MG Car Repair: 1980 MGB idle, gasoline engines, secondary ignition

Question
Idles good for 1 minute or so then wants to stall. Happens most of the time but not all. Engine runs well except for this.
Thanks
B.B.

Answer
Hi Bosko,
Your symptoms narrow the problem down to only one thousand possible things. So testing is the ONLY way to find such a problem.

All gasoline engines only require three items to run, Compression, Fire and Fuel with conditions on each. Compression is not likely to come and go so you can look at Fire and Fuel as the most likely fault. The simple test for a loss of fuel is easy except at idle, because at idle the engine dies almost instantly. The test for Fire is also easy and can catch a fault at idle. So you should run the test for Fire first.

Since you don't have any problem except at idle all tests must be run at idle.

Connect a timing light up to the COIL wire, not a plug wire like setting timing. Tape the trigger down so the light flashes all the time. Lay the timing light on the windshield under a wiper arm so you can see the flash while sitting in the car. Start the engine and let it idle and watch the flash carefully and note when the engine dies if the flash stops first or continues to flash as the engine spins to a stop. You will need to watch carefully because the engine does not spin many times after it dies at idle. Make sure your idle is from 800 to 1000 RPM. You may have to do it several times to be sure of what you see. Another thing to look at when doing this test is to watch the tach. Note if the tach coasts down with the dying engine or does the tach drop first.
The timing light flash dying first then the engine coasting to a stop is either the primary or secondary ignition failure. The tach dropping first is a Primary ignition failure.

If the engine dies and the flash of the light continues down with the coasting engine then you most likely have a fuel problem. Since you do not complain of any running problem You should look at fuel pressure to the carburetor and at float level. Also check to see that the air cleaner and air cleaner box is secured to the carburetor with no air leaks. For a test remove the vacuum hose from the anti dieseling valve at the charcoal canister and plug the hose.
Let me know,
Howard