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MG MGB Technical - engine missing

Took the car on a 500 mile trip this weekend. It ran fine for the first leg of the trip and we spent the night. In the morning it started fine and for the first fifty miles it was fine but then I noticed when I was not pushing on the throttle it would seem to be acting like the key was being turned on and off. When under full throttle it would pull fine but when just lightly touching the throttle it would miss quite a lot. The next morning it would not start. The car is a 1980 and it has a weber carb with some kind of header for the exhaust and an electronic distributor. Does anybody have any thoughts where to start?
GK George

Sounds like an ignition problem, so start by checking the ignition electrics. Look for loose/broken wires or loose connections. Did the tacho go to zero when these "missing" episodes occurred?

John
J Tait

Reading the title, I thought you'd had a nasty surprise when you opened the bonnet (hood) one morning!

D Balkwill

I see how you could get that from the title but , no it is stil in there, just not running like it should. I did not notice if the tach was working or not bt I will check.
GK George

"had a nasty surprise"

Reminds me of the Quantas pilot's report on a piston-engined aircraft and the ground engineers response - "Problem: No.3 engine missing. Action taken: Engine found on Port wing after short search".

John's point about the tach should always be the first step when any missfire or cut-out occurs, even before coming to a halt, knocking it out of gear or dipping the clutch. While the momentum of the car is still spinning the engine, and the ignition is switched on, the tach should still be registering. If it isn't the problem is in the ignition LT circuit i.e. feed to coil, coil, points, distributor ground wire, or connections between them. If the ignition warning light is also lit, then the ignition feed from the ignition switch as been lost.

If the tach is still registering then the problem is HT or fuel, although it could be the condenser. If stopped with the ignition on you can still hear the fuel pump click occasionally then the pump is OK. With the ignition off carefully remove a fuel feed pipe from a carb and you should get a spurt of fuel from it, so use cloths to stop that splashing onto a hot exhaust. If the pump doesn't click, or the fuel doesn't spurt, then that is most likely to problem. If they do then it is likely HT.

Clip a timing light onto the coil lead and each plug lead in turn and watch for regular and consistent flashing as you crank. OK on the coil lead but not the plug leads indicates a cap or rotor problem. Not on either indicates a coil, coil lead or possibly condenser problem.
Paul Hunt 2010

This thread was discussed between 24/05/2010 and 25/05/2010

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