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MG MGB Technical - Carb problem

I have a weber 32/36 dgev. Trying to adjust idle mixture but when adjusting the screw either way, no difference - engine warm, choke disengaged. The reading on exhuast gas CO meter over 9%! Stumped as to what to do next. Checked my Weber manual, but no help there.
Any suggestions?
/Moss
Moss

Hello Moss. I assume that when you say the engine is "warm" you mean it is above zero considering the weather you Europeans have been experiencing over the last week or so......

Tony
Tony Oliver

Moss. Check the power valve. This is located inside the top cover of the carb and is held in the off position by vacuum. Problem is when the diaphragm ruptures it supplies extra fuel--you can actually see it dribbling into the primary venturi. Depending on how old the carb is, and when it was last rebuilt, the power valve can be a significant problem, over riding any adjustment of the idle mixture screw. Remove the top, dry it, apply a suction to the vent to the power valve and see if it moves upwards. If it remains downwards, the diaphragm is bad and the valve needs to be replaced. Several years ago, this part was not contained in the carb rebuild kits. But, the current (EMPI, part number 00-2361-0, Carb Repair Kit) rebuild kit, which comes in a white box rather than laminated to a piece of cardboard, does contain a new power valve.

Les
Les Bengtson

Hi Les, I recently serviced the cab with a kit made by webcon - no powervalve was included. The diaphram you name, where is this in the carb?
I'll check the power vavle...
Thanks for response...and Tony engine warm as in normal operating temperature.
/moss
Moss

Moss. The power valve is the item with a spring wound around it that is attached to the top cover of the carb with the long axis of the valve pointing downwards. Item number 93 on my illustration of the carb disassembled. The upper end is a "rubber" diaphragm which is used to hold the valve closed when there is significant manifold vacuum. When the manifold vacuum drops (e.g. when you open up the throttle to pass someone) the valve opens and supplies extra fuel. It works in combination with the mechanical accelerator pump to enrichen the air/fuel mixture. Pop the cover off your carb and you will see the spring loaded valve hanging down from the cover. Check it by applying vacuum to the vacuum port in the cover or by physical inspection. Mine was quite obvious in its failure, both in the condition of the diaphragm and in the dripping of fuel into the venturi rather than the invisible mist of air/fuel you should find.

Les
Les Bengtson

Hi Les, thanks for the info/advice. I'm checking it today. I have changed the mechanical accelerator pump when I did the service a few weeks ago. There was no power valve in the kit. So I'll have to buy one. To be honest the car has never run as I thought it could. And now with your advice it sounds like the root of the problem is this power vavle.
More later...
/Moss
Moss

This thread was discussed between 17/02/2012 and 19/02/2012

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