Spark question.

rjc2rjc · Feb 2, 2015

Okay I have read several times now that the spark should jump a 1/4 inch gap. I have taken the spark plug out and held it against the block well a 1/4" or so from the block. I get a blue spark on the plug where the spark should be but nothing jumps to the block. should I be doing this test with a bolt or something or should there still be a spark jumping from the plug. I have watched my father years ago do this on vehicles but I cant remember if it was the plug or a bolt.

Nebraska Kirk · Feb 2, 2015

Try taking the plug wire off the spark plug and holding the end 1/4" away from the block and see if the spark jumps the gap.

old · Feb 2, 2015

If the plug wires have a boot on them take and old spark plug and gap it out as wide as you can to make that 1/4 inch gap. Or find an old plug with that does not have a boot or cut the boot off and use it

rjc2rjc · Feb 2, 2015

okay will try that with the old plug

schumy ia · Feb 2, 2015

I use a phillips screwdriver with insulated handle
in place of the spark plug,stick it in the wire and
hold it close to a good ground while cranking the
engine

John T · Feb 2, 2015

The spark is looking to eventually jump to frame ground. Usually you don't use a plug but instead have the spark plug wire to 1/4 from frame to check for a spark. If you use a plug its usually grounded direct to frame so the spark is across the plugs gap. However if the plug were 1/4 inch away the spark could first jump the normal plug gap and then from the plug to frame, i.e. two sparks. The 3/16 to 1/4 inch gap in open air (atmospheric pressure) is to simulate a plug gap of say 0.030 under compression. Actually depending on the ignition system I've seen sparks jump nearly an inch but they grow thinner and more yellow instead of blue as the gap increases.

John T

Spark question.

rjc2rjc

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Nebraska Kirk

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old

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rjc2rjc

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schumy ia

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John T

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