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MG MGB Technical - Weber Carb on 74 MGB

Recently my car has been stalling when I come to a stop after driving for a while. It does not do it every time and does it without warning such as a stutter or roughness. I cannot restart it immediately but by waiting about 5 minutes it will start (with a strong smell of gasolne). I think it is flooding out and that is why it will not restart. The carburetor is a 32/36 DVAG. I rebuilt it but the stalling still exists. Any thoughts on what is causing the stalling and what I can check?
R Volkwein

"I rebuilt it but the stalling still exists" Are you competent to rebuild it? Have you rebuilt other, similar, carbs? This may sound like a somewhat insulting series of questions, but, it is central to any assistance we may provide.

Thus, let us assume, for a moment, that you are a novice at this and begin at that point.

Was the power valve replaced with a new one? Did you verify the functioning of the power valve. (The power valve is held in the off position by suction from the intake manifold and is spring loaded to the on position, providing massive amounts of extra fuel. Thus, when the rubber diaphragm ruptures, through age and wear, the effect is to flood the carb with excessive amounts of fuel causing it to run far richer than intended.)

How is the choke set? A Weber DGAV has a ??? type choke? If it is anything other than a manual choke, has the choke function been tested and verified? What is the position of the choke when, after the engine stalls, you quickly remove the air cleaner and inspect the choke's position?

Both the primary (smaller) and secondary (larger) venturis have a piece known as the "auxiliary venturi" which bridges the opening at the top. This auxiliary venturi has a brass tube which runs from side to side and allows fuel to flow into the center of the carb venturi (barrel). These brass tubes have been known to shift backwards, exposing the open end and allowing excessive amounts of fuel to flow into the engine resulting in an excessively rich mixture. This can be seen as a dribbling of fuel into the primary venturi as the engine is running at idle. (Normally, you would only see the finest of mists being drawn into the intake manifold. Actual streams of fuel, other than from the accelerator pump circuit, indicate some form of problem.)

Is the fuel float level set correctly? If not, it can allow an excessively rich mixture and, when coming to a stop, spill over and flooding the carb's venturi with a sufficient extra amount of fuel so that the engine is stalled out and cannot be easily restarted until some of the excess fuel evaporates.

All of these enumerated areas can cause, or contribute, to the problem you describe. Often, it is a combination of several areas. Thus, the entire system must be inspected and each area checked to see what, if any, impact it might be having on the problem you are experiencing.

Les
Les Bengtson

R before you pulled your carb off what tests did you do? Before I go into the great detail of step by step tests I'll pass along a little quote from a long tims ago "90% of all carb problems can be traced directly to the ignition system."Think about it.Keep us in the loop. RIC
RIC LLOYD

This thread was discussed between 10/09/2010 and 15/09/2010

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