Spark testing

[HenryB]

I’m using an adjustable gap spark tester. When I attach one end of the tester to the plug wire and clip the other end to the plug I get a healthy spark, three out of four plugs. One plug, with the tester assembled the same way, no spark. Tried another plug, no spark, but when I attach one end to ground a good spark. When I remove the plug and attach the plug wire and ground the plug I get what seems to be a good spark. Should I expect the spark to jump with the plug installed?

 

[nwcove]

your method is a bit confusing, but if you have an open air adjustible spark tester, there is no sparkplug involved in the test. one end of the tester plugs into the plug boot, the clamp goes to a GOOD ground on the engine.

 

[metriccrescentwrench]

..... but when I attach one end to ground a good spark.....

Yep, you gotta' properly ground one end of the spark tester otherwise all those electrons have no place to go.

 

[HenryB]

That makes sense, and I’m not arguing, but three of the plus tested with the ground clip attached to the plug and the electricity jumped the gap in the tester and the plug. On one plug the electricity did not, but the electricity did jump the plug gap with the tester removed (and the spark appeared healthy).

I get what appears to be a healthy spark on all four leads when I attach the ground clip to a ground. I’m wondering if that is the way to use the spark tester?

The gap is set at 7/16"

I have another tester with a bulb and it attaches to the plug and the plug wire.

 

[Tim Frank]

That makes sense, and I’m not arguing, but three of the plus tested with the ground clip attached to the plug and the electricity jumped the gap in the tester and the plug. On one plug the electricity did not, but the electricity did jump the plug gap with the tester removed (and the spark appeared healthy).

As stated, your explanation is confusing.
Are the plugs screwed into the engine or loose?
If they are just loose and grounding on the block, you arelucky that 3 of 4 tested OK.

I get what appears to be a healthy spark on all four leads when I attach the ground clip to a ground. I’m wondering if that is the way to use the spark tester?

Yes.

 

[HenryB]

The plugs are screwed in tight.

With the adjustable gap tester set at 7/16", and the ground clip attached to the spark plug terminal, the spark jumps both gaps (the plug gap and the 7/16" tester gap) on three out of the four plugs.

The spark does not jump both gaps on the fourth plug with the tester set the same way. But if the ground clip is clamped to a ground there is a spark at the tester gap.

The voltage passing through the fourth plug is less than the other three, but will it fire the plug enough to ignite the fuel?

Is the 7/16" gap that the tester is set to sufficient to test a plug voltage with out the plug in between?

 

[F_R]

You need to understand the basic concepts of electricity. When it is time to fire, voltage builds up in the coil until it is high enough to force a current (flow of electrons) through whatever resistance is in it's way. That resistance consist of the plug wire, the air gap in your tester, the internal resistance of the plug, and the air gap across the plug's spark terminal. It then has to flow back through the engine block, to the source (coil). Now, understanding that, once the voltage is high enough to force the electrons across BOTH of the gaps you are presenting, it flows and you see it as a spark. Once that happens, the energy is dissipated and it's all over till next time.

Emphasis on BOTH air gaps in the circuit. If all other things are equal, it would make sense that the one plug has a higher resistance for some reason. On the other hand, the voltage may never be rising high enough to jump the resistances of both air gaps.

I understand where you are coming from with your question. However, if spark voltage is sufficient to jump just the 7/16" tester gap to ground (not through the plug also), that is the standard test. You are trying to combine that with the plug gap, and that is going beyond the normal test. Knowing the normally available voltage in the systems, I'd say it should be capable of jumping both though. May I suggest a possibly bad plug? Or a weak coil? Or a high resistance in that plug wire?

 

[HenryB]

Thanks for the come back.

It is probably me but I feel that the motor is not up to snuff, maybe a bit more vibration.

I’ve switched out the plug and got the same result, no spark with the ground clamp on the plug terminal. I am waiting on a used coil from ebay, ten bucks; rather not change out one of the other coils. I can switch the wires then.

But if that don’t do it, the next step involves big money parts. I’m pretty sure I can use my manual and an ohmmeter to test the parts under the flywheel. Then I think comes the power pack change out. The thought is depressing but I probably will go that route even though I probably have enough spark.

My motor is a 1998 Johnson 130.

 

[Tim Frank]

You haven't provided any history or why you are checking the spark, but if you are doing a normal troubleshooting sequence, IMO, the test that you performed as per the norm rules out spark as a problem....if you are actually having a problem.
The "invented" test that you performed, at best suggests that you may have a weak component....that may....or may not fail in the future; at worst has you chasing your tail.

At this point, i'd be more worried about compression and fuel diagnostics.

 

[Bosunsmate]

A timing light would tell you if its firing when in use but since its jumping that gap it should be.
Like Tim asks have you something wrong with the motors running or are you looking for imperfections? Most motors have them