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MG Car Repair/zenith carb

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Question
Hello I have a question about a triumph spitfire carb issue and seeing you are knowledgeable on mg I figure the mechanics are almost identical. I have a stock single zenith stromberg carb and recently the fuel pump was not working so I cleaned it and it is working but fuel flows out of the carb where the air filter is attached when it is off and was wondering it that is normal, I don't have an air filter yet so I was not able to test  it with one on to see if it stops the flow but was curious if that is normal. Thank you

Answer
Hi Daoud,

The spitfire has a mechanical fuel pump so when the engine is shut off the pump stops. However, the pressure that the pump produces to keep the carburetor float bowl full is supplied by the return spring in the pump not the stroke of the cam that drives the pump.
That means that there is still from 1.5 PSI to 3 PSI fuel pressure applied to the carburetor after the engine stops unless the cam stopped while pressuring the pump spring.

That is just an explanation of how a pump can still apply fuel pressure after it stops pumping.

Another, and more common cause of fuel exiting the jet and carburetor is heat expansion. When the engine is running air is circulating under the hood and fresh fuel is entering the carburetor and fuel is vaporizing and cooling the intake manifold.

When the engine stops, all of the cooling stops and the extreme heat of the exhaust and the heat of the engine itself goes into the intake manifold and carburetor and over heats the fuel and since fuel boils at a very low temp it causes the fuel to turn from a liquid to a gas and since the float chamber is closed off and the vent for the float chamber is a long hose to the charcoal canister, the expanding gases can not escape and cause pressure in the float chamber thus forcing fuel up and out of the jet into the throttle area and and in your case it dribbles out the front where you have the air filter off.

In your case, run these tests to see if that is the cause. First disconnect the vet hose from the carburetor to the charcoal canister and get the engine to operating temp. Now with the hood up set any kind of electric fan next to the car and blowing directly onto the carburetor. Then shut the engine off and note if it still dribbles fuel out of the carburetor.

If it does not, you should consider fabricating a sheet aluminum plate to go between the carburetor and intake manifold that will shield the radiant heat of the exhaust manifold and engine. At the same time try to locate a fiber block to fit between the carburetor and intake manifold to slow down heat transfer of engine heat to the carb. Also get some tube type heat shield and fit it over the fuel line from the carb to the fuel pump.

Also Triumph changed the fuel pump on later model spitfires from a short operating arm to a long operating arm. The reason was that they added a thick fiber block between the fuel pump and the engine block to slow down heat transfer to the pump and fuel.

Many other brands of cars had the same problem and they referred to it as "Heat Soak". Some even put a small 12v fan close to the carb and activated it when the engine was shut off and it would run until a small thermostat turned it off. Many cars put a thermostat on the radiator electric fans to make them keep running after the engine was shut down and that would run until a thermostat turned it off. (many people returned to their car to find a dead battery due to a failure of the thermostat to turn it off). Some cars put louvers in their hood to exhaust the heat of the engine and many had to go to great lengths to correct this problem.

Howard  

MG Car Repair

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Howard M. Fitzcharles III

Expertise

MG from 1956 (USA versions only) up and Engine theory.

Experience

Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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