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MG MGB GT V8 Factory Originals Technical - engine stops

my Mgb V8 (Rover SD1 with 2 SU carbs) has run perfect after I had it adjusted on a power bank but now after 1,5 y and probably 7.000 km the engine once in a while stops when open the trottle after cornering or driving away from the traffic lights. It takes a long start to make it run again, and the engine also has troble picking up at low revs and high gears.I looks like the engine is getting over-fuelled or the spark is to weak.
Needles look fine, renewed the spark plugs, readjusted the CO level but it is still not as reliable as it was. any body an idea??

A.E.J. van Dulmen

It could be. Hard to tell when you are so far.
It could be electrical, if you have points check the condenser, if you have electronic ignition check the module. When hot they have the tndency to short and cut of the engine. Also check your wire from the starter to the Ign (Ign Sw or distributor) coulod be grounding. If your floats are ok in the carbs and the needles look good (adjusted correctly) then it has to be electrical.

Hope this helps you.

Bill Guzman

Good advice from Bill - I would certainly change the coil too. I once had a faulty pressure regulator (on Carbs) that reduced petrol flow and created similar symptons so while ignition electrics are the most likely culpret - fuel system (blocked filter, faulty float or tired pump) could also be the problem. My guess 'tho is that the 'long time it takes to get it to run' again is the time the amp or coil need to cool down.

Good luck



Unless you have twin exhausts you can't adjust by exhaust emissions alone, even for mixture and certainly not for air flow. Have you balanced the carbs for air flow and mixture from first principles? Have you checked the dwell and timing? Centrifugal and vacuum advance operation? Checked the damper oil level?

Check how far the spark will jump with the coil lead pulled out of the coil before the engine stops. Should be at least 1/4" and could be more. The spark should be blue and make a sharp 'crack' when it jumps the gap. If it will barely jump a plug gap and is yellow and fizzes the condensor could well have failed.

Have you confirmed you have the correct coil for the loom? Chrome bumper 4-cylinder cars used a 12v coil, rubber bumper a 6v coil with loom ballast. Factory V8s always used the latter. Don't know what SD1s used. A 12v coil should measure about 3 ohms across the primary (could be 2.4 ohms for a Sport coil) and about 1.5 ohms for a 6v coil. If with the ignition on and the points closed you have 12v at the coil +ve you have an unballasted system and need a 12v coil. If you see 6v you have a ballasted system and need a 6v coil. A 12v coil on a balalsted system will give a low HT, a 6v coil on an unballasted system will overheat and burn the points.

Many Lucas distributors have a very flexible ground wire inside to ground the points plate as it twists under changing vacuum conditions. Eventually it can break, as can the conductors of the coil wire inside the insulation where it goes through the distributor body. This can be a prime cause of the engine cutting out as the throttle is opened to pull away.
Paul Hunt

The last thing you mentioned, broken insulation somewhere in the wiring from the distributor to the ( new original SD1 coil)makes sence to me. If i take the HT lead between the coil and distributor of i stiil hear a 'crack' wich I can't locate. I assume that nothing should be heard with the HT lead off.
Thanks to all for your reply.
A.E.J. van Dulmen

Hi Arno,

By removing the HT lead from the coil you are allowing the HT to rise to 20kV or higher on a good ignition system. If you can hear a 'crack' with this wire out it indicates you *do* have a good HT voltage and hence a good condenser, the HT is finding its way to ground across the outside of the insulator and through the air, any dampness in the air will make this easier. With everything connected when the plug fires, which is at about 7kV to 10kV, the HT is automatically prevented from rising any higher, which is why it doesn't short to ground or give you a shock when holding the insulation. *Not* connected to a plug the HT rises to a point at which it will break through the insulation of the HT lead to ground, or to you if you should be happening to hold it. Which is why I say to remove the lead from the coil and not from the distributor cap or a plug :o) Removing a plug lead from the distributor would also be safe.
Paul Hunt

Hi Paul,

thanks for your safety advice. you say that hearing a 'crack' witk the HT lead out indicates that the voltage is OK as I hoped to have a cause of my problem. I still think that the spark is to weak, I compared the spark from a plug lead to ground from my V8 with my other MG and with my landrover.
The spark from my v8 is about 5mm long angd more orange as the other 2 are 8 yo 10mm long and a bit more blue.I dont mind buying a new electronic ignition but I hate investing and not solving the problem.


Are you saying that the maximum you can draw the spark out to on your V8 is about 5mm and it is orange? That *would* indicate a weak-ish spark and posibly a bad condenser but contradicts the indication of the 'crack' you can hear if the coil lead is out altogether. How far can you draw the spark out if you remove the coil lead from the *distributor* and hold it by a ground? If that is significantly more then I'd say your rotor and or cap are the problem, or the coil leads.
Paul Hunt

This thread was discussed between 20/10/2004 and 27/10/2004

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