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MG MGB Technical - Dash Pot

Ok, I am sure this is not new. What are the symptoms of a low oil in dash pot? Just what does the dash pot do? My MGB 1975 with super charger started to just stop running every so often. Like It became too rich and motor stalled. Sit for 2 seconds, restart, runs rough then leans out and takes off. I added oil to pot but have not run enough to know if problem was this. How much does this pot hold. I could not get it to really fill. After adding about 4 ounces I stopped and took it for a short run. Initionally it smoked a bit out the exhaust but cleared shortly...
Ronald Milostan

The dashpot is a damper to prevent the carb piston from "fluttering" as it rises and falls. The spring tries to push it down, the engine's inlet depression makes it rise. The dashpot controls it. Without any oil, I suppose the piston might rise too fast initially on opening the throttle, which would over-richen the motor.

The oil in the dashpot really doesn't have anywhere to go as it is in a sealed tube only open at the top. Most folk overfill them, but they quickly discard the excess into the inlet and find their own level. What sort of oil are you using. The manufacturer recommended the engine oil (20 - 50), but since I have no experience of a supercharged car, I can't say what you should use.

Mike Howlett

See the thread "dashpot oil servicing" on the Spridget tech board for Gryf's excellent description of how the dashpot works. Also tells you how to fill it.
Lack of oil makes it go lean on acceleration, leading to stumbling and coughing until it gets going.
FR Millmore

Have you checked for gas tank venting?
Dan Robinson

Low oil rather than no oil is not always evident when driving gently as it still functions to damp slow piston movement. First time I went to Silverstone, drove 60 miles there with no problems. Running the parade laps on the circuit as a passenger to my wife still no problem. When it was my turn, flooring the pedal out of the corners made the car stutter and practically stall. Very embarrassing. Dribbling a little extra oil into the dashpots from the dipstick restored the pickup on hard acceleration.
My SU's normally only require 2-4 pumps from a std oil can for topping up.
V Todman

Ok, today will try running longer. Also will look at gas tank venting. Still looking to find how much oil the dash pot can actually hold. I put in about 4 or 5 ounces and some burned out the exhaust. I will now watch and add a small amount on a regular basis since I drive this car regularly during the summer.
Ronald Milostan

The purpose of the oil reservoir and damper piston is actually to *enrich* the mixture on acceleration, because when the throttle is suddenly opened the air accelerates faster than the fuel droplets, which causes the mixture to weaken. The damping system counteracts that. The symptom of insufficient oil is stumbling and hestitation even with small openings of the throttle. You can check if you have enough by unscrewing the damper piston cap, lifting it up, and pressing it back down again. If you feel the resistance of the oil before the cap reaches the piston cover then you have enough. You shouldn't have to top up in many thousands of miles. Putting in too much isn't a problem, it will just burn off any excess. Normally just to the top of the inner cylinder is enough.

A rich mixture could be caused by a leaking float valve, check the carb vents for overflowing with the ignition on but the engine stopped. If you have the OE SU fuel pump it shouldn't click more than once every 30 secs like this.

The tank should vent through the charcoal canister but thi can become blocked which will cause fuel starvation. A quick check is to remove the fuel filler cap when the problem happens and if you get a gasp then the tank vent is blocked. Note that if the original tank vent system via the canister has been removed and sealed you must fit a vented fuel filler cap. Often these have a spring loaded valve which can still give a small gasp when removed even though it is not causing starvation. Another check of fuel delivery is to remove the petrol pipe from a carb and direct it into a container. With the ignition on it should deliver at least 1 Imperial pint per miniute with negligible bubbling.
Paul Hunt 2

This thread was discussed between 05/09/2006 and 06/09/2006

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