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MG TD TF 1500 - Dashpot Oil

Can someone please explain "dashpot oil" to me? Apologies for being too simplistic about the question.
My TF was idling very smoothly, until all of a sudden, I came to a stop and she would barely idle, then stall. She will start and run but will not idle. I have not touched the carbs since it was professionally tuned up. I seem to recall reading something about dashpot oil and it's role in smooth running carbs. I suspect my dashpot oil might be, low? How do I know? Would this effect the carbs ability to idle?

Thank You In Advance
MAG Graham

You have some other problem than dash pot oil. I would suspect an ignition problem, posibly a condencer that went south.

The SU carburetor is a variable venturi carburetor. By that I mean that as the engine speed incerases, increasing the amount of air flowing through the carburetor, it causes a greater vacuum or depression at the choke area in the throat over the jet and would cause a much larger amount of fuel to be drawn in as the engine speed increased, much more than is needed. To control the amount of fuel being drawn into the engine, the piston above the choke area and part of it, is drawn up by the vacuum developed above it in the vacuum chamber. This action opens the choke area, which in turn causes a smaller amount of vacuum to develop in the choke, thus pulling in less fuel. The piston also carries a metering needle that in moved out of the jet to provide a fairly accurately metered amount of fuel to be available to the engine. The whole arrangement made for a carburetor that was pretty efficient over a broad range of engine speeds. It's downfall was that it could not be cleaned up sufficiently to meet US air quality standards, but that is another story.

When one pushes down on the accelerator to get the car to move out smartly, there is a sudden, large demand for extra fuel into the engine, which a carburetor that is just ideling along can't respond to. Standard US carburetors (Holly, Carter and such) use an accelerator pump to squirt extra gas intot he choke area and richen the mixture a bit (actually a lot) to meet this demand. The SU carburetors use, insead a damper on the top of that piston that we talked about earlier. This damper acts just like a shock absorber and resists the rapid rise of the piston for a long enough period, theat the choke area remains the same size as at the lower speed, thus increasing the speed of the air flowing through it (basic fluid dynamics from Phsysics 101 if you remember that) and thus decreases the pressure or causes a greater depression (vacuum) in the choke are over the jet. The net result is that the mixture is enrichend sufficiently to keep the engine from stumbling in an effort to keep up with the greater demand placed on it. The dashpot oil is the hydraulic fluid used to cause the damper to act like a shock absorber. If the dashpot oil gets too low, it will cause the engine to stumble when trying to accelerate, but once the new speed ir reached, will not affect the running. Dashpot oil is also nothing more exotic than 20 weight oil, or auto transmission fluid or any of about a dozen favorites that people use in their carbs.

This is a very basic description of the SU carburetors and the role played by the dashpot oil. Hope it helps mor than confuses. Good luck - Dave
David DuBois

The big brass hex nut on top of the carbs is connected to the damper, a kind of shock absorber for the piston in the carb. You just unscrew that, pull the damper out with the brass nut, and nearly fill the little reservoir that the damper thing fits into. Put the damper back in, your'e done. You can get a bottle of SU carb oil that will last forever, and has a nice nipple to keep from spilling. Most folks agree that whatever oil you use in your engine is fine for the carbs.
Tom

I use engine oil. (20/50 multigrade)
J James

Thank You All for the great info.

David - If a condenser went south, is it as easy as just replacing the condenser? Will there be adjustments that need to be made?
MAG Graham

I've had T-Series cars for over 40 years and always used "Singer" sewing machine oil in the dashpots.
Its lighter than engine oil and reletively cheap.
Wayne Styles1

If lighter oil works better. I have some spare Mobil One 0/40 synthetic in the garage. I'll try it on the next top up.
J James

The condenser can just be swapped out with no concern about having to readjust. Cheers - Dave
David DuBois

Marvel mystery oil works great (the red stuff) and it smells good too. This oil also freed up my rack (mgtf) steering after it became so stiff it would hardly turn one cold morning. Apparently some uninformed service station monkey used chassis grease in the alemite fitting instead of gear oil. Steering is now like new.
SEAMUS
F. HEALY

Just don't bother to buy the dash pot oil that's sold by Moss. You can get lots of good stuff from them, but their price for this item is way over the top! (IMHO)

See http://www.omgtr.ca/technical/dash_pot_oil/dash_pot_oil.htm
Ted Jackson

This thread was discussed between 26/09/2005 and 17/10/2005

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