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MG MGB Technical - engine cutout

I have a 73 mgb gt. Lately, the car runs well at start up. After about 10 minutes of driving the engine starts to sputter. This is most notable when starting from a stop sign. It seems to get worse as the drive continues, sputter in 3rd or 4th gear. I gave it the gas on the last stop and it had no power at all. After dropping into neutral, the engine revved and gave me some acceleration when back in gear. After arriving home, the car wouldn't start for a few minutes.

I have an electronic ignition and put fresh gas in it. Any thoughts on what the problem may be?
Tom Thompson

Tom - if the symptoms go away after a period of time with the engine shut down, try removing the fuel tank filler cap. If that relieves the symptoms, then you have a clogged tank vent line from the charcoal canister to the tank. If the symptoms are continuous, see the article, Fuel Delivery Troubleshooting in the SU fuel pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Thanks Dave. Removing the gas cap did not change the problem. I'll try some more of the things you mention on your web page.
Tom Thompson

If you have the original HIF4 carburetors, you may have a problem with them running an improper fuel / air mixture as a result of the temperature compensating mechanism of the jets failing to react. Rather than employing a manual choke mechanism, its thermosensitive fuel / air mixture control makes for easier cold weather starting. In order to accomplish this, a bi-metal blade in the form of a L-shaped bracket is used adjust the height of the fuel jet as needed according to the operating temperature of the engine. The L-shaped bracket is held in place with a shoulder screw and a spring so as to be an articulated assembly. The mixture adjusting screw bears against the longer leg of the L-shaped bracket, thus thrusting against it in order to change the pivoting angle of the bracket. The straight plate on the short leg of the L-bracket serves to relocate the fuel jet upward or downward as the L-bracket pivots. Additionally, the flat plate is also a bi-metallic strip which is designed to distort according to changes in temperature. This allows for compensating alteration of the fuel / air ratio in relation to temperature changes. This is an advantage because when the temperature in the engine bay increases, an uncompensated fuel mixture tends to become lean. In the case of the SU HIF design, with increasing temperature, this bi-metallic strip will distort downward in order to lower the fuel jet slightly, consequently increasing fuel flow, and thus maintaining the proper fuel / air mixture over a wide range of operating temperatures. A positive secondary effect of this feature is that this temperature compensation also makes the HIF carburetor less prone to suffer from vapor lock, which is sometimes a problem with the predecessor H and HS type carburetors during hot weather and very slow driving. This precise fuel-metering control means that once correct fueling is established by appropriate fuel-metering needle selection, the fuel / air mixture is maintained over a very wide range of operating temperatures. The thermally compensated fuel jet also gives a consistent idle speed over a range of operating temperatures, whereas SU HIF Series carburetors can tend to stall in a long idle in summer where everything heats up, and can need a tweak of richening in winter and leaning (weakening) in summer. Drivability with SU HIF Series carburetors is consequently enhanced and emissions are kept within tighter limits during the cold start and warm-up period.
Stephen Strange

This thread was discussed between 25/08/2012 and 26/08/2012

MG MGB Technical index

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