"newshound" wrote in message
...
I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the HT
side to earth. That sounds to me like a failed coil, or am I missing
something?

I have already disconnected the "kill wire" as other gremlins have got
into the LT side over the winter, but the starter motor and solenoid are
fine and I can spin it up nicely with appropriate jumpers. It's a simple
"magnet on the flywheel" type, no contacts, no accessible LT side. There
is still some magnetism left and, as I say, it ran fine last year.

Change the spark plug.
IME 99% of the problems in these systems revolve round the spark plug.

I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the
HT side to earth. That sounds to me like a failed coil, or am I missing
something?

I have already disconnected the "kill wire" as other gremlins have got
into the LT side over the winter, but the starter motor and solenoid are
fine and I can spin it up nicely with appropriate jumpers. It's a simple
"magnet on the flywheel" type, no contacts, no accessible LT side. There
is still some magnetism left and, as I say, it ran fine last year.

TIA

On 28/05/2015 20:11, newshound wrote:
I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the
HT side to earth. That sounds to me like a failed coil, or am I missing
something?

I have already disconnected the "kill wire" as other gremlins have got
into the LT side over the winter, but the starter motor and solenoid are
fine and I can spin it up nicely with appropriate jumpers. It's a simple
"magnet on the flywheel" type, no contacts, no accessible LT side. There
is still some magnetism left and, as I say, it ran fine last year.

TIA

Might have answered my own question. Since it obviously gives sparks
once per rev, perhaps each end of the HT coil goes to one plug, so that
the circuit goes coil - plug1 - ground - plug2 - coil. Would be nice to
confirm though. I found one of those "spark gap" plug HT voltage testers
the other day, might try hooking it up to see what I am getting on the
starter.

In message ,
newshound writes
I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the
HT side to earth. That sounds to me like a failed coil, or am I missing
something?

I have already disconnected the "kill wire" as other gremlins have got
into the LT side over the winter, but the starter motor and solenoid
are fine and I can spin it up nicely with appropriate jumpers. It's a
simple "magnet on the flywheel" type, no contacts, no accessible LT
side. There is still some magnetism left and, as I say, it ran fine
last year.

I have a Kohler motor in my Jonserad mower. The manual and cct. diagram
are no help to you but I note the raft of safety features, any one of
which shuts off the motor! Viz operator presence relay, seat switch not
occupied relay, bagger interlock!!

--
Tim Lamb

newshound wrote:
On 28/05/2015 20:11, newshound wrote:
I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the
HT side to earth. That sounds to me like a failed coil, or am I missing
something?

I have already disconnected the "kill wire" as other gremlins have got
into the LT side over the winter, but the starter motor and solenoid are
fine and I can spin it up nicely with appropriate jumpers. It's a simple
"magnet on the flywheel" type, no contacts, no accessible LT side. There
is still some magnetism left and, as I say, it ran fine last year.

TIA

Might have answered my own question. Since it obviously gives sparks
once per rev, perhaps each end of the HT coil goes to one plug, so that
the circuit goes coil - plug1 - ground - plug2 - coil. Would be nice to
confirm though. I found one of those "spark gap" plug HT voltage testers
the other day, might try hooking it up to see what I am getting on the
starter.

It does sound like a classic wasted spark type system where there is no
connection from the HT side of the coil to chassis.

On Thu, 28 May 2015 20:58:40 +0100, Bob Minchin wrote:

It does sound like a classic wasted spark type system where there is no
connection from the HT side of the coil to chassis.

The engine isn't earthed? How does the starter motor work...?

Adrian wrote:
On Thu, 28 May 2015 20:58:40 +0100, Bob Minchin wrote:

It does sound like a classic wasted spark type system where there is no
connection from the HT side of the coil to chassis.

The engine isn't earthed? How does the starter motor work...?

That is not what I said. We are discussing the HT circuit. irrelevant to
a starter motor

In article ,
newshound wrote:
I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the
HT side to earth. That sounds to me like a failed coil, or am I missing
something?

If it is wasted spark - ie both plugs fire together - it's common for the
plugs to be at either end of the coil winding. In other word, the coil
winding is between the two plug central electrodes.

