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MG MGA - dash pots

what weight oil should one use in the carb. dash pots???
gei irwin 1

Straight 20SAE weight oil.

Automatic transmission fluid (ATF) is recommended by most, but some say it is too light for anything other than standard tune?? Otherwise fully synthetic 20 weight engine oil. Or you can pay a bit more for a nice plastic bottle with SU on the side! (from about $5 up to $13 for 125ml)

Neil
Neil McGurk

Another thorny topic! Here in Oz with our warm weather, engine oil (say SAE 30) is unquestionably the best (holding the piston down). The books say light oil but they are mostly written in the UK with much cooler climes. None of our MG's accelerate properly with light oil in the dashpots. I know that raising the piston brings about greater needle/jet clearance, but the speed of the air over the jet (creating "suction") results in richer mixture which is what is necessary for "optimum" acceleration.
Barry Bahnisch

I'm using 20wt motor oil.
Ed Bell

I've always used 30WT oil, even when I drove the car in the winter. Works fine.
Cheers,
GTF
G T Foster

I've tried lots of kinds of oil and never noticed any difference.

Malcolm
Malcolm Asquith

I have normally used 3-in-1 (general purpose light oil - bicycle chains, door locks etc) but have now started using my Myford lathe oil since I have gallons of it! (Esso Nuto H32). I tried grade 30 but found it too heavy in the UK climate. Engine was slow to respond; carb pistons struggled to rise in concert with my right foot.

Steve
Steve Gyles

Does the amount of oil you add make a difference. I normally fill to 1/2 inch below the top of the damper. Is this correct?

Gordon Harrison

I use 3-in-1 oil like Steve and it works ok but it also seems to disappear after a day or two's driving.
Is this usual and if so,where does it go?
Colyn
Colyn Firth

According to John Twist of University Motors you should use 90 weight gear oil. I have never tried that. I have allways used ATF. Works great and the carbs respond very well. I never use the "Dash Pot Oil" that is sold by Moss or Vick British as it is over priced and ATF works just as well.
JP Jim

I have alway used the theory that if I meet resistance anywhere near the top as I install the dashpot cap, then there is no need to fill it. If I try to keep it topped off, then just like coolant level in the radiator, I will forever be topping it off. Based on my "theory", it never needs filling. In fact, I can't remember the last time I added any.
Chuck Schaefer

As you install the damper you should feel some reistance as it enters the oil. This should happen when the screw top is about 1/2 inch from engaging the top of the threads. It's not much oil and over filling is easy to do. I never have to "top up".
Ed Bell

Colyn,

Can I ask the obvious? You do only fill the the hole in which the damper goes, do you?

I work at a carleasing company and a few weeks ago a client called that he had overfilled the engine with oil. That was somewhat of an understatement as he had filled the engine right to the oilfillercap....

Willem
Willem vd Veer

Colyn

What everyone is trying to say is only to fill to near the top of the piston cylinder, not to the top of the chamber. If you fill to the top of the chamber two things will happen:

1. During operation some oil may migrate out through the screw cap hole (if your carb has one).

2. The oil above the level of the piston cylinder will seep down the outside wall of the piston, into the main piston chamber and eventually migrate into the combustion stream of fuel and air.

See my schematic.

Steve

Steve Gyles

Hi,

I just use the oil I use for the engine, (10W40) and My MG looks happy with it. Throttle response is OK. Too.
Once I tried a thinner oil, like 3-in-1 or tranny oil, and it seemed to have disappeared halfway a Sunday trip. Leaving me with a really unpleasant driving car. Luckily I had a litre of spare engine oil in the booth ( or trunk, or whatever, we call it “koffer”) so I could fill it up again, and had a pleasant drive further on. Since then, I kept using plain engine oil, and I’m happy with it.

Rik.
Rik de Krom


Barney Gaylord says if he can feel resistance when starting to remove the damper, he just screws it back in. It's got enough.

I'm guessing that those who are having disappearing oil are over filling, and it's going the route Steve Gyles is suggesting.
JM Morris

Yes, I agree on that, Barney is certainly right here. If you feel a resistance when you screw in the damper, it's still ok. Any overfilled oil will either spill or get burned away in the engine. I try to fill up until half an inch from the top of the tube. (If I succeed in that) The thinner oil, however, seemed me, to disappear in a wimp. With the ordinary engine oil I only need topping up a few drops every 2-3000 KM, and it drives ok to me.

