coil at fault not fuel vapour lock

[staghunt]

For two years I have had the problem of on hot days after stopping stag would not start very well and then would run really roughly, having to over rev it to pull away. well I always thought it was fuel vaporisation due to latent heat from engine boiling fuel and then getting vapour lock as there seemed to be no fuel in filter and I had noticed just a little dampness on bottom of overflow pipe, it never happened on normal driving and would be perfectly fine after a short period to cool down. So I fitted Viton tipped float valves and set up float heights. This year though it started to run rough when just driving and It actually konked out on my way home, left it a while got it started only to stop again. So i thought I would check and set up timing vacuum and centrifugal advancement and then check and set up carb balance, while i was on the last stages of this the tick over stated to drop and run rough. This was the first opertunity were i could test the coil to prove it was at fault, i did this by taking a wire direct from battery +12v therefore bypassing the lower voltage supply which comes via resistance wire. This instantly corrected the rough tick over. I had already ordered and was waiting for an Optronic Megaspark which is supposed to be best suited to optronic ignition which i have now fitted and all my problems seem over (well stag ones anyway). cheers greg

[V Mad]

You say the coil was at fault, but are you sure you had the correct coil type? If your coil was the correct type (ballast or 6 volts), and failed due to overheating then applying full 12 volts to a coil rated at 6 volts would seem to overload it even more.

Conversely if you had an incorrect coil fitted ie a non-ballast (12v) coil, then this would give out a weak spark until the full 12 volts was applied.

Just an observation but it might just help identify the true cause.

[Staggard]

I agree, I reckon if you have a 6volt coil breaking down and you run 12volt to it will make it worse. conversely a 12volt running on a 6volt feed will obviously give you a weak spark and when you then run 12volt it will work correctly.

[73stagman]

You say the coil was at fault, but are you sure you had the correct coil type? If your coil was the correct type (ballast or 6 volts), and failed due to overheating then applying full 12 volts to a coil rated at 6 volts would seem to overload it even more.

Conversely if you had an incorrect coil fitted ie a non-ballast (12v) coil, then this would give out a weak spark until the full 12 volts was applied.

Just an observation but it might just help identify the true cause.

Possibly the ballast resistor wire in the loom was damaged causing a larger than acceptable voltage drop. When the temporary bypass wire was installed the result would have been immediate and clear cut but I don't think Greg has left the wire there permanently so no damage to the coil necessarily. But the ballast resistor wire could still be faulty!

[staghunt]

I agree, I reckon if you have a 6volt coil breaking down and you run 12volt to it will make it worse. conversely a 12volt running on a 6volt feed will obviously give you a weak spark and when you then run 12volt it will work correctly.

Well i had no trouble for twenty years only the last two, perhaps some of the windings had shorted together giving less magnetic field and by increasing the voltage brought it back up ? cheers Greg

[Staggard]

Well i had no trouble for twenty years only the last two, perhaps some of the windings had shorted together giving less magnetic field and by increasing the voltage brought it back up ? cheers Greg

All I was saying Greg is a 12 volt coil will work on a 6 volt feed ( I know because I have done it to get me out of trouble on several occasions. If you have had the same coil for 20years it has done very well, but that would depend on how many miles has been done in that time. Also I think the winding shorting is more likely if the coil was being over powered and not under powered.

[KOY 23]

There seems to be confusion over 6v, 12v, ballasted and non ballasted coils.

Just measure resistance between brass terminals (disconnect wires from one side) it should be about 1.5 ohms for a Stag's ballasted system. If it reads 3 ohms it's not the right coil. Some high output coils are less than 1 ohm so check suitability with your electronic ignition module.

Make sure that the ballast resistor is in circuit, do a search if not sure how to test.

As previously stated, fit a good coil from someone like LD parts. Saving a few quid is not worth the risk of an breakdown/accident caused by cheap parts.