the bottom of the piston that perishes and lets the oil out. I've never quite figured out where the SU oil goes. The only thing I can figure is that as the damper piston falls, maybe some oil gets "pumped" upward and escapes between the tube and the wall....
Rob Edwards

Yes, SU - but not the same year as car, older if I remember right. I'm just surprised how much oil it took this time to bring them back up, 15 drops or so.
R.W Anderson

As for "where does the il go?", some will come out through the breather hole at odd times, but most finds it's way between the piston and dash pot and the low pressure pulls it into the combustion chamber. That's why some people used to put upper cylinder lubricant (RedEx) in the dash pots.
Allan Reeling

It's curious - some folk say they have to top up the SU dampers from time to time, but my cars have never needed it. The oil is always there, just where it should be. It is in a sealed container which is only open at the top, so there aren't too many places it can go.
Mike Howlett

That's what's got me mystified -- it shouldn't go out the vent as long as it's below it's "normal" level, and it shouldn't go down the outside of the piston tube once the level is below the top of the tube (unless as I say above, it get pumped up as the piston falls). But I too notice the level falls slowly over time....
Rob Edwards

If you overfill them, they suck the oil out until they are happy. The correct level is just above the tops of the damper pistons - anything more does nothing except get sucked into the vacuum chamber where it gums up the works. There are various official publications which have you filling them too high - they are simply incorrect. When not overfilled, most cars will go a year or two or three (12,000 miles) without needing any. Since you should be removing and cleaning the air pistons and vac chambers by that time, and you invariably spill the oil, that's when I fill them, and rarely ever need to add any. If the dampers stop just short of threading in, then sink, you are good. How fast they sink is a function of oil viscosity, so thinner oil or hotter engine will let them drop faster. If the car develops a hesitation on acceleration, check the damper oil - that is the sign.

"Drops" is silly - I use a pump oil can with ATF, and about three pumps from a hesitation is fine to a little much, and the car will soon fix it.

FR Millmore

The oil is there NOT just to damp. The reason why "publications" show the oil level above the top of the piston, and this includes both BMC/Leyland and SU notes, IS to lubricate the running surface between the piston and the dashpot. Probably less necessary on ball bearing suction chambers, but I would still top up above the level of the piston to ensure that friction doesn't do part of the damping!! As for gumming up the works, the amount of oil usage is hardly going to do that. If you fret about it, use RedEx or similar.
Allan Reeling

There is no, zero, not any contact between the piston and the chamber wall, unless they are damaged, so they do not need any lubrication, and oil there screws up the calibrated leakage between them, and attracts and holds dust, upsetting the entire carburettor function. That is the entire point of the "drop test". Walls should be dry and clean, when the unilateral clearance will be on the order of .0001/.00015.
That leaves the piston center tube bearing surface, and in probably ten thousand carbs, I have yet to see one sticky or rusty, unless it was off the car in a swamp.

FR Millmore

That's right FRM. The only way to make the piston work properly is to have it and the chamber walls scrupulously clean by wiping with petrol (gasoline). Any lubricant will hinder the operation of the components.
Mike Howlett

I think I've woken up in a parallel universe. The centre of the piston and the dash pots are bearing on each other , hence the reason why there are ball bearings on the later models. Wakey, wakey!!! We all know the outer diameters don't touch. Could it be you haven't seen a sticky or rusty centre tube is precisely because they ARE lubricated by the damper oil!! Wake up and smell the coffee.
Allan Reeling

Should the vacuum chamber (dashpot) bushing and / or the piston damper tube become badly worn from lack of lubrication, the vacuum chamber (dashpot) will quickly duct the oil above the damper tube into the intake manifold.
Stephen Strange

I'm still wondering where the oil /below/ the top of the tube goes. And it does slowly go...
Rob Edwards

I think you haven't woke up at all. I clearly acknowledged that there is contact on the center tube. Since it seemed obvious to me, I did not elaborate on the fact that it does get lubricated as much as is required by the one drop of oil I put on it when I service carbs, and the tiny amount that creeps up the tube wall and/or gets splashed around as things move. That's on carbs that I fill to just above the damper pistons.

"We all know the outer diameters don't touch. "
Not true, by observation of people lubricating the chamber wall deliberately, as well as polishing them
"so that they move smoothly".

And my coffee consumption is likely far beyond yours, as well as my carburettor experience, or at least, understanding.