Rik
Rik de Krom

I dont fill the carb damper to the top, I usually put enough in to be able to see it at the bottom of the tube.
Probably the oil is going along the route that Steve suggested so I think I will try some slightly thicker oil just to see if the car still runs ok and if the oil stays around a bit longer.
Colyn
Colyn Firth

Penrite suply an oil for the job.
http://www.penriteoil.com.au/products/classic-car-oil/su_damper_oil
Advantage is that it comes in squeeze bottle with a spout so you don't spill it everywhere but in thw hole!
Mike
Mike Ellsmore

Maybe it is known to all the readers but it might be interesting to discuss the function of the damperoil.

As I understand it, the damper regulates the movement of the piston AND creates an accelerator-pump effect.

The thicker the oil the slower the piston will rise when the throttle is opened, thus creating a greater vacuum that will suck more petrol into the mixture, which will in turn help to accelerate better/smoother.

You can learn a lot by driving the car without dashpot damperoil and see how it behaves, that way you can spot (in a magnified way) the behaviour caused by thinner oil.

In theory a multigrade oil would be better than a single grade, in the same way you wouldn't use a single grade in the engine anymore?
Willem vd Veer

Willem

I understand what you are saying. However, what confuses me with your logic is that there is a tapered jet needle at the bottom of the piston. It will only deliver maximum fuel to the air when the piston is fully raised. If the piston is too slow to rise then there will be insufficient fuel (and air) for the demands you have put in via the throttle pedal.

Or am I missing a trick?

What I have noticed is that with insufficient oil in the dashpot the idling RPM tends to creep up.

Steve
Steve Gyles

Steve,

When the tapered SU needle rises and falls normally it matches the fuel to the airflow.
However, for acceleration you require the mixture to be richer for a short period.
The delay in the piston rising, caused by the dashpot damper oil, causes the air across the jet to have a higher than normal velocity, as the restriction caused by the piston does not match the requirement of the throttle opening.
This higher velocity of air across the jet causes a greater than normal amount of fuel to be drawn into the airstream. Greater than the tapered needle would normally allow.
After a short time the piston and needle reach the correct position to match the aiflow and hence give a normal mixture.

Mick
M F Anderson

Steve,

I should have mentioned that with the restriction of the air across the jet, caused by the piston, compared with the larger cross section of the openings in front of and behind the piston, a lower pressure is caused above the jet.

See Bernoulli's Theorem:

http://classicairshows.com/Education/Aerodynamics/BernoulliAT1243.htm

You must have been down at the local pub when your RAF instructor was covering this subject.

Mick
M F Anderson

Mick,
I could not have explained it better!

(actually, I don't think I would have managed more than 50%, but we don't have any RAF instructors)
Willem vd Veer

Mick

As Willem says, well explained. I am happy with the theorem, it was just the physical restriction to fuel flow caused by the needle that had me wondering. In short and, going to the extreme, if you have the bath plug in, no water will pass, regardless of pressures acting above it. How much do you need to lift the plug to make any significant flow?

Now which way does the fuel swirl past the needle, clock wise or anticlockwise and does it make a difference? These systems were designed in the northern hemisphere. Do they work okay in the southern hemisphere?!!

Steve
Steve Gyles

Steve,

It is a lesser known fact that directional needlefuelswirl was extensively researched by Abingdon engineers during the post-war recordbreaking period. It was found that it isn't a problem with the fixed needles with 0.90 jets as per SU H4. On the hypertuned (supercharged) recordbreaker engines however, at full power the needle deflection was as much as 1,25 degrees on the exotic alcohol fuelblend, necessitating the use of titanium needles. Interestingly with 5-star Shell fuel the (brass)needle deflection was 1,31 degree.
It may also influence wear in the biased needles that are fitted to later (HIF) SU's.

OK, it was a long day at work.
Willem vd Veer

Willem

I was just having a bit of fun with the swirl. I had no idea it was actually researched!

Steve
Steve Gyles

Steve,

That makes two of us. ;-)

Willem
Willem vd Veer



Well now. I'm glad to see this has come back up, since I never got an answer in a different thread regarding piston oil reservoir bore diameters.

The oil has to push past the plunger. Assuming both carb plungers are the same diameter, if there is a differing bore size, then one piston will rise faster than the other (because the smaller bore creates a greater oil restriction). The question is--how much bore size variation is too much?

