Proper grounding for SM-IV

[23T Hemmee]

Hi Spud,
I know this is basic but just want to make sure.  Been running a SM3 grounded to an oil pan stud on KB block with big red button kill switch.  Recently bought someones spare SM-4 that you hopped up in 2008 with the big yellow capacitor on the outside.  Since changing over, it seems the mag tries to fire sporadically with the switch off.  Pulled the switch to see if contacts looked bad and they looked brand new, no evidence of arcing anywhere so I don't think the switch is bad.  I see on the notes, at the bottom of the paperwork that you hand wrote to ground both heads to the coil, am assuming these grounds go to the yellow wire side of the coil, but just want to make double sure.  I ran a Vertex for years because it only had 1 wire and even I could figure that one out.......lol. Thanks!!
BTW using 2-wire harness and 28900A redtop if that makes any difference.

[Spud Miller]

 The best ground is going to be to the heads since the plugs are screwed into them and that is the end of the line for the spark.

 If the kill switch is off, is properly grounded and you're still getting a spark occasionally, then the switch is BAD and is indeed arcing inside. Not all switches are cut out to be kill switches. If they are only rated for 12VDC, then the contacts aren't far enough apart inside and voltage is jumping.

 Find a good switch with a 12VDC rating *AND* a rating at 220/240 VAC. Usually, those will work provided they are rated for enough current. I wouldn't waste my time with anything rated less than 20 AMPS @ 220VAC, even for a 3 amp mag.

[slingshot383]

Ground the Mag to the heads, or the coil tot he heads, or both?  And if you ground to the heads, do you still chassis ground the coil?

[Spud Miller]

 Ground the negative side of the coil to one head. Jump the heads together so they're both grounded to the coil. The mag is already grounded to the coil through the yellow wire.

 If you do that and also run two wires to your kill switch (one from each side of the coil), then there's no need to ground the coil to the chassis and all that.

 If you only run the hot wire to the kill switch, then the kill switch won't work if you don't ground the coil to the chassis.

 It's much better to run + and - to the kill switch...then you know it'll work when you need it.

[slingshot383]

So the kill switch needs to be double pole, single throw?

[Spud Miller]

 A single-pole, single throw switch is all that's needed.

 When the switch closes and connects the two terminals, it connects the + and - of the coil together...that grounds it and it's dead.