Let's test your ignition skills

[frit91]

I am working on a four wheeler for a guy and am kinda confused, dumbfounded, and slightly amused all at the same time. Let me explain. It showed up with an intermittent spark issue, and did not run, which turned out to be a bad kill switch. To verify this I unplugged the kill switch wire from the cdi and It had spark again. the issue I have is that with a plug installed in the head it has no spark. Verified by a functioning spark tester. Remove the plug and ground it against the head and it has spark. Install a plug in the head and ground a 2nd plug with plug wire attached to the head and no spark. remove 1st plug from hole and it has spark again. WIth the wire connected back to the cdi and a plug installed and the bad switch set to off if I hit the start button the plug fires once. Same deal but with the kill switch set to run absolutely no fire. I know that the switch is bad, but with it removed from the equation and the spark plug firing outside the head, why will it not fire inside the head? Tested coil it is good but replaced with a new one anyway. Stator tested out okay. Either a cdi works or it dont so it is good. Let me clarify that this thing didnt run when I got it dropped off. Could the bad switch be causing a ground issue or a draw that screws with the spark? I am waiting on a new switch to be delivered. Also I changed the ground from the battery to the frame and from the engine to the frame so I know those are good also. I know this was a long winded post but I am just befuddled. Any ideas will be considered and a fix will be happily accepted. And no...setting it on fire is not a fix.

 

[moby]

when it's sparking outside the head, is the spark a bright blue (strong spark) or a dull yellow/orange (weak spark)? a weak spark most of the times won't spark under the compression of the engine I'd be leaning towards a bad cdi. the capacitor is supposed to build up a charge and discharge it for ignition.

 

[Aviation-nut]

We saw this a lot with coil packs on Briggs & Statton units. A weak coil pack would loose spart when warm, but if you removed the plug from the cyclinder it would fire. Under compression the weak coil pack could not do it. Heat was a big destroyer with many of these units. No one would clean the unit from grass clippings then the coil pack would start to fail from excessive heat.

 

[frit91]

when it's sparking outside the head, is the spark a bright blue (strong spark) or a dull yellow/orange (weak spark)? a weak spark most of the times won't spark under the compression of the engine I'd be leaning towards a bad cdi. the capacitor is supposed to build up a charge and discharge it for ignition.

It's a good spark. What gets me is the fact that a plug grounded to the head will spark but take another plug and screw it into the head and the plug grounded to the head stops firing. Unscrew the plug from the head and the other plug grounded to the head will fire. I replaced the coil and tested the new coil before install and it ohmed out in the proper range so I assume that it is good. I am leaning towards the kill switch being the problem. WIth all wires connected and the plug installed in the head and the kill switch off the plug will fire one time when I push the start button. no spark when in the run position. I think the switch is grounding out somehow or I hope so at least. I will find out when I get the new switch. And to be clear I know the plug fires once because I am using a spark tester. I have seen a plug fire outside of the head and not fire installed due to weak spark but never seen what happens when doing what I just described with 2 plugs. I guess its the switch since it fires when it is not suppose to and dont fire when it should.