testing spark plugs under compressio.

[AUSDOK]

Hi was shown this method of testing spark pugs in engines to see if they are firing under compression.
1:You will need 2 good spark plugs tested on the magneto you are using.
2: a piece of  of spare lead .test before using.
3: a piece of insulating material to lay one spark plug on.
Okay place one spark plug on the insulating material.
Connect the ht lead from the magneto to the body of this plug. I use a spring clamp and connect it around the thread.
Connect the spare lead to the top of that spark plug and the other end to the spark plug in the engine. Turn the engine over,
for you to see a spark in the plug that is outside the engine it must be firing in the engine under compression to complete the circuit
I hope this is clear. Regards Rudy

[cobbadog]

I can only see that showing you that the spark is reaching the spark plug in the engine not that the one in the engine is firing under compression. 
It is similar to just putting a plug inline going to the plug in the head. In order for the spark to reach the engine it has to pass the spare plug and spark at that point to make it to the engine plug.

Unless I totally confused.

[asw120]

If this is two plugs in series, that should work. Bit hard to tell from the description how you're connecting them. This is actually what is in the old Champion U series spark plugs. They were used where fouling was a problem. You can actually hear them on a quiet engine.

This assumes, of course that your unknown spark plug isn't cracked, or otherwise shorted.

Jarrod.

[wee-allis]

Yes Rudy, I read your plan. Two plugs in series with the first one insulated from earth. For the first plug to show a spark, the second one must be working, or as Jarrod says shorting to earth, but this would show up as a mis-fire on that cylinder.

[AUSDOK]

Thanks for the replies. I posted the info in good faith and only to assist. Hopefully it will be of use to someone.  Rudy

[cobbadog]

Thanks for doing so Rudy, it is just my pea brain not engaging as to what is happening and how it works.