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MG TD TF 1500 - Dash Pot Oil - yes or no, or ???

Still having problems with the TF. Coughs and farts occasionally, and doesn't like sudden throttle. I can't easily lift either piston. Should I remove the (standard SU) oil? Could be the coil too ... but one thing at a time!

What do you run in your dashpot? Don't forget that we run our MGs into the cold weather of autumn, sometime as low as +5°C.

Gord Clark
Rockburn, Qué.
Gord Clark

I run SU oil, about 20W, but in your cold climate I'd run 10W or even 5W. ATV might be thin enough, but I've never used it. PJ
PJ Jennings

I would suggest trying heavier oil first. It won't do any harm if "damping" is not the problem.

It is not only the oil which can differ. The damper pistons themselves can vary. For a while after I supercharged my race TC in the mid 70s, I had to "feed" the throttle on gently as I accelerated out of corners, because it didn't like sudden throttle. Then one day I bought a new piston. I experimented with a few other s/h pistons too. As I remember it, the new piston was not the strongest! With the strongest piston in my spares box (i.e. most resistant to piston lift) the problem was solved because the car responded instantly to full throttle.

I guess it is possible that you need the opposite, but the symptoms you describe sound like the experience I had.
Bob Schapel


R L Schapel

I have an air/fuel gauge installed and notice wide fluctuations during acceleration. I've experimented with Marvel Mystery Oil (20W), 30W, and 90W gear oil, and have found the 90W to have a smoother gauge response. Pressing up on the piston, I notice that there is a lot of difference between them. At one point I put 30W in one and 90W in the other, and they balanced better.

I think the lift effect on the pistons is quite strong, and their momentum carries them past the optimum point. They may even bounce around a bit. This is conjecture on my part, but I think the heavier oil and increased damping is better. John Twists also recommends heavier oil.

My carbs work, but I think they could be much better, and getting the damping right is a big part of that. Not there yet.

Jim
J Barry

Gordon
What spark plugs are you running
William Revit

I'm betting it isn't the dashpot oil. I can run mine with or without dashpot oil and it accelerates fine. You can test this by simply unscrewing the damper and accelerating down the street. If the problem changes for better or for worse then you have some data to work with. I'm assuming you've done the usual tune-up items including synchronizing the carbs and setting float heights?
Steve Simmons

Running N5C. Just put in a clean set today, hoping that would solve things ... it didn't. I'm just about ready to change the coil, but want to eliminate any spark plug issues first.

Gor Clark
Rockburn, Qué.
Gord Clark

... sorry, meant to say - wanted to eliminate any carb/piston problems first.

gac
Gord Clark

Gord,

Yesterday I was preping my carburetors to install, and noticed more resistance when lifting the pistons than I thought desirable. I removed and cleaned the chamber and piston. On the piston I found a slight rub and buffed that spot a little, and wiped the chamber with very thin trumpet valve oil. I also found the needle was not perfectly perpendicular so I adjusted that as well. Altogether, there was a huge difference in the action. Perhaps one of these steps would help you.

Charlie
Charlie Adams

Gord - when you entirely remove the dashpot dampers, do the carb pistons then move more freely? If not, then you have binding of the pistons, and you need to disassemble, clean and check both dashpots and pistons.

I worked on a car recently where I found that the two pistons had been switched the last time somebody cleaned things, and when I got the car neither piston moved smoothly, both with rubbing needles. I re-swapped the pistons and replaced the needles, and the change was remarkable.

Just a thought.

Tom Lange
MGT Repair
t lange

If it is oil, I have always used MMO. Marvelous Mystery Oil.
TRM Maine

On the subject of oil in the pistons. How much does one fill? Also I had a similar problem and it was solved with a new condenser. All seemed OK at static but obviously something was happening under driving conditions.
JK Mazgaj

JK: There are two tubes - the inner tube in the piston into which the dampener fits, and the dashpot tube in which the piston tube slides. The proper fill level is the top of the inner piston tube.

Many people mistakenly fill the entire chamber to the very visible top, to the dampener screw threads - the excess oil will drain down and remain on the top of the piston. Fill to the top of the inner piston tube, which you can see by looking in from the top.

Tom Lange
MGT Repair
t lange

Every day learning something, I was never aware of the presence of an inner tube, will have to look for that next time I am checking the dashpot oil.

Thanks again to Tom, I am learning from these forums.

Peter
P G Gilvarry

It's a bit complicated to explain. The piston has a tube, which slides in the dashpot tube we all see sticking up on the outside. The inner piston tube is what gets filled with oil of some weight - opinions differ.

Put another way, if you remove the dashpot to clean it, the tube you see remaining sticking up is the piston tube, which should have oil in it. I give up trying to explain it any more clearly!

Tom Lange
MGT Repair
t lange

Got it, thought there was a narrower part down lower, so I have been doing it right.

Thanks.

Peter
P G Gilvarry

First place I'd look is your ignition timing. I don't know how long it's been since the engine was rebuilt or the distributor as well, but I suspect you might find the issue there. As for damper oil... heavier oil will make the mix a little richer on WOT. Lighter oils less so. I run 90W in the carbs of both of my MG's that are supercharged. Prior to that I ran 20W motor oil. Similar effect can be made by changing the piston springs.
L E D LaVerne

A minor correction if I may... the correct fill level is 3/8" from the top of the damper tube. If you fill to the top, it will spill over as soon as you insert the damper, and further spill over from engine vibration until it's down about 3/8 anyway! Most manuals seem to say 3/8" but I've also seen reference to just under that (or was it over?).
Steve Simmons

Lots of great suggestions. Unfortunately, I'm not an ignition guy, but some fresh carb ideas. Sandy Sanders called me this PM, and made the same suggestion as Tom Lange. So tomorrow, I will address the rising piston issue. Sandy also suggested using ATF oil in the dash pots.

Right now, I can barely lift the pistons with my little finger.

Gord Clark
Rockburn, Qué.
Gord Clark

That is the way they should be - it requires persistent strong pressure to raise them. Engine oil works fine.
Dave H
Dave Hill

This thread was discussed between 11/08/2019 and 17/08/2019

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