--
*When you get a bladder infection urine trouble.*

Dave Plowman London SW
To e-mail, change noise into sound.

On 29/05/2015 00:36, Dave Plowman (News) wrote:
In article ,
newshound wrote:
I have an ancient Laser ride-on mower with a Kohler 18 twin cylinder
engine. Fine last season, but no sparks now. Checking for continuity, I
have 26 kilohms between the two plug leads, but no connection from the
HT side to earth. That sounds to me like a failed coil, or am I missing
something?

If it is wasted spark - ie both plugs fire together - it's common for the
plugs to be at either end of the coil winding. In other word, the coil
winding is between the two plug central electrodes.


Thanks.

I will try a pair of different plugs as Harry suggests, it's just that I
would not expect these to go "suddenly". Ohmeter check shows many megohm
"across" the plug but I agree they could be arcing internally. I'll try
the Megger, too, when I try new plugs.

Thanks all for the suggestions.

In article ,
newshound wrote:

I will try a pair of different plugs as Harry suggests, it's just that I
would not expect these to go "suddenly". Ohmeter check shows many megohm
"across" the plug but I agree they could be arcing internally. I'll try
the Megger, too, when I try new plugs.

If one plug had failed to the point where it won't spark, it's very
possible the other won't either.

Ford EDIS, which is the wasted spark system I've fitted to my old Rover,
is also quite sensitive to the actual plug leads and plugs. They do need
to be the correct resistance plug leads and resistor plugs to function
properly. I'd measure the resistance of the plug leads. Should be
something like 5-10,000 ohms. A burnt out one will be perhaps 100,000
plus. And make sure the new plugs are resistor types if the originals were.

--
*When I'm not in my right mind, my left mind gets pretty crowded *

Dave Plowman London SW
To e-mail, change noise into sound.

On 29/05/15 15:19, Dave Plowman (News) wrote:
In article ,
newshound wrote:

I will try a pair of different plugs as Harry suggests, it's just that I
would not expect these to go "suddenly". Ohmeter check shows many megohm
"across" the plug but I agree they could be arcing internally. I'll try
the Megger, too, when I try new plugs.

If one plug had failed to the point where it won't spark, it's very
possible the other won't either.

Ford EDIS, which is the wasted spark system I've fitted to my old Rover,
is also quite sensitive to the actual plug leads and plugs. They do need
to be the correct resistance plug leads and resistor plugs to function
properly. I'd measure the resistance of the plug leads. Should be
something like 5-10,000 ohms. A burnt out one will be perhaps 100,000
plus. And make sure the new plugs are resistor types if the originals were.

I had a mower that wouldn't start.

I put fresh petrol in it and after a bit it did.

a winter with a badly fitting fuel cap can result in petrol that has
lost its ability to ignite


--
New Socialism consists essentially in being seen to have your heart in
the right place whilst your head is in the clouds and your hand is in
someone else's pocket.

"The Natural Philosopher" wrote in message
...
On 29/05/15 15:19, Dave Plowman (News) wrote:
In article ,
newshound wrote:

I will try a pair of different plugs as Harry suggests, it's just that I
would not expect these to go "suddenly". Ohmeter check shows many megohm
"across" the plug but I agree they could be arcing internally. I'll try
the Megger, too, when I try new plugs.

If one plug had failed to the point where it won't spark, it's very
possible the other won't either.

Ford EDIS, which is the wasted spark system I've fitted to my old Rover,
is also quite sensitive to the actual plug leads and plugs. They do need
to be the correct resistance plug leads and resistor plugs to function
properly. I'd measure the resistance of the plug leads. Should be
something like 5-10,000 ohms. A burnt out one will be perhaps 100,000
plus. And make sure the new plugs are resistor types if the originals
were.

I had a mower that wouldn't start.

I put fresh petrol in it and after a bit it did.

a winter with a badly fitting fuel cap can result in petrol that has lost
its ability to ignite

Yes, good point.
I always try to empty the tank on my mower for over-wintering and put in
fresh petrol whenI need it next.

