I have recently unearthed my old Suffolk Super Colt petrol mower after it laying dormant for 20 years. I have cleaned the carburettor, checked the compression is O.K., cleaned and reset the points and plug but there is no visible spark at the plug gap. I have substituted the spark plug with a new one but still no spark, even with the suppressor resistor shorted out.

However a neon timing lamp connected in series with the spark plug flashes weakly which suggests to me that the magneto may not be working properly. The resistance measured between the HT plug lead and chassis is around 4500 ohms.

I do not know much about magneto ignition but can anyone tell me if there should be DC continuity between the HT lead and ground and does this depend on whether the points are open or closed?

Regards - Alan S

I must confess to being mightily impressed with the speed with which the previously described problem was solved after posting it here. After attempts over many months, the engine sprang into life within an hour. It runs but hunts on anything less than full throttle. I do not know if this is due to the fuel that I am using (3 year old lead free) or the carburettor settings

However I would appreciate a reply from anyone who can advise me on the carburettor settings and whether using lead free petrol will damage the engine or nor. Or should I play safe and buy leaded stuff?

Alan S

On Jul 2, 12:12*pm, weathereye
wrote:
I have recently unearthed my old Suffolk Super Colt petrol mower after
it laying dormant for 20 years. *I have cleaned the carburettor,
checked the compression is O.K., cleaned and reset the points and plug
but there is no visible spark at the plug gap. *I have substituted the
spark plug with a new one but still no spark, even with the suppressor
resistor shorted out. *

However a neon timing lamp connected in series with the spark plug
flashes weakly which suggests to me that the magneto may not be working
properly. *The resistance measured between the HT plug lead and chassis
is around 4500 ohms.

I do not know much about magneto ignition but can anyone tell me if
there should be DC continuity between the HT lead and ground and does
this depend on whether the points are open or closed?

Regards - Alan S

--
weathereye

off the top of my head, i would think that there should be continuity
between the HT lead and ground, and that it shouldn't depend on the
points, if you have any, which if it's a magneto you probably don't.
if there is no continuity, then it would seen the magneto has
opencircuited.

if there is continuity, then my guess is that the insulation has
weakened and the spark voltage is being limited at some value where it
can penetrate the insulation. i find that car ignition ht wiring often
degrades before 20 years.

"z" wrote in message
...
On Jul 2, 12:12 pm, weathereye
wrote:
I have recently unearthed my old Suffolk Super Colt petrol mower after
it laying dormant for 20 years. I have cleaned the carburettor,
checked the compression is O.K., cleaned and reset the points and plug
but there is no visible spark at the plug gap. I have substituted the
spark plug with a new one but still no spark, even with the suppressor
resistor shorted out.

I would think that if it has points then it has a condensor. Maybe the
condensor is bad. OTOH I had an old edger that nobody could seem to get
running. Presumably the magneto had lost it's magnetism.


However a neon timing lamp connected in series with the spark plug
flashes weakly which suggests to me that the magneto may not be working
properly. The resistance measured between the HT plug lead and chassis
is around 4500 ohms.

I do not know much about magneto ignition but can anyone tell me if
there should be DC continuity between the HT lead and ground and does
this depend on whether the points are open or closed?

Regards - Alan S

--
weathereye

off the top of my head, i would think that there should be continuity
between the HT lead and ground, and that it shouldn't depend on the
points, if you have any, which if it's a magneto you probably don't.
if there is no continuity, then it would seen the magneto has
opencircuited.

if there is continuity, then my guess is that the insulation has
weakened and the spark voltage is being limited at some value where it
can penetrate the insulation. i find that car ignition ht wiring often
degrades before 20 years.