My motorbike with an electric starter runs an 11ah battery so yours should be OK. Maybe you have a duff coil, measure it's current consumption it should be no more than 3 – 4A with the points closed and engine stationary. It will be less than this with the engine running.

Derry.

Hi Bob,
I'm surprised that the battery voltage drops from its fully charged voltage (Which is probably about 13 volts.) to 11.9 volts after just one minute. The ignition circuit should still work well at 11.9 volts. When STARTING the engine in a car the battery voltage will probably be less than this as the starter motor draws a few hundred amps. I suggest checking the current that is being drawn both with the engine stopped (in a position with the contact breaker closed. ) and with it running. I would expect it to be less than 5 amps. The current with it running should be less than when it is stopped. I have an ignition coil which is probably similar to yours so I will do some current measurements tomorrow for comparison. Are you sure that the engine stops due to ignition problems and not a fuel problem ? Could it be it runs for a minute with the contents of the float chamber ? One other thing is I suspect your engine runs at a much lower speed than a car engine so it is possible the coil may be heating up. Check if the coil has heated up at the end of the one minute run.

Les.

Bob,

Another thought, are the points closed until needed or open until needed, if shut you maybe shorting the battery.

Martin P

hi Martin I'm very intrested in your sugestion about using flouresant light fitting chokes on the ignition of stationary engines, I have never heard of this before some more information would be much appreciated. Regards Farmer Dave

I've used the ballast from light fittings but only on a low tension setup with an ignitor, don't know how they would fair for a HT sparkplug.

J

Hi Bob,
Here are some measurements I took from the ignition coil that I have . (Lucas but model number not known.) Primary resistance 3.3 ohms (This would draw 3.6 amps when the points are closed.) Secondary resistance 4.5 K ohms. I am using it with a 0.22 uF capacitor but I think about 0.5 uF is normally used. I am using this with a pulse generator for igniting the burner on my furnace. At the point when the points (High voltage transistor in this case.) the peak voltage on the primary reaches about 180 volts My driver circuit generates a pulse every 80 mS (12.5 Hz) The current is switched on for about 2 mS each cycle. It is driving a spark plug with the gap opened up to about 70 thou and even with this gap it works OK even with the voltage reduced to about 10 volts. Anthony's (LAD machining) point about the coil being designed for use with a ballast resistor is worth checking. (If it has a low primary resistance it would suggest that it is this type.) Lofty's comparison of dwell angle is not a fair comparison as the mini is a 4 cylinder 4 stroke engine and yours is a single cylinder 2 stroke so 40 degrees on a 4 cylinder engine would be the same as 80 degrees on a single cylinder 2 stroke. If your engine is slow revving then it should work OK with a smaller dwell angle which would reduce the average current consumption. The points only need to be closed long enough for the current to have reached a high enough value. The rise of current in the primary is initially controlled by the primary inductance but then is limited by the primary resistance.

Les.

btw.. many renault ( and others) use a on plug fitted coil..about 30 mm dia ..this might prove more fitting in scale to your engin..**LINK** for example and http://www.qxcomponents.co.uk/CoilCatalogue.pdf

Edited By jason udall on 01/07/2013 17:08:12

There is a thread on the 'on the plug' coils in another site to do with model engines, I'm just an occasional visitor to that site, I think there are two types, one twelve volt, the other I think is higher.

Ian S C