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MG TD losing power

Sorry Tommy, but you really were way off on this one. Dave from Steamboat Springs complained about his '51 MG TD losing power on steady driving. The carburettor (yes, that is the British spelling) dashpots are only for dynamic conditions as Ray explained. By retarding the rising of the carburettor piston, they momentarily enrich the mixture for acceleration. They do nothing for steady state driving.

The loss of power is more likely to be due to the improper operation of the carburettor piston. The top of the piston has a rubber diaphragm, which lifts the piston in response to manifold vacuum. The position of the piston controls the fuel metering through a long tapered needle on the bottom of the piston. If the piston does not move to the proper position, the fuel mixture is not correct, and can lose power. On an old side-draft carburettor, it is likely that the rubber diaphragms are worn out and cracked. Luckily, replacing them is very easy. Typically you just unscrew the top cover, pull out the piston, and replace the diaphragm. That is much more likely to be the problem than the dashpots.

I was wondering about that ‘put oil in them’ advice, it didn’t seem right. Dave needs to get a good manual for the care and upkeep of those carbs. From what I understand they’re easy to get right, if you know what you’re doing.

Is This Vehicle Being Operated On Unleaded Gasoline ? Has The Engine Been Modified Or Is A Lead Sustitute Being Used ? Lead Acts To Lubricate Valve Stems And Guides.

I vaguely remember some of the symptoms from the radio show while I was driving down the road. I believe the symptoms appear after the car is warmed up and running for a period of time.

Also, has the engine been checked for carbon deposit build-up ?


Dave, leave the carbs alone. Your problem is simply the fuel cap vent is clogged. As you drive at high speedmit creates a vacuum and stops fuel flow. When you stop the air seeps back in and you are good to go for a little while.If you were to remove the fuel cap after it stops you would hear the air rush in.

If not the fuel cap vent, this engine also has a primitive form of Positive Crankcase Ventilation, a rubber hose from the valve cover to the intake manifold. If this hose collapses the engine will lose power and erp oil all over the exhaust manifold at highway speeds.

Too bad “Dave? Dave? Dave’s not here!”

A '51 TD has all kinds of possible problems…guess we’ll never know.

WE DO KNOW! I have seen this problem too many times. DAVE if you come back the fuel cap vent is the problem. Once those carbs are set they are very reliable and should never give you any real problems. But if you get them all out of adjustment then you will have problems.
wayne, waynescyclegarage

Trouble is, Dave was never here…

Frustrating! We come to the rescue and the victim is missing:)
Call Click and Clack (not possible)
I also have a couple of Ideas why his engine kept running too. ( Click or Clack) Not as certain as the fuel cap though.

there does seem to be lots of problems with those I have read alot about the loss of power and I think you hit the nail on the head. local classified ads

Tommy and RickNY both have valid views. Skinners-Union (SU) carbs were never meant to work outside England, preferrably on flat sand beaches at ideal temperatures and humidity levels for speed time trials. Traveling across country, from high and dry, to low and humid, is beyond the average driver’s and young mechanics skill level. “SU” carbs can be set up for limited operating conditions, but are not computer controled plumbing fixtures, as are modern fuel injection systems, and must be coddled and stroked, just like “Click and Clack.”

Never owned an MG TD but I did drive an 1965 MG 1100 for about six months. The 1100 was an early econobox…transverse four cylinder, front wheel drive, seating for four. A Rabbit before there were Rabbits. This was in 1977 or maybe '78. During a rainstorm the car was running rough and getting no power. I pulled off the highway and into the parking lot of the Foodtown and turned the car off. Or I should say tried to turn the car off. The cute little Wilmot Breden key turned but the engine did not stop working. I knew enough about cars at the time having done my own oil changes, changed the points and condensor and adjusted my own timing on all my own cars, I lifted the hood and in the twilight I was treated to a spectacular display of St. Elmo’s fire running along all the electrical wiring under the bonnet. Summoning my courage, I grabbed the primary wire on the distributor and yanked. The engine continued to run. I finally killed the engine by using a screwdriver to pry off a spring clip on the distributor cap and lifting the cap. I had to walk back to campus as doing this had cracked the cap and the car would not start until I got a new one. All I can think of is that British electrical systems of a certain age no longer care much about on or off or what path the spark takes to ground. I guess I could have stalled it out but clutch plate was bad enough as it was.