Rob- The answer is there; the depletion drops off exponentially as the oil is less overfull.
"When not overfilled, most cars will go a year or two or three (12,000 miles) without needing any." That's on my own cars and hundreds of customer cars I have serviced, in some cases the only person to touch the carbs over many years. When I was filling them "by the book", I was adding a bit at every oil change or other encounter.

FR Millmore

>"When not overfilled, most cars will go a year or two or three (12,000 miles) without needing any."

All I can say is that that has not been my experience with my personal car. It WILL over time drop well below the top of the tube.
Rob Edwards

Well FRM, we will both continue to do what we have always done and if running them for 2 or 3 years without a top-up seems sensible to you....words fail!. My experience is only in the hundreds, serviced, re-built and so on, so each to their own. As for coffee, you can have too much!!!
Allan Reeling


FWIW, I need to fill my dashpots every run.

The bl***y stuff keeps running out!


Roger T

Good by me.
Words might fail but my carbs never have.
For the record, after a few years at various Britcar shops, I was hired as an SU specialist by a Pittsburgh Volvo/Triumph dealer, in 1968, because none of the "mechanics" his son had hired could fix the customer's cars; some weeks I tuned 15 or 20 Volvos and TR. I have probably owned a hundred SU on my own cars and most of them got rebuilt, and there are likely a couple hundred lying about my shop - that's just the ones that moved in. Add in ten years of fulltime British work and 35 more part time, much of it specializing in Jaguars where the SUs came three in a bunch.
Needs coffee!

FR Millmore

I'm with FRM on this. I used to suffer the same problem--"where does the oil go?" I then took a tip from Barney Gaylord--the MGA guru. He says all he does is unscrew the damper and start to pull. If there's resistance at all, he doesn't take it out. Just screws the damper back in. IE, if you feel resistance, it's good to go.
Since I started doing this, I no longer have to top up the damper. It's always good to go. Two years now and so far so good. No top ups.
JM Morris

I ran an MGB for 12 years as my daily drive, covering tens of thousands of miles. I don't suppose I put oil in the dashpots more than twice in all that time, and then that would be because I had dismantled them for servicing. If you think the level is dropping it must because you have overfilled them in the first place. If overfull it will find its own level, just like the earlier radiator tank.
Mike Howlett

>FWIW, I need to fill my dashpots every run.

Ok, that has NOT been my experience. Several times a year, tops.

I'll grant that my experience has been that maybe it's seeking it's own level. But "it's own level" seems to be about where it's shown in this section (oddly enough!) -- well below the top of the tube. And yes, by Barney's measure there's enough oil. It performs its damping function. But I like the idea of a little lubrication on the sliding surface there.

Rob Edwards

Apparently I appear to be able to get some good long threads going.

As I mentioned way above, I topped mine off to top of chamber the piston slides in. I haven't gone back.

One main difference between topping SU this year than last, before I simply used Castrol 20w-50, now I use a free bottle of Moss oil for carbs. I think this is much thinner than the 20w-50.

Not sure if I've eliminated the originally noticeable hesitation upon initial acceleration from stops.

I also think I may have complicated things by almost running the gas tank to empty the other day and introduced possible debris into fuel system. Time for a can of carb cleaner in the tank and time to have a look inside float bowls.

But thanks for ALL the feedback and discussion. I was worried most thought my subject line was inquiring about where to add the oil, not where does it leak off to.

Thanks Again
MN MG Group Gazette
R.W Anderson

You're right in your belief that lubrication is important. If you invert the vacuum chamber (dashpot) and examine it carefully, you will notice a bushing inside of its neck. The damper tube of the vacuum piston is a precision fit inside of this bushing. Because of the limited airflow capacity of the vent hole in the damper cap of the damper rod, a small amount of the oil that is above the damper tube will be pumped down the exterior sides of the damper tube by the air compressing above it slightly when the vacuum piston rises, providing essential lubrication to the bushing inside of the vacuum chamber (dashpot). The maintenance of this fine-tolerance interface is critical to providing the appropriate amount of vacuum so that the vacuum piston will rise to a position in which its attached fuel-metering needle can meter fuel to the correct fuel / air ratio.
Stephen Strange

RW asked "Where Does Carb Damper Oil Go?"

That's why my engine is so oily underneath!
Mike Standring

This thread was discussed between 21/08/2012 and 04/09/2012

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