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MG MGB Technical - fuel problem down hill?

Took a long trip this weekend car started missing and seemed to be worse after or when going downhill. Then on a long steep grade engine stoped. After coasting the next 5 miles I found a place to stop. Tried to start it but no luck, then after disconecting the fuel line at the carb and reconnecting it the engine fired and I made it home (no more hills) but it is still missing. The after market fuel pump is working.
Could the pump be sucking air instead of fuel from somewhere that gets worse downhill.
Could the float flood the engine more going down hill and cause missing. ANY IDEAS???
Bob R.

Bob. Knowing the year, the carb(s), type of fuel pump and whether there are any fuel filters in the line would help. This assumes that it is actually a fuel related problem. I loose wire could also have the same effect if it were part of the low tension ignition circuit.

The first thing to do would be to run a pressure and volume check on the fuel line going to the carb(s). You need an assistant, a vacuum/fuel pressure gauge and a small bottle. Connect the gauge to the fuel line and have the assistant turn the ignition key to the run position to engergise the fuel pump. You should see at least 1 psi pressure. If not, bad pump or block in the line. Ideally you should see about 1.75 psi on a good pump, but Colin Campbell, in his book "The Sports Car Engine", states that 1 psi is the minimum acceptable pressure for SU carbs. If you have good pressure, you can have low volume. Insert the end of the fuel line into the bottle and see how long it takes to fill it. Campbell says the pump should take no more than 22 seconds to pump 1/4 pint of fuel. The Facet pump on my daughter's car pumped a full liter in 56 seconds. Have not run this test yet with an SU pump. But, if you do not pump at least a quarter pint in 22 seconds, look into why the delivery volume is low. If your pump is a Facet, it may have a fuel filter built into it. If this becomes plugged, it will show proper pressure, but low volume. If you have any fuel filters in the line, and you should, replace them. I am finding a lot of crud in the local gas and have to clean the filter on a regular basis. If the fuel pressure and volume are good, the car should run properly on level ground. A bad pick up tube, inside the tank, could fail to pick up fuel when going up hill or down hill. This must be removed to inspect it.

If fuel pressure and volume are good, it could be a carb problem. You have to remove the cover from the float bowl and inspect to see what is happening. You can also have a clogged jet on any type of carb and need to inspect that.

If the fuel system checks good, check out the ignition system to see if there are any loose wires and how strong the spark is at the coil lead and the spark plug leads. Post what you find and we can provide more help. Les
Les Bengtson

Right! I guess it would help---It's a 70 B WITH HS 4 SU's the inline clear glass filter is clean and there are no leaks, the pump is of the type sold by Moss as a solid state pump for $89.00, however it was installed by PO. and has no label. The pump has never stoped ticking while running and now I have it off the car and it seems to pump fine.
I guess it could be somthing other than fuel but with the way things get worse going down hill it seems to be a fuel problem (have checked all obvious connections).
Should the pump fill up the fuel filter or should there always be air there when looking at the clear glass filter?
Bob R.

Bob. It seems that my cars run with the clear filter about half full. If you are running a clear, glass bodied filter, make sure you have the gray, plastic piece on it that makes sure it cannot come apart or be broken easily. The fuel pump sounds like the Facet or Fawcet pump and it does have a fuel filter attached. My daughter had the same problem down in Tucson a couple of weeks ago. Good pump pressure and low pump volume. Bad filter, which I did not know the pump had. Ernie's Brit Cars found the problem for her. Once again, you really need to do a volume and pressure test to see what is happening. Her pump is showing 1.75 psi and will fill a liter bottle in about 56 seconds. You might see what yours is doing. If it is not close to that, check the filter, the line from the tank to the pump, the line from the pump foward and the pick up tube in the tank. Les
Les Bengtson

Does it worsen as the fuel level drops? Can you park on a hill and check fuel flow then? If the pickup pipe inside the tank is rotting it could be sucking in air at certain orientations and fuel levels. If the tach is steady during the missfire then you can eliminate ignition LT problems, could still be something in the HT though. Cap, rotor and all leads secure?
Paul Hunt

Thanks for the ideas guys. The carbs are off the car and when I get them cleaned up and back on. I will check eveything you have suggested. I have drained the tank(in case of bad fuel). I now remember that the 1st indication of a problem was a slower idle to the point of almost dieing right after filling the tank.
Bob R.

This thread was discussed between 19/11/2002 and 20/11/2002

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