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MG MGB Technical - engine stalling

my b has started to do an odd thing. if you start it up and try to let it idle the rpms will decrease from 1000(it's usual idle) and the engine will turn off. If you push down the gas just a little bit it will stay on. Nothing elts out of the ordinary. It started happening when I was waiting at a light to turn into the gas station(and was quite low on gas) so my first thought was that I might have pulled something out of the bottom of the tank into the gas line. I let the car sit over night and today it is still doing the same thing. The car has a weber downdraft. Any ideas?
Jason Smith

Hi.

I am not familiar with your carb, but if it has a tickover jet I suspect it is blocked.

An air leak into the manifold is another possibility.

Don
Don

A manifold air leak usually causes the idle to rise, unless it is a gross leak like a Welch plug blown out of the inlet manifold as a result of a backfire. Pretty-well *any* other departure from a previous tuning state will cause an idle reduction, so the usual thing is to go through the valve clearances, plug gaps, points gap/dwell, timing, and only then carbs. But a Weber with a blocked idle jet is a distinct possibility. However I have never subscribed to the 'running out causes blocked jets' theory. The pick-up remains at the same height above the bottom of the tank from full to empty. And whilst the ratio of crap to fuel will be higher when the pick-up can no longer suck fuel up than when the tank is full, it is a linear and gradual progression from one to the other. I regularly run my cars down close to E, have run out a number of times (once accidentally due to a new tank sender giving an incorrect reading and the other times to check and adjust the calibration!) and never had any problems with dirt affecting the carbs. However I do only have SU's, and as an aquaintance said many years ago "You can drive a coach and horses through them and not get a blockage".
Paul Hunt

Thanks for the responces. I was tinkering with the car some more and noticed a few things. When I first started the car up it would idle fine. I had the air filter off and I worked the accel cable some to see that was happening. Everything was as usual for the first couple min. Then it changed. When I let the accel cable back into resting position I noticed a whisling noise coming from the card air intake. I also noticed that the two flaps(Im not sure what they are called) were remaining at a steep angle. The engine would then run rough for a sec and then turn off. Should these flaps remain at this steep angle?
I also had a question about the fuel filter. Should their be any air in the fuel filter? Should I disconnect it and get the air out?

thanks for your time
jason
Jason Smith

Jason,
The two flaps are called choke plates, they will be in an upright position when the choke is not engaged. When the choke is engaged the plates will close and cause less air to enter the carb creating a rich mixture. Check out this website for info on how to tune and identify your carb, http://www.redlineweber.com/
The Weber has only 2 adjustments, an idle speed screw and a mixture screw. They are fairly easy to adjust and despite what some will say they give as much power as an SU. The web site I linked will give directions on how to get the proper mixture and then idle.
The fuel filter will always look like it's half full. That's the norm. However if the filter is old you may think of replacing it.
My Weber will also make a hiss (actually sounds like the suction tube a dentist puts in your mouth) when at idle but I know it is not a air leak.
As for starting and not staying at idle. How is your fuel pump, when you turn the car on and before starting does the pump begin its familiar clicking and then stop? If it keeps clicking, or not clicking it could be a problem. I know the pump motor has electrical contacts which can become gunked up and not work properly. Cleaning them helps.
It sounds like your car is becoming fuel starved, I would guess fuel pump but would check all the easier things first, good luck
John

john-
The fuel pump will click 100% of the time the key is in the on position. The fuel pump is one of those little square ones. Also the fuel filter never has more then 1/10 of its volume filled. does that sound like a fuel pump problem? (maybe in addition to some other problems).

thanks alot
jason
Jason Smith

Jason; If it runs ok at speed the fuel pump is not the problem. It takes considerably more fuel to run a car at speed than is required for idle. My filter also looks empty at times but the car runs ok. I suspect your low speed carburetor circuit needs cleaning. To clean it properly you need to disassemble the carburetor. Your engine runs ok when you first start it because the coke is on and the fast idle setting and richer fuel mixture keeps the engine running.

Clifton
Clifton Gordon

I have always found a can of carb cleaner spray works wonders on fixed jet carbs .
S Best

This thread was discussed between 20/04/2005 and 26/04/2005

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