Spark, but no spark to plug

[Poodle_Bruce]

Howdy folks,

I just finished a cylinder rebuild on a Stihl 025. I got everything back together and fired it up and was tuning it in. It died, and I couldn't get it to fire off at all. I pulled the plug and turned it over and I had spark. I put the plug back in the cylinder, and it won't fire. I grabbed a plug tester and I get no spark to the sparkplug when it is installed. The plug tester does light up if I ground the end to the cylinder. I have the same result with a different plug.

Is it a bad connection between the cylinder and plug? Is there something wrong with the ignition circuit? Would compression affect spark? Do I have two bad plugs?

Thanks for helping a guy out.

 

[Arnaud91]

Hi,
Check the connector in the boot, sometimes the hook side is separated from the spring side due to vibrations or while pulling, twist before to avoid that.

 

[HarleyT]

Did you get all of the grounding wires hooked back up correctly?

 

[Poodle_Bruce]

Hi,
Check the connector in the boot, sometimes the hook side is separated from the spring side due to vibrations or while pulling, twist before to avoid that.

That is a good thought. I will pull the boot off and take a look

 

[Poodle_Bruce]

Did you get all of the grounding wires hooked back up correctly?

I think so... I only took off the two wires connected to the cylinder.

 

[The Stig]

Are you using a new plug ?

 

[Woodslasher]

I had this happen with two saws so I asked a local saw guy who's been fixing them since Pioneer, McCulloch, and Homelite ruled the woods, he told me the coils were bad. Sure as shootin', I put a new coil on one and it fired right up. According to him, it might be good enough to make a blinky bulb flash but it's too weak to produce spark under compression.

 

[Poodle_Bruce]

I had this happen with two saws so I asked a local saw guy who's been fixing them since Pioneer, McCulloch, and Homelite ruled the woods, he told me the coils were bad. Sure as shootin', I put a new coil on one and it fired right up. According to him, it might be good enough to make a blinky bulb flash but it's too weak to produce spark under compression.

Interesting! That is easy enough to replace.

 

[Poodle_Bruce]

Are you using a new plug ?

Yes. I picked up a NGK

 

[The Stig]

Yes. I picked up an NGK

Is the switch ON ? Is there a coil gap adjustment ?

 

[Poodle_Bruce]

Is the switch ON ? Is there a coil gap adjustment ?

Yes the switch is on. Good to check! I have pulled on a saw with the switch off for an embarrassingly long time.

I can adjust the coil. I had not because it was working. Do you have an adjustment recommendation? As close as possible without touching?

 

[J D]

I can adjust the coil. I had not because it was working. Do you have an adjustment recommendation? As close as possible without touching?

Slide a standard business card between the coil & flywheel magnets, let them clamp together then do the bolts up... gap will be spot on

 

[stihltech]

This is why I do not use the inline spark tester with a light.. Use a good sparkplug with the side electrode removed.and shell grounded. If it sparks that, the coil is good. Sorted out many a weak spark that way.

 

[bobsbigsaw]

There is a dozen things that can go bad or just not quite right.
1. The coil winding can go bad inside.
2. The connection from the coil to the spark plug wire can be loose, so it fires sometimes.
3. The plug wire can be wet or oily and the sparks leaks into the grounding of the case.
4. The plug wire is old and has cracks, so it looses its insulation value.
5. The connection between the spark plug wire and the spark plug can be loose same as #2.
6. The spark plug can have a carbon buildup of oil residue, causing a short internally to the spark plug.
7. The gap of the spark plug has to be set correctly, sometimes better to set it smaller to get it started and diagnosed.
8. If the pulling of the cord is too slow, the coil doesn't make enough voltage to fire the plug.
9. The gap between the flywheel and the coil has to be set correctly or a close as possible without hitting the flywheel to make maximum voltage.
10. It takes much more voltage to fire a spark plug in a high compression engine than it does in open air, so use the decompression buttons.
11. Check the ground wire on the coil.

When possible, I use NGK spark plugs and wires on all my equipment. They run better, even on my old Chevy.

I guess its not quite a dozen things.

 

[davidprivett]

speaking of spark plugs has anyone had good luck with the autolite extreme start iridum plugs? I picked up one and put it in my 610 mac it seems like it has better throddle response and picked up some top end rpm. Or am I dreaming?

 

[Poodle_Bruce]

I took apart the spark plug wire. The clip was loose, so I twisted and squished it tighter, and mounted it 90 degrees from the old connection point on the wire. The boot was a bear to wrangle back in place.

I looked at adjusting the coil to the flywheel, but it was perfectly placed already.

I took a crappy no-name spark-plug that worked and trimmed off the arm. The saw had no problem making sparks with my test plug. I put the real plug back in the saw and it immediately fired.

I did order a cheapo replacement coil before I took the plug wire apart. I'll keep it as a spare.

Thanks to everyone for the help!

 

[Tomos770]

. The boot was a bear to wrangle back in place.

Silicone spray....wd40...ect

 

[bobsbigsaw]

glad to see it all runs.