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MG Midget and Sprite Technical - lumenition on 1500 Midget

My son has recently bought a 1977 midget 1500 and as part of a general fettling, along with new distributor cap, rotor, leads and plugs I have fitted a Lumenition MTK007 Magnetronic ignition system and a couple of problems were noted.

After removing the points, condenser etc and cleaning the advance plate the adaptor plate and magnetronic module were fitted with no problem, however after fitting the magnetic trigger disc over the cam the clearance between the module and disk was only about 10 thou. (0.25mm) instead of the 60thou ((1.5mm) recommended. After re-checking the advance plate and general installation, ensuring that the module and adaptor plate were properly seated, I resorted to making a 50 thou (1.25mm) plastic packing ring which fits under the two small lugs inside the hub of the trigger disc. The packer sits between the shoulder of the cam and the lugs, maintanimg the correct clearance. This means that the rotor arm is 50 thou higher but clearances inside the distributer cap were checked as ok. The car at first refused to start and that is when problem two emerged.

The car has a ballast resistor ignition circuit and the instructions state that the power feed to the module must be on the ignition switch side of the ballast resistor. We could not find the ballast resistor but the number of cables connected to the coil and their colours corresponded with the wiring diagram for such a circuit, as did the 6v measured at the coil with the ignition on. Connection was made to the ignition circuit at the fusebox.

When carrying out the usual trouble shooting tests it was noted that the 12v feed to the module was present until the starter was operated. Feed for this comes from the only convenient point upsteam of the ballast resistor on the fusebox terminal 5, white cable. It would appear from the wiring diagram that this is the feed to the main circuit when the ignition is on, powering the ignition via the ballast resistor and the wipers, heater fan etc via the fusebox. The problem is that when the starter is cranked this circuit appears to be isolated (ignition switch?) while the coil is fed with 12v from the auxiliary contact of the starter solenoid. This I think became late 70's common practice, ensuring all available battery power went into the starting process and the 6v coil was fed with 12v while the starter cranked.

The installation was re-checked and the timing set statically, and the engine eventually fired at the point the key was released to stop the starter cranking. It has to be stated that it then ran very well indeed, but you will see the dilemma, when the starter is cranking the ignition is effectively off. The tecnique appears to be to spin the engine briefly and hope the ignition then kicks in before the the engine stops turning, which it does if the battery is ok, less easily if the battery is a bit down.

I have made enquiries with other classic car buffs before and during the Classic Motor Show at the NEC and the problem is known to them. Some MG buffs thought that the ignition circuit power (white cable) should stay on when cranking, others say not.
It could be that the car has been fitted with a different ignition/starter switch which isolates that part of the circuit when cranking. The car was bought only a few months ago and the doors/boot/ignition all have different keys, so who knows.

General consensus is that depending on the make and car some circuits do maintain voltage when cranking, it all depends how complicated the circuit is, the more complex the more likely there is to be a bit of circuit maintaining 12v. As you know being complex was not what the Midget was about! A sample of the suggestions received is to do away with the ballast resistor circuit and fit a 12v high performance coil, or fit a simple push button circuit which puts 12v onto the Lumenition module while cranking (both hands required) or fit a relay circuit to do it automatically, or as I had already considered, a pushbutton for the starter or even a manual pull starter contactor as per early Minors!

The solution in the end was to fit a pushbutton starter switch just to crank the engine. Disconnect the white-red trace cable from the ignition switch and extend it with an in-line connector. Connect another cable to the white cables on the ignition switch (there is a convenient spare tag on the ignition switch) and connect these cables to a momentary contact pushbutton, some even have "start" printed on them. Turn the ignition on, but not all the way round to start, then press the button, works a treat and very retro. The start button is temporarily mounted under the dash until the radio console is installed.

Matthew is a Chartered Accountant, I am a furnace engineer. I chip in with a bit of technical help when I can remember it from the days when we all had to do this stuff in order to drive to work, but I'm too big and old to get out of the Midget!

Thanking you in advance, Roger Plant
R L Plant

Sorted that out very easily and might I say very competently as well Roger.
I wouldn't like to think what kind of advice you would have received here but a sample would have been "change the condensor" LOL.

Anyway I like the push button start that was a nice retro way to sort it. I think I would have gone for getting rid of the ballasted coil and gone 12 volts everywhere.

A few years ago I had a Metro van that the solenoid lost the ability to give the ballasted coil 12 volts on cranking. I fitted a "hot Switch" that allowed me to switch 12 volts to the coil when starting. This was fine until I forgot to switch it off, as when I switched the ignition key to off the engine remained running.. :-) Didn't damage the coil however!!
Robert (Bob) Midget Turbo

BTW there isn't a ballast resistor, as such, only a balasted wire to the coil - White/Blue IIRC.

Also, the instructions for the Magnetronic advise removing a small amount of material from the bottom of the rotor arm to allow for the thickness of the trigger. I didn't read the instructions until AFTER fitting and destroying my new dissy cap and rotor arm!
Dave O'Neill 2

Thanks for the fast response and complimentary remarks, much appreciated. Bob, the general advice was to convert to standard 12v but there were some odballs as well. Didn't know about the ballast wire Dave, but it makes sense as we couldn't find a resistor. Regarding cutting off the bottom of the rotor, good idea, I didn't pick that one up from the instuctions but there still has to be a packer under the trigger disc to stop it getting too close to the module.

Thanks again fellas. Next question, how do I get rid of the play in the collapsible bit of the steering column?

Roger Plant
R L Plant

This thread was discussed on 08/12/2009

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