Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Basics - SU Damper oil

I have a '53 TD that I bought 4 years ago as an abandoned restoration. Great car with many new parts. I've done a few MGB's before but none equipt with SU's. This is my first hands on experience with the SU's. I read the books and rebuilt them and they appear in excelent condition. Picture shows from and to progress. I even got the engine runing this weekend for the first time in at least 15 years. Sounded great. But my confusion is with the damper oil. I put in 30 weight, and I could not get the damper rods down the tubes. They hit the oil and stopped. So I pulled out the oil and replaced it with 10W30. The damper rods still stopped when hitting the oil, but I was able to work them in with some tilting and pushing. Once down all the way, it was very hard to raise the pistons in the carbs. Eventually, one got better than the other. It now takes 2 seconds for me to lift (with just a finger and considerable effort) one piston all the way up, and 5 seconds for the other. They both drop as I would expect them to do normally. What am I missing?

MJ Rotsky

Sounds like you have way too much oil in the damper. You only need a small amount, I'd guess less than a teaspoon. You shouldn't have any resistance when you put the dampner in until just before the bottom of the cap is about to thread in. You'll have to remove some oil, you can stick in the dampner, remove it and wipe the oil off, repeat until you get to where the the cap first show resistance just above when it starts to engage the threads. They will still be hard to move by hand, these dampners works well!

That's how I've set my SUs, there maybe others that have other guidance.

BVS Van Sloun

Another oil for your consideration:
Mort 1950 TD Möbius


'I beg to differ', the damper oil can/must be filled to the top, any exces oil will overflow harmlessly.

I would inspect the damper piston carefully for contamination/blockage. 10W30 should give 'normal' damping. 2 seconds with effort seems a bit too much for me?

Thick oil gives more enrichment on acceleration
Willem vd Veer

I'd recommend going to a motorcycle shop and getting fork oil. It's available in all weights from 5 up to 30 I think. I run 20w in my Florida car and it works great.

L Rutt

I use Marvelous Mystery Oil in my 51. Works fine
Tom Maine (TD8105)

I used to use 3 in 1 oil in my old MGB, but I have a container of Moss oil that is labeled specifically as carb damper oil. I believe that it calls for 20. 30 weight motor oil is definitely too viscous. You should be able to push the damper rods into the bores with light resistance.
Steven Tobias

You'll get plenty of opinions/alternatives on what oil to use. ATF is a 20 wt oil that I use. Yes, 30 wt is too heavy, so that may contribute to your problem.

However, another issue may be your pushrods might not be vented properly. Check to make sure you have the original brass-capped pushrods with the hole in the top. Some of the later versions weren't vented. Without a vent the air in the chamber has nowhere to go. See the following for an explanation:

The best way to fill the dashpots is to remove the pushrod, lift the piston to its highest point, then fill just the tube you see. Yes, you can fill the dashpot with the excess pushing out the vent tube cap, but it makes a mess.
David Littlefield

Thanks for your comments. To respond - As I said, I have already switched to 10W-30 oil thd thought that would help. It did just a bit but not nearly enough. I can try ATF and Marvel Mystery oil and see what happens. As to the venting, I can't even get the threads of the cap close to the carb without it stopping, never mind the vent hole in the cap, which I do have. Any other thoughts are welcome.
MJ Rotsky

You may check that the parts of the cup assembly at the bottom of the dampener rod are not gummed up and are loose so that they open up on the rod descending.
I generally insert the rod to the oil and let it settle on it's own, which takes a minute or so. It will generally sink down to the top of the threads. Allowing it to sink on it's own keeps from pushing oil out.

I fill with 20w oil to 1/2 inch from the top of the tube, and then insert the dampener.
Dallas Congleton

Hi Mark,
I have used an automatic transmission fluid for over 40 years in the SU carbs, and it has worked just great. 30 wt. oil would be too heavy from my experience.
George Raham [TD4224]

I had a similar problem. Before you put the oil in the dampers did you do a drop test on the pistons and centre the jets properly? If so then the pistons should both fall in unison when there is no oil in the dampers. Did you achieve this? John Twist has an excellent video on his site that shows how to do this.

