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MG Car Repair/mgb starting problem


QUESTION: hi barrie,
i have a 1979 mgb roadster which refuses to start basically there is no spark at the plugs when cranking,i have fitted accuspark stelth electronic ignition,changed the coil,a 12v ballast,cap,rotor,leads but the strange thing is that when the ignition is turned on i get 12v at the positive side of the coil,where i understand ther should only be about 9v as it is a ballast system.i dont know if the resistor has been taken out or not in the past,there is some evidence of tinkering could i solve the problem by changing the coil to a non ballast one if i suspect the resistor is faulty,or could you suggest something else.the car  ran ok duing the summer although to the end there was a misfire,and it would not start if it was hot and finally it wouldnt start at all
hope you can help

ANSWER: Hi Rob.  Your 1979 MGB has a ballast resistor wire hidden inside the wiring loom.  When the starter is operated the resistor is bypassed by an extra wire attached to a tiny terminal on the starter motor.  If any part of this wiring is faulty, then the system will struggle to provide eneough energy to start the car.  Also, you have fitted an AccuSpark system that really needs a 12 volt power supply at all times.  

I would remove both the white/green wires that are attached to the + terminal of the coil and run a new power supply directly from the WHITE side of fuse number 3 to the + terminal.  I would also ditch the 9 volt coil and fit a non-ballasted Lucas 12 volt Sport coil or an AccuSpark coil.  

If the car still misfires, please get back to me because you may have additional problems.    

---------- FOLLOW-UP ----------

QUESTION: hi barrie
thanks for getting back to me so soon,i will try what you say, could the wire at the starter motor be faulty ,bad connection ,or simular.
i have a non ballast coil arriving on the weekend will let you know how i get on

I would do nothing until you fit the 12 volt (non-ballast) coil.

There is a White/Green wire that is connected to the smallest terminal on the starter motor.  This wire is only powered whilst the starter is operated, and it will be redundant when you fit the bypass wire directly from fuse 3 to the coil.  I would cut it off and throw it away.

Note that there are two White/Green wires attached to the + terminal of the coil.  You can safely cut off and remove both of them before running a fresh power supply directly from the White/Brown side of fuse number 3.  

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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