Spark plug testing

[jj4091]

Not getting a spark on petrol lawn mower, read on line that if you measure between electrode tip and plug body a resitance reading of 5 - 20 Meg ohms is ok. I have 2 which both read open circuit which I would have thought was better than any resistance at all. There is continuity between the cap and the electrode tip so not a problem there I don't think. Am I missing anything and there is something buried in the porcelain insulator or are both plugs ok. Going to set up the magneto and check wiring next if plugs ok. Thanks
Never thought to test a spark plug before but can't get a new one locally and quickly.

 

[Mottie]

Surely if you have a resistance between the electrode tip and the plug body, you won’t get a spark to jump the gap between the electrode tip and the body? There should be a resistance between the terminal tip (what the plug lead goes on to) and the centre electrode tip.

 

[Burnerman]

I try for a spark with the end of the HT lead close to the engine.....if it has a rubber cap, shove a screw or similar in the cap and try that way.
Next, isolate any cut out switch before going deeper!
John

 

[Harry Bloomfield]

Not getting a spark on petrol lawn mower, read on line that if you measure between electrode tip and plug body a resitance reading of 5 - 20 Meg ohms is ok. I have 2 which both read open circuit which I would have thought was better than any resistance at all.

There should be infinity, between top contact and the body, otherwise the HT will be leaking to ground. I understand - Some plugs include a resistor, between plug top contact and the centre electrode.

 

[Mottie]

Get someone else to put their finger in the plug cap to tell you if they can feel petrol coming through when you turn the engine over. You’ll soon know if there's any HT voltage coming down the lead.

 

[Burnerman]

I guess any grot between the centre electrode and ceramic may give some conductivity but it’s not a test
I bother with.
John

 

[jj4091]

Surely if you have a resistance between the electrode tip and the plug body, you won’t get a spark to jump the gap between the electrode tip and the body? There should be a resistance between the terminal tip (what the plug lead goes on to) and the centre electrode tip.

https://www.google.com/search?q=can...cEMzkuNKAH_fUCsgcEMzkuNLgHlxw&sclient=gws-wiz I presume there is room for some resitance that a spark will overcome. Must admit it puzzled me also.

 

[jj4091]

There should be infinity, between top contact and the body, otherwise the HT will be leaking to ground. I understand - Some plugs include a resistor, between plug top contact and the centre electrode.

Thats what I thought, but please see my reply to Mottie

 

[jj4091]

I try for a spark with the end of the HT lead close to the engine.....if it has a rubber cap, shove a screw or similar in the cap and try that way.
Next, isolate any cut out switch before going deeper!
John

The only switch is attached to and operated by the dead mans handle and shorts the HT to earth to shut down the engine. It is being actuated/deactuated as it should.

 

[Harry Bloomfield]

Thats what I thought, but please see my reply to Mottie

Even a with a very high value of resistance, you will find the tiny amount of current involved, will disappear, at those voltages - hence, no spark.

When measuring resistance values, often the voltage at which the value is determined is important.

 

[Burnerman]

I guess the only thing you can test now is the card thickness gap between the flywheel and the igniter, unless the machine is very old with contact breaker points.
Sounds like a new igniter needed to me, although failure is pretty rare.
John

 

[jj4091]

I guess the only thing you can test now is the card thickness gap between the flywheel and the igniter, unless the machine is very old with contact breaker points.
Sounds like a new igniter needed to me, although failure is pretty rare.
John

Thanks, I had already done that John and measured the coil resistance which I think is OK but I can't find out exactly what it should be. I guess I'll just have to wait until I can get a new plug as I'm not convinced they are OK. I'm not sure a multimeter is the right tool to test with, a megger would probably be better but I don't have access to one.

 

[jj4091]

Coil rsistance is 4 Mohms which apparently is in the range it should be, so really it can now only be the plug, fingers crossed.