On Fri, 29 May 2015 15:19:02 +0100, Dave Plowman (News) wrote:

In article ,
newshound wrote:

I will try a pair of different plugs as Harry suggests, it's just that
I would not expect these to go "suddenly". Ohmeter check shows many
megohm "across" the plug but I agree they could be arcing internally.
I'll try the Megger, too, when I try new plugs.

If one plug had failed to the point where it won't spark, it's very
possible the other won't either.

Ford EDIS, which is the wasted spark system I've fitted to my old Rover,
is also quite sensitive to the actual plug leads and plugs. They do need
to be the correct resistance plug leads and resistor plugs to function
properly. I'd measure the resistance of the plug leads. Should be
something like 5-10,000 ohms. A burnt out one will be perhaps 100,000
plus. And make sure the new plugs are resistor types if the originals
were.

Ordinary plugs usually develop leakage faults which divert the energy
away from the spark gap. This is particularly true for inductive
discharge HT systems (Kettering (including the transistorised contact
breaker type) and magneto where a contact breaker suddenly interrupts the
LT energising current in order to generate the 3 or 4 hundred volt back
emf which is multiplied up by the HT winding into the 25 to 40 Kilovolt
region) but not so much in the case of capacitive discharge systems where
such leakage paths can't interfere with the 'inductive kick' process of
generating the nominal 3 to 400 volt primary winding impulse voltage,
merely in the case of more extreme leakage cause a greater volt drop in
the HT windings (the primary still gets hit by the 400v excitation pulse
regardless and the reduction of voltage at the spark plug becomes simply
a matter of volt drop due to leakage inductance and winding resistance
alone).

The resistive plugs use a high temperature ceramic resistor of about
5000 ohms or so and so it's possible for these to go high resistance
(open circuit at typical DMM test voltages aren't necessarily a sign of
failure - you should test with a Megger to more properly test for 'open
circuit resistor').

It's easy enough to test whether the plug is a standard or resistive
plug type using a simple test meter. If such a test shows open circuit
you'll need to use a high voltage tester to properly prove whether the
plug's built in resistor has failed sufficiently enough to compromise the
spark performance.

Getting back to the issue of the 'wasted spark' system, it's actually a
very necessary requirement that the double ended HT winding (magneto or
coil) is floating with no leakage paths to frame (earth). This
arrangement effectively places each plug's spark gap in series thus
allowing the single HT winding to produce a spark at both plugs
simultaneously.

Such double ended HT coil windings need only to generate another 5 to 10
percent more voltage to compensate for the volt loss across the plug
which happens to be 'wasting its spark energy' at that part of the cycle.

The waste spark is timed for when the cylinder pressure is low,
typically near the end of the exhaust stroke, which reduces the breakdown
voltage to a fraction of that required to jump the gap when subjected to
the much higher pressure developed towards the end of the compression
stroke. IOW, the spark plug that's currently using its sparking energy to
ignite a fuel air charge, always gets the Lion's share of the HT voltage
(at least under normal working conditions).

Incidentally, this is the reason why you need to use a spare plug to
provide an earth return when using the classic "detach each ht lead in
turn' to discover which spark plug is failing to ignite the fuel/air
mixture on a rough running engine that's misfiring on one or more of its
cylinders. You can't just simply detach the HT lead since this will
compromise the remaining good plug's sparking performance as well as
subject the HT winding to much higher voltage stress than normal.

--
Johnny B Good

In article ,
Johnny B Good wrote:

Incidentally, this is the reason why you need to use a spare plug to
provide an earth return when using the classic "detach each ht lead in
turn' to discover which spark plug is failing to ignite the fuel/air
mixture on a rough running engine that's misfiring on one or more of its
cylinders. You can't just simply detach the HT lead since this will
compromise the remaining good plug's sparking performance as well as
subject the HT winding to much higher voltage stress than normal.

There's another interesting feature of wasted spark I don't quite
understand. I have EDIS wasted spark on my V-8 Rover. My posh timing light
with an inductive coupling and a dial in advance setting doesn't work with
it. My older simpler one which had a direct connection to the spark plug
does.

--
*Until I was thirteen, I thought my name was SHUT UP .

Dave Plowman London SW
To e-mail, change noise into sound.