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MG MGB Technical - Carb trouble!!

I took the 73 MGB out for a ride yesterday and it was running great. After a few miles the car was racing as if the accelerator was stuck. I couldn't get it to stop so I pulled over and cut the ignition. Upon inspecting the carb I found that the one idle screw had twisted itself down and had the throttle wide open. I took the screw out but the carb was flooded and it would not start. When I took off the air cleaner and lifted the piston, gas was pouring into the carb. I checked it today with the ignition on and gas is still pouring in. I know nothing about these carbs, where do I start to try and troubleshoot this? I give up, if I get it running its going up for sale! Thanks for any advice.
Steve Sekaz

I experienced the same problems at various times. As far as the high engine speed goes, the problem was caused by a lack of a spring or nut under the head of the idle speed screw. I did not believe it myself that engine vibration alone could move the screw down to that extent when the nut or spring is missing.

The fuel flooding into the carburetor is probably caused by bad float needles or bad floats (maybe both). Check these parts, you may have to replace them, if your car has the original carbs, SU HIFs, you will have to remove them to replace the parts.
Willie

Steve. There is a fuel bowl attached to the carb. This has a needle valve which is activated by the pressure of a float. When the fuel level drops, the float drops and allows the needle valve to open and fuel to flow into the fuel bowl. If this valve fails to seat, or if the float is bad, fuel continues to flow as you are describing. This can be caused by bad parts or crud in the fuel system and is the most likely cause of your problem.

A less common cause is the use of an incorrect fuel pump having too high a pressure which does not allow the needle valve to seat and shut off the flow of gas. If you do not have an SU fuel pump, this would also be an area to check for. Les
Les Bengtson

All of which reminds me that I did not update for a solution to a somewhat similar problem that was haunting me. I solved my problem after much elimination, by replacing the needles and seats.
Ken R
Ken Rich

Like Les says, check the needle and seat in the fuel bowls. Last December I got a brand new set of carbs and they started pushing gas back through the lines even into the evaporative cannister. I finally found a tiny piece of clear plastic packaging material stuck to the needle of the rear carb. Problem solved!
Ken T
Kenneth Thompson

This thread was discussed between 11/06/2003 and 13/06/2003

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