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.

 

[SteveSr]

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.

Actual coil failures on these newer (OEM NOT Aftermarket) electronic coils is very rare. You need to be in a dark place and pull the rope fast to be able to see the spark.

More likely you just flooded the saw as the 250s are prone to flooding because the operator misses the "pop". If it was flooded adding more gas via the plug hole will only make it worse.

 

[Coralillo Lo Pro]

Actual coil failures on these newer (OEM NOT Aftermarket) electronic coils is very rare. You need to be in a dark place and pull the rope fast to be able to see the spark.

More likely you just flooded the saw as the 250s are prone to flooding because the operator misses the "pop". If it was flooded adding more gas via the plug hole will only make it worse.

I was told that about my 251 and a new AM coil fixed it just fine. Worth ruling everything else out first, but this insistence that OEM Stihl coils rarely fail can delay diagnosis by chasing everything but that. In this case, an AM coil did nothing for it (cheap under $10 test I can return) and I do see good spark in the dark so not really my tester lying in this case. I know the flood prone issues of the 250 but most all my issues with flooding saws I've found that throttle on the trigger usually gets around it. I've cleared things by starting repeatedly with the plug out, let dry, switched plugs, and still nothing. Would be inclined to agree with you about flooding, but if so, man, this is the trickiest saw to unflood I've ever used. No such issues with my 251. Cleared the cylinder with a lot more pulls, will let dry out longer. Plug was definitely drenched as could be last time I pulled it out. Did a sloppier quick job of diagnostics than usual but that's because I just thought this saw was a starter cord repair and worked fine one day after repairing it. Then it didn't, then it did, then it didn't again. Without concentrating on flooding as the most likely culprit, I jumped too quick to coil without a really good spark test of it.

 

[Oletrapper]

Actual coil failures on these newer (OEM NOT Aftermarket) electronic coils is very rare. You need to be in a dark place and pull the rope fast to be able to see the spark.

More likely you just flooded the saw as the 250s are prone to flooding because the operator misses the "pop". If it was flooded adding more gas via the plug hole will only make it worse.

Easiest to flood saw I have ever owned. MS 250. Put on choke. Pull twice. You may or MAY NOT get the pop. Take off choke and pull till it fires. jmho OT