HELP! Stihl KM90 (4 Mix engine) no spark

[Crazykartman]

I’m an experienced OPE mechanic but I’ve never run across this issue and I’m stumped. Was given this machine that won’t start, no spark. The strange thing is I get spark in my spark tester if I ground the end of the tester to the block, but I get no spark from it when it’s attached to the plug. Put a brand new OEM Bosch plug in and still no spark. What am I not seeing?

Thanks all!

 

[Firewood Hoarder]

Does the flywheel to coil air gap measure right? Flywheel magnets feel strong? Do you get spark if you close the plug gap to almost nothing?

Is your spark tester the lamp style, or does it have an air gap the spark must jump?

I would consider replacing the coil.

 

[Crazykartman]

Does the flywheel to coil air gap measure right? Flywheel magnets feel strong? Do you get spark if you close the plug gap to almost nothing?

Is your spark tester the lamp style, or does it have an air gap the spark must jump?

I would consider replacing the coil.

Well you called it. I was using a light bulb style spark tester. Are those less reliable in your experience? I’ve never run into an issue using one.

I smashed the plug electrode and did get a small spark. I purchase an aftermarket coil and it runs great! It just goes to show there is always more to learn.

Thanks!

 

[Firewood Hoarder]

Glad you got it sorted out!

I've seen it a couple times where the lamp style spark tester will illuminate, but the actual secondary current made by the coil won't jump a gap.

I no longer use a spark tester because of it; I just pull the plug and start pulling the recoil while watching the plug gap for signs of life