Although I never got an answer, it later occurred to me that maybe this is one reason why you hear of tuners experimenting with differing oil viscosities between the front and rear carbs.
JM Morris

JM

Surely all that is taken care of by the standard SU balancing process. The SU balancing kit also comprises rods that are inserted into the dashpots and you check that the pistons rise in unison. Although I concede that the dampers are removed for that particular check.

I would have thought that the oil reservoir diameters are all the same for specific H series types. Do you have proof that they vary?

Steve
Steve Gyles

JM

If you look at the materials used and the level of finish, I can hardly imagine that tolerances in manufacturing would have significant effect on the performance. And if there was an effect, the plunger looks more prone to differing than the bore.

Differences or damage in the dashpot bore is more likely to cause variation that has influence on performance?

Willem
Willem vd Veer

Steve--I assumed the same as what you have stated (the SU balancing kit showed--as best as I could tell--that the pistons were rising in unison). But what got me to checking into this was the fact that it took almost twice the force to raise the front piston vs the force required to raise the rear piston (engine not running--fingers inserted into the mouth of the carb).

The damper diameters, size, etc. were exactly the same. But there was a .005 difference in bore diameters between the front and rear carb. Unfortunately, that was measured at the top of the bore. I then discovered that there was another, "step down", concentric bore in the area where the plunger operates, and was unable to measure if there was a difference there (no measuring device that would reach that far down into the bore).

I know this is nit-picking. But the curiosity is "killing" me :) HOW can this be, that there is so much variation in the force required to move the pistons (oil past the plunger) and yet the pistons seem to move in unison during operation.

The carb internals are clean as a pin, correctly assembled, no air or oil passages blocked, etc. Checked and rechecked all of that.

This is an exercise in theory only, to try to expand my understanding of how and why the carbs work the way they do. I don't lose sleep over it, I just go on. But I would like to understand the reason. There's something I'm missing.
JM Morris

JM,
When using the SU balancing kit, the dampers are removed therefore the pistons should move with the same amount of effort. If not, then the difference may not be caused by the dampers. Does the force required to move the pistons seem more equal with the dampers removed? If they are the same without the dampers, you might try switching the dampers from the front carb to the back and the back to the front to see if there is any change.
Ed Bell

Thanks, Ed. I'll give that a try.

By the way, to answer the original post, see the following. It was taken from "Calvers Corner" on the Minimania.com tech article section:

This subject is a regular message-board poser. Many of us have touted our preferences and discoveries whilst playing about over the years. One problem with trying to deal with it on the message board is the sheer volume of information offered and available against the time available to post it. Personally I try to keep my answers as short and informative as possible. Apart from the fact it costs a fortune to be active on the web site here in good old 'Blighty', long dissertations can put folk off from reading it all. I've posted information as I have found it on this subject several times, but not always the same answers as the situations and engine configurations vary so much. There are no hard and fast answers, but I gather from what I've heard recently some of my earlier advice posted has been misconstrued. So, to try and set that right, and give a good working basis on what to use where, here are my choices - SU Dashpot oil - manufactured for the specific application of SUs in STANDARD applications. That is on engines that are completely standard as the manufacturer produced them. It is a little on the thin side, but the whole engine tuning data for each engine is based on using this oil. Used as such it works very well, especially where economy is the main goal and the car is driven very sympathetically. ATF - 'automatic transmission fluid'. A very good substitute where the genuine SU dashpot oil isn't available for use in STANDARD spec engines. 20W fully synthetic engine oil - not to be confused with 20W50 engine oil of any sort. Just a 'straight weight' oil. I use this on practically anything where any modifications at all change the spec away from original. This includes any induction/exhaust changes no matter how small. Largely because the engine is likely to see more 'aggravated' use. Active use of the throttle pedal requires a more stabilizing effect on the dashpot piston along with slightly more resilience to give the desired pick-up. The main benefit of the synthetic type is its consistency over a very broad operating temperature range. 10W/40 engine oil - gives very similar results to the 20W fully synthetic oil as defined above. It is certainly easier and cheaper to get as most modern automotive engines use this oil. Semi-synthetic is best, although fully synthetic is as good but rare in the price/'what you have in the garage' stakes. Standard mineral oil is OK, but gives poorer performance until the engine warms up unlike the synthetic oils as outlined above. Again, this is by no means the 'be-all and end-all'. Others have distilled (literally in some cases) their own 'snake oil' that gives them what they're looking for. The problem with 'special brews' is that they're hard to repeat consistently. I just hope this list saves some agony.
JM Morris

This thread was discussed between 14/06/2010 and 20/06/2010

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