I did all of that and still found when I put the oil in that one piston dropped more slowly than the other. I took the Suction Chambers back off (don't disturb the piston or you'll need to re-centre the jets) and found that oil had overflowed and one of the piston shafts was covered in oil. I then screwed the damper into the Suction Chamber and checked from the bottom of the Chamber to ensure the damper was concentric with the bore of the Chamber. I found it was bent and dragging on the bore a little.

Also there's a small washer on the bulb of the damper that acts as a non return valve. Make sure it's not in upside down or gummed up somehow.

The vent on my carbs isn't in the damper screw top it's actually in the Suction Chamber itself. Have you examined the suction chamber for vent holes?

Just a few thoughts you might try.

I ended up using 3 in 1 oil which is very light. Works fine.

A R Jones

Did the slides move freely before adding the oil? Might trying dumping the oil and checking free movement sans oil.

Jim Northrup

I think the discussion on how much oil is of interest. I filled my dampers to the top for years. Last year I was informed by one of the members on this web that rebuilds SU's for others to only put in a small amount.

According to my "Tuning the SU Carburetor" manual, it says.

"Top up the piston damper with the recommended engine oil until the level is 1/2 inch (12mm) above the top of the hollow piston rod.


On dust proofed carburetors identified by a transverse hole drilled in the neck of the suction chambers and no vent hole in the damper cap, the oil level should be 1/2 inch (12 mm) below the top of the hollow piston rod"
Bruce Cunha

Jim, the slides move freely withough the dampers in. I did do the needle centering process described by John Twist and they both move as prescribed and at the optimal drop rate. I emptied one carb and refilled it with ATF. No difference. I tried both carbs at various oil fill levels and little variations but still a bear to raise. No better than 3 seconds. Even the one that went up in 2 has gone backwards.(By the way, the Jag CLub website has a nice description of the carbs and they specify that the oil should be filled to 1/2 inch BELOW the hollow piston rod.) I found that I had another damper and cap assembly in my spares kit and I tried that in the carbs to. No difference.

Again, any suggestions are welcome.
MJ Rotsky

WSM: "oil to Ref A" = engine oil
20 below freezing, 30 above.
or 20W-30 or 10W-30.

Or as many of us said and do, ATF. In my case since Volvo told me to c1967, when I was hired as the SU carb expert at a Volvo-Triumph dealer. It works year round without changing the oil in the carbs or your "carb oil can". Before that I did as the WSM; but, 10W40 is fine, 20W50 is a bit much in below freezing temps. 5W or 3 in 1 is generally too light, unless the mixture is already too rich, or you have a very responsive engine with a very light flywheel.

Regarding fill levels, and notwithstanding the various published recommendations, The action of the dampers is only affected by the oil being just above the damper piston when the air valve piston is fully down - anything over that is just reserve to make up the tiny amounts that get lost by working down past the piston rod. The less there is above the damper, the less is lost, and the less you ever need to add. BUT: any airspace below the damper will cause a stumble on acceleration. Excessive oil will cause a mess, on the outside of the carbs on externally vented, or inside the vac chambers on "Dustproof".

I do as Dallas - fill the tubes to 1/2" down, drop the dampers in, put the oil can away. When you get back, the dampers will be sitting down on the threads, ready to screw in.

By now you should get the idea that it is not very critical; the only thing affected is throttle response, and in general the carbs are more responsive than the engine within the ranges given. If and when you get very good at understanding the fine points of SU operation and tuning, you MIGHT find some barely perceptible result of changing between these options. In the meantime, stop worrying yourself about something that is not specified and nobody told you to attempt measuring.

FR Millmore

When I did our TD carbies, one of the damper assemblies was upside down, as AJ above stated,,,, but you seem to have adifferent problem,,,, S0ooo a few more questions,
When you had the car running, how did the engine respond to aceleration? how was the throttle response?
Did you take the pistons out and try to insert the dampers (with and without oil) so you could get a better visual on what is (not) happening?
I'm sure that you checked to see that the tube is clean as Jim suggested above,,
It sure seems as if something in the damper assembly is the culprit,,,
Where in Mass are you?? I'm in Ct and might be able to take a ride up..



