What resistance should be measured on lawnmower spark plug?

I'm not sure that 1 Megohm IS a lot for what is essentially an open circuit.

The spark is the result of a very high voltage jumping across a gap.

It's a long time since I gapped a set of plugs, but *if* the plug's gap is set correctly, I'd suspect whichever component is supposed to be supplying the HT.

But check that gap first...

I think you have misunderstood the OP's question. He is saying that the series resistance between the connection on the top of the plug (which connects to the coil or whatever) and the electrode at the bottom of the plug (which the spark jumps from) is 1MOhm. The series resistance indicates that the plug has a built-in RFI suppressor and the question was whether 1MOhm is a typical value for this.

What the value was is lost in time, unfortunately. However, a quick internet search comes up with values between 5000 and 20000 ohms. At one time there were suppressor ignition leads, but these were even more unreliable! You can still get suppressor spark plug caps, and typically these have a resistance of 5kohm (eg

I recently measured the resistance of the resistive ignition leads on my 25ish year old Volvo V40. They were about 600 ohms over a length of about 25cm. I megohm definitely sounds far too high. John

Plug has some sort of short then

It appears to be a European style plug

Its likely that a film of carbon would be a lot less than that, which combined with the 1M would effectively divert all the current from the electrode

No idea.

He has two plugs, Do either of them spark?

The sparking plug failed in my petrol Flymo. I found the grass cutter worked perfecly well with an old sparking plug from a car.

I bought a new plug the same as the old one on ebay.

My seldon used petrol chain saw would not start.

It worked much better when I put the old petrol into the grass cutter and filled the chain saw withn new petrol.

The specification for the NGK BPR5ES indicates an internal 5k ohm

The claim here, is 2mA flows during the conduction period. 2mA through a 1 megohm resistor, is a drop of 2000V. You could probably hear a change in engine note with that (perhaps the spark duration would be slightly different). The voltage drop might affect the current flow. The plasma itself, when the gap conducts, is a relatively low impedance.

A scientist might collect both a voltage waveform and a current waveform, to better characterize the circuit. You get some idea then, as to what potential power is available to do various things (like, make the ignition wires warm). To do current in the circuit, you'd want to do a low-side current monitor, but sometimes the physical implementations prevent easy circuit measurement. There are some other high voltage circuits, where low side current monitoring is used for control purposes. You can also tell quite a bit from watching the primary side behavior on a coil, and not even need to look at the secondary side for load characterization.

Paul

I wonder if one can still get 18mm spark plugs.

I think you can get just about every spark plug type ever made. Right down to 5mm

The agricultural test for a working spark is to lay the connected plug on top of the engine cylinder and crank the motor. Wire brushing the plug interior and warming to remove conductive debris can be a fix. Single cylinder horticultural motors of my youth had a contact opened by the flywheel initiating the spark impulse. These almost invariably needed a clean for the first post Winter outing. I assume modern tackle uses a different system.

Oops!

Fair enough.

It's shown in this circuit diagram. I think. :)