This could be as simple as carb iceing; the cold air @ high altitudes causes ice to form in the throat where the air passes through at the venturi. The ice melts at an idle in about 20 seconds a few more seconds for the water to clear the engine,and your back to running smooth. If you hurry and remove the air-cleaner you can see the ice before it melts. Pipeing heat from exhaust manifold into air-cleaner will help.

Carburetor Ice Can Form In The Venturi, Even In Warm Weather. In Fact It’s More Likely In Warm Weather Because Warm Air Can Hold More Moisture Than Cold Air.

Air pulled through the venturi increases in speed, decreases in pressure, and decreases in temperature. The incoming air decreases in temperature by as much as 70F degrees.

Most carburetor icing will occcur above 50% (some say above 80%) humidity and from 20F to 90F degrees temperature.

Bruce, however you look at it, you could be onto something with the icing theory. I wonder if this vehicle has a warm air-to-intake system that has been removed / defeated or is no longer functioning ?


If I remember this call correctly, Dave said his car ran well around town but had problems when he went on the road. Since Steamboats Springs is at 6,700 feet, it stands to reason that his carburetor mixture is tuned for the thin air at that altitude. When he descends to the surface of the earth, he encounters our thick terrestrial atmosphere and his mixture is way too lean. I bet if he made an idle mixture adjustment at lower altitude, the car would run much better. Of course, he would have to change it back when he went back to his mountain home.

Let’s get him back on Stump the Chumps.

The SU carburetors did not have rubber diaphrams. They had aluminum pistons and those pistons would rise to open the venturi for air flow while metering the fuel appropriately. The secondary on a Rochester Q-jet is a very crude use of the concept.

And rust accumulation in the tank can cause the problem but if it sooner when the tank is full and later when near empty the vent is the most likely cause.

The vent is a tiny hole that clogs easily on these. I had this problem years ago with my MGB and with a Sprite. Being a motorcycle mechanic and working on mostly vintage bikes I see this problem a lot as these old bikes are similiar to the old MG’s. It creates a vacuum and you run out of fuel and as you stated with a full or nearly full tank it happens sooner, and running at high speed makes it happen quicker.
wayne, waynescyclegarage

I have this problem, the car occasionally beginning to lose power, or run heavily not making power, with my '54. The cause, in my case, has always been a float-needle sticking.

The old S.U. carburettors on T-series MGs have a separate float-bowl which has an overflow drain-tubeoff the cap-fastener, in the middle of the top of the float-bowl. The tube, copper, runs a short way down to drain overflow when a float-needle sticks.

To check, pull to the side of the road, open the driver’s door (suicide door), lean out so your head is on the ground and look forward for fuel dripping under the car.

If you see fuel dripping, shut the car off, open the bonnet, take a spanneror something from the toolbox there and give a sharp rap to the leaking float-bowl. Turn the ignition back on and listen for the fuel-pump to stop after two or three ticks. If the ticks continue fuel continues to flow. Rap a few more times to seat the needle, until the ticking stops. Start the car as flooded and carry on.

If it happens often, or irritatingly inconveniently, clean the fuel system and add an in-line fuel filter. You can recondition the float-needle by removing it, removing the float from it, fitting it CAREFULLY and LIGHTLY in an electric drill motor, then dressing the tip with fine emery as it spins. Do NOT sharpen the Angle of the tip, as it then may lodge and cut off fuel flow.

This sounds like a good candidate for “Stump the Chumps”.