I think that you may be doing a test that is giving you a false indication. If the slides move up and down freely without the dampner in, and you've gotten to where you can screw the dampner down, you might be just fine. Various oil viscosity will tune the respnsivness of the carb -in use.- It takes a surprising amount of effort to lift one of these slides, the vacuum on the backside of a running engine is significant. Doing your test of lifting up the slide with the dampner in on my car would take a few seconds using finger pressure. I'd test a couple of my others, but they all have filters on. Perhaps other members can let you know what they feel for resistance on their carbs.

If the slide falls normally, the valve in the dampner would appear to be working.You may just want to do a test run and find that everything is fine.

BVS Van Sloun

As Fletcher stated you are trying to measure something that is not defined and is not necessary to measure.

To understand the purpose of the dampener, you need to know that the dampener is a one way check valve to slow the rise of the piston. The piston will still fall at its rate defined by its weight (or weight plus the spring induced force). Once the throttle plates are balanced (with the dampeners not installed and the engine running) the airflow thru the two carburetors is theoretically the same. The air pressure (at steady state) at the suction chamber and the bridge of the carb is the same. If you increase the airflow through the carb, to even the pressure, the piston must rise, providing a new location on the needle from which to meter the fuel in the annulus opening between the jet and the needle. The idle mixture is about 13.2:1 (rich, compared to a stoichometric mixture of 14.7:1) and somewhat leaner as you open the throttle and cruise, due to the needle stations selected for the carburetor.

Under acceleration, if you dampen (retard) the upward movement of the piston, the air pressure under the piston (at the bridge) will decrease as the air flow increases (Bernoulli’s principle) This drop in pressure pulls additional fuel into the airstream enrichening the mixture for accelerated load, despite the area of the annulus being smaller due to the thicker portion of the needle being in the jet. This increased mixture is about 12:1 or more, depending on the needs of the engine.

Despite the one way check valve in the dampener, the piston rises, the pressure increases over the jet and less fuel is pulled into the airstream, while at the same time the needle is allowing more fuel in as it rises. Eventually, the air pressure under the piston and that inside the piston chamber evens out for steady state, and the needle station for that airflow is in play.

So you see, the rate at which the piston rises may be variable, but the improtant thing is that the dampeners resist the rise for enriching the mixture under acceleration until a steady state is achieved (and that is why the carbs are called Constant Depression).

Dave Braun

Thanks again for your comments. I guess I'm just stuck on that old theory that what goes up, must come down. Or in this case, the opposite relationship. And if one direction is measured, why shouldn't the other be? Ah, the mysteries of life with LBC's. So I guess I should just focus on getting the two carbs to react the same way and go on from there for now.

Unfortunately, the fact that the engine runs does not translate to the car being drivable. That would reguire a floor, seats, and maybe even a steering wheel. All I did was fill the float bowls and run it out of gas. But that certainly added enthusiasm into the restoration.

I still think it would be very interesting to see what others find when they try to raise the pistons like I did. Would be nice to know if mine are really reacting similar to proven performers.

MJ Rotsky

It takes some effort to raise my pistons when the dampers are full of oil. Especially the first 1/4- 1/2 inch but gets progressively easier as you get your fingers under the damper. I guess there's a lot of air trying to get out of the small vent hole.

If they pass the drop test then I think all you need do is to make sure that the dampers don't rub on the walls, the damper non-return valve works the right way and that they both fall at the same rate when the dampers are full of oil.

A R Jones

One more little item with all this. I found that I had 3 different versions of the dampers. I've attached pictures of pictures (originals too big to post) which shows the different styles. Interestingly, the nasty looking one on the left was the one that worked the best. Probably because it would get misaligned at times and let more oil through. Which is the correct style for a 1953 MG TD?


MJ Rotsky

I have the one on the right in my TF.

A R Jones

The original ones had one type of end clip, more recent productions the other type of end clip. That should not change the function at all. To sum up the excellent posts above: the pistons must rise and fall with no drag without the dampers in. With the damper and oil in place, it should be very stiff to raise upward, but should fall pretty quickly back down- maybe a touch slower than with no damper or oil. Time of rasing will depend partly on how strong the lifting force is that is applied, so I don't know that you can compare directly to other carbs/people, etc. However, carbs must be the same (ie damper rise time)for correct running. You may need to order at least one replacement damper to match one or the other, or a new pair. Make sure the spacers/etc. are assembled in the right order- could be the photo, but it looks like a brass washer on the right one and none on the middle under the clip. George
George Butz

Like AJ's, my TF's is like the one on the right in Mark's photo.

