Spark on tester but not on plug

[Pioneer]

Had this happen to a saw I just picked up, it would give me an indication on the neon tester but not with a spark plug. I checked the kill switch by disconnecting it, no luck there. Then I checked the coil gap, seemed excessive. I adjusted it down to where it should be with a shim and got a nice fat spark. So the tester was actually indicating a weak spark where the plug would give you none at all, in this way the tester actually helped with the diagnostic.

 

[Bob Hedgecutter]

If you know the plug you are suspecting is 100% good- like good strong spark in another device- then yes.

 

[stihltech]

Neon testers don't impress me. I have had them show spark but not start. I like a tester that is basically a plug without the side electrode. If it sparks that, ignition output is OK.
Unless it is an L77 plug. Get a real plug first.

 

[sean donato]

Plenty of good cheap gap type spark tester out there. I favor my opama brand but you can get cheap version for under $15.00.

 

[Mad Professor]

Have somebody hold the plug and pull the recoil.

 

[Okie]

neon spark testers suck big air. They will lie to you about spark being ok.

I do not use neon ignition testers.

I suspect if you had to shim something to get a good fat spark something is still not completely correct.

 

[SteveSr]

Neon testers don't impress me. I have had them show spark but not start. I like a tester that is basically a plug without the side electrode. If it sparks that, ignition output is OK.
Unless it is an L77 plug. Get a real plug first.

It only takes about 80V to flash a NE-2 neon bulb. Spark tester result depend on exactly how the bulb is wired or if ti is just capacitively coupled.

 

[Coralillo Lo Pro]

I should know by now from an old Bosch electronic ignition that was kind of shorting out but still lighting my tester not to place much faith in cheap inline testers. Have kept placing way too much faith in my tester though. My Stihl MS250 has been starting some days easily and not a trace of a cough other days and got a new coil for it and still won't start. Tester showed spark, compression at 140, tried gas in the cylinder, couldn't get a cough at all. Was baffled til I read this thread and remembered testers lie. Almost certainly no spark at the plug. Have to check the coil wires more carefully, one's probably raw and shorting out.