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

 

[SteveSr]

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.

Keep in mind that the 025 / MS250 is a TOTALLY different saw than the MS251. The MS251 that I rebuilt last fall was WAY too lean from the factory and hard to tune with the limited coil. I think that I opened up the "H" and additional 3/4 turn before resetting the limiter cap. Didn't notice hardly any loss in power.

 

[Coralillo Lo Pro]

Keep in mind that the 025 / MS250 is a TOTALLY different saw than the MS251. The MS251 that I rebuilt last fall was WAY too lean from the factory and hard to tune with the limited coil. I think that I opened up the "H" and additional 3/4 turn before resetting the limiter cap. Didn't notice hardly any loss in power.

Yeah, noticed that comparing them. Went to borrow a plug from the 251 and was totally different size. Thought it was meant to be an evolution, but guess not, it's nearly a pound heavier and they still sell both saws. Finally got the cursed 250 sputtering slowly to life today after a long dry out. Took a bunch of pulls of sputtering and dying before finally got it to hold and gingerly bounced the throttle til I got it revving. Then it idled and ran fine. Just wanted to be sure it was okay before returning it to owner and getting paid for what was just a starter rope repair. His problem to deal with the temperamental flooding nature of 250's, I know it works. I like the 211 I fixed up, it's a neat little 35cc saw.

 

[Oletrapper]

Keep in mind that the 025 / MS250 is a TOTALLY different saw than the MS251. The MS251 that I rebuilt last fall was WAY too lean from the factory and hard to tune with the limited coil. I think that I opened up the "H" and additional 3/4 turn before resetting the limiter cap. Didn't notice hardly any loss in power.

Just curious. Why would you replace the limiter cap? I throw them away. lol
They are nothing more than the govment sticking their nose where it doesn't
belong. Again. jmho OT

 

[Oletrapper]

Yeah, noticed that comparing them. Went to borrow a plug from the 251 and was totally different size. Thought it was meant to be an evolution, but guess not, it's nearly a pound heavier and they still sell both saws. Finally got the cursed 250 sputtering slowly to life today after a long dry out. Took a bunch of pulls of sputtering and dying before finally got it to hold and gingerly bounced the throttle til I got it revving. Then it idled and ran fine. Just wanted to be sure it was okay before returning it to owner and getting paid for what was just a starter rope repair. His problem to deal with the temperamental flooding nature of 250's, I know it works. I like the 211 I fixed up, it's a neat little 35cc saw.

The 250 will probably come back if he can't start it. As I have said before, easiest saw to flood out there. I honestly believe it has to do with something in the carb and pressure in the fuel tank allowing fuel to get in crankcase while just sitting on the shelf. I set mine on its side with caps up and crack the fuel cap to prevent pressure from building up. Seems to work and it will start 99% of the time. jmho OT

 

[SteveSr]

Just curious. Why would you replace the limiter cap? I throw them away. lol
They are nothing more than the govment sticking their nose where it doesn't
belong. Again. jmho OT

This saw is being used by volunteer trail maintainers and I try to add operating margin (richer) and remove opportunities (keep limiter caps) for mis-adjustment to prevent a saw from being burnt up.

 

[Coralillo Lo Pro]

The 250 will probably come back if he can't start it. As I have said before, easiest saw to flood out there.

He's a tree service guy who has had it awhile and knows the drill on it flooding. He apparently hasn't had that much problem with it since he figured out throttle-depressed starts.