electrical question

Helping a neighbour get her '74 Vespa 150 started, she left it in the
damp winter weather and I think a little corrosion caused it to stall
the other day and not start afterwards. This is an import from India
with a home-made starting switch, the simple toggle kind, attached in
the lockable left-storage compartment. By checking continuity I was
able to determine that the switch allows the bike to start when the
switch is off. When the switch is 'on', one of three wires from the
stator is shorted to one of the other two. (The third stator wire goes
to the lights.) The thing is, the switch isn't inline, ie., in series,
with the high tension coil.

(Have tested resistance on the HT coil and I see about 1 ohm on the
primary and about 13,000 ohms on the secondary so I'm guessing for now
that the HT coil is okay. At the moment, can't get at the stator since
I don't have the right flywheel puller, am trying to source one.
Suspecting corrosion or old age has ruined the igntion winding or maybe
the condenser or points.)

My question is a more general one, whether it is a dumb idea to put the
ignition 'switch' in parallel with the HT coil? Eg., while the starting
'off' position of the toggle doesn't seem wrong, could the
'on'/stop/short-to-groound position allow some small transient current
that would harm the ignition winding or the HT coil or
condenser/capacitor/points? (I can think of two reasons for wiring it
this way: 1) ignorance and 2) a desire not to lengthen the wires from
the stator to HT coil which are on the right-hand side of the scoot
while the storage compartment is on the left-hand side. Maybe there are
others I haven't thought of but I think if I were jury-rigging a more
secure, ie., less easy to steal, switch, I would have mounted a keyed
ignition switch from some scrap bike on the right-hand side.


Comments appreciated in advance.

 

Not really the point of my question but omitted to mention that
regarding the non-starting problem, there is fuel but no spark. After
six months outside under a tarp', with some new gas-oil mix in the tank,
it started first kick but died the next day. Light wire from coil works
fine, also took handlebar switch cover off in case kill switch was
shorting. Don't have a peak voltage adapter so I don't know if any
current is reaching the HT coil, but I can light a 6/12 volt test lamp
from the stator light wire but not from the primary or secondary HT wires.

 

paul c wrote:
....

Also forgot to mention that this is the battery-less kind original style
of Vespa (which I get a kick out of, so simple, even if it does have
those pesky points). Another thing that may not be stock for all I know
is that it does have a regulator (labelled 'Bajaj Regulator') only
within the light circuit. Suspect this was a sensible add-on.

 

THIS is slightly off-topic, but you seem to be voluntarily repairing a lot
of bikes for neighbour lady bikers. Are you, like ,getting a lot??

 

With that type of ignition, a handy test to check the points are closing
correctly is to disconnect the ign lead from the coil - put a headlamp in
series with the points/mag lead and borrow the battery + lead to light the
bulb, there will be a noticeable dip in brightness as the points open and a
small amount of voltage is dropped by the DC resistance of the mag coil, if
unreliable running is caused by an oxide layer on the points faces the
current passed may clear it, in any case the difference in resistance
between points open/closed is difficult to see with a multimeter.

On most ignitions of that type I've seen, the ignition switch has been
arranged to short the ignition wire to chassis in the stop position -
apparently the contact resistance of a switch in series has a noticeable
effect on spark efficiency.

 

ian field wrote:
....

Thanks for the tip. When you say 'disconnect the ign lead from the
coil', I presume you mean the spark plug lead and the main/'high
tension' coil, not the lead to the coil and not the ignition coil on the
stator, right? (Of course, in this case there is no battery, no spark
either and the . I've tried a test light in the plug lead, thinking I
might see something even if there is no spark, might try a peak voltage
adapter, aka 'dvm' I got recently, but in any event, at least assuming
I'm right that the high tension coil is okay, I'll be stuck until the
flywheel extractor I had to order arrives - this flywheel appears to be
of a somewhat ancient variety, the pullers that work for p-series vespas
are too big for it, still not sure if I ordered the right puller but
won't have cost much if I'm wrong.)

ie., most of the current goes to ground?

 

Before giving that advice I should have asked you first to actually look
through the access slots in the flywheel and check there actually is a set
of points - some modern electronic types incorporate a high voltage winding
in the stator assembly (does the same thing as the transistor inverter in a
CDI) and also don't need a battery to make a spark.

You can leave the ign coil connected in parallel with the points/stator coil
if you want, but the dip in brightness of the test headlamp bulb (with
battery) in series will be much less visible as the points open because the
2 coils in parallel will produce much lower resistance - the difference in
resistance to drop some of the bulb voltage will be smaller so the bulb
won't dim as much when the points open.

As the points open and the current is allowed to flow through the stator
coil, the coil will behave as an electro magnet so you should be able to
feel it pulling the magnets in the flywheel,
this is another indicator of points open or closed.

 

And obviously, it's getting some kicks out of you, as well.

Steve

 

Apparently vespa is the Latin name for wasp.

 

It's points alright, I'm from the generation that used to always carry a
nail file in the glove compartment.


Now, this evening, what looks like the right puller arrived, about 25 mm
wide on the outside thread. Won't get to neighbour's place until Monday
to take it off.


Of course I could be wrong that the non-starting has something to do
with the stator's ignition coil or the condenser or the points. Besides
examining the points as you advised above, any other tips besides
looking for black wires, testing resistances and so forth? Perhaps I
should have paid thirty bucks to order new ones, but until I get that
flywheel off, I can't be sure of what to order. I suspect this engine,
even though it was made in India in the 1970's, resembles an early
1960's Italian Vespa.

cheers,
p

 

Years ago I used to use a cheap & nasty battery charger to pass current
through the bulb and stator coil/points as the "rough DC" caused the
flywheel to hum when the points were open, this made it easy to check the
timing as the slight vibration was easy to feel as the rotor was turned past
the timing mark.

Unfortunately there is a slight possibility that the alternating field from
the stator coil can affect the permanent magnets in the rotor.