On a related topic (I decided not to start a new post), regarding the vent hole in the brass damper cap. I quote the following write-up from

"Some carburetors have the top vent hole, but not all. Some carburetors have the lower vent hole, but not all. The trick here is that you need exactly one of these holes but not both."

And, "If you have both vent holes the engine will run really bad as it spoils the chamber vacuum, and the piston may only rise a little or non at all (always lean under power."

In checking the carbs on my '54 TF, I have both vents on each carb. Should I now seal off the vent hole in the damper cap? The engine seems to run well after rebuild (although it does have an annoying miss I'm trouble shooting), but then, I haven't had it out on the road for a full test.

John Brickell

Here is another picture of the three dampers in the same lineup. Looks like the two on the right have the same washer/spacer setup. Assuming close or same performance, can these two then be considered to be a match? If so, can the washers/dampers be swapped between rods? The unit on the right has a hex cap while the other two have round caps.


MJ Rotsky

You will note that there is a brass shoulder solidly attached to the rod, with a tapered face toward the rest of damper (washer, piston, clip). Then the brass washer, which has a matching tapered ID; these two parts form the damper valve, and must be assembled correctly. Then comes the actual piston; these come in at least two different lengths. A critical function is that the "slack" in this assembly allows the air piston to rise a bit, immediately on throttle opening. This allows acceleration to begin. This is a fairly subtle difference, and is rarely changed, but I think there are generally two lengths of damper piston, and they should be matched. There was a change in this length in mid MGA production, recommended for retrofit, but I do not know of such a change/variance for TD/TF. There are two different piston lengths for the later plastic cap dampers also, and not a few have been modified. It is entirely possible that any of these variants or home brew remedies have been mixed up here.

The pictured damper on the left is clearly wrong, as there is not enough clearance between the piston and the clip; it is possible that the valve washer is upside down, which would decrease the damping rate and possibly make it erratic, or the piston is too long. The center one may be missing the valve washer, and appears to be a short piston one. The RH pic is the latest type damper and appears to be a long piston version.

Note that TD dampers had knurled brass caps, then they change to hex brass, then to plastic grip knobs. Any of these could be correct in function, so long as they have the right piston and vent/no vent holes. Currently supplied ones from Moss etc. are plastic caps, but you can press those off and press on the knurled brass ones if you have them.

You could take better pics (from the side) and measure piston lengths to help us sort this, but I suggest you contact/send them to Joe Curto to sort this out, as it will drive you nuts, and it looks like you need at least one new one, which Joe can supply. But be aware, the correct ones will still be stiffer than you seem to have decided is correct!

FR Millmore

OK, you put up the 2nd pic while I was writing. The two on the right will likely function as a pair. At some point you should still contact Joe to get it right.

FR Millmore

I just pulled out a spare damper, and FRM is right that the left one is wrong- likely the washer is upside-down. Not at all like the electronically thickened "oil" in the GM/Ferrari shocks! George
George Butz

You did hit on something. The one on the right, with the hex cap is longer than the other two. So I switched the damper from the right rod (longer) to the left. This way I at least have two rods the same length with similar dampers. But I compared them again and I noticed that there seems to be more space from the lock ring to the damper on the one I didn't touch than on the one I just switched. Looks like the washer on the one I switched is just a bit thicker than the other (Middle unit in pic). Looks like a call to Mr. Curto is in order as you suggest.

I guess I won't know the final result of all this until I actually start driving the car and get a chance to start tuning it for real.

Thanks again,
MJ Rotsky

Getting someplace!
Note that there are two "piston" lengths in this discussion:

The brass damper piston, about 3/8" long, which comes in two common lengths. This together with the other damper parts determines the initial lift of the air piston, which will be equal to the axial clearance in the damper assembly.

The overall damper length of about 3", determined by the thin steel rod. In general, all SU 1.5" or smaller use the same length damper; if you have a longer one it is from an H*6 or H*8. So far as damper function, it is only necessary that the damper be in the oil at all times, just as with the oil level. But, too long an assembly might restrict the upward travel of the air piston on the smaller carbs.

FR Millmore

This thread was discussed between 12/12/2011 and 14/12/2011

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.