Lark ignition

[Cadillac-ack-ack]

Progress is progressing with this motor. A small set-back found me replacing a coil. Not because it was a cracked original, but because as I was installing new points I discovered the green point wire had been re crimped using a hardware store terminal and all but 2 strands were broken. Cutting the connector off and replacing it would have left the wire too short and I did not think I could successfully splice in a new section of wire. Setting points @ .020 got me pretty close and then fine tuning the adjustment using the flywheel and timing marks got me dead-nuts-on. Wrapping a rope once around emergency start groove in the flywheel gave me one nice white spark on one spark plug and one on the other. Before you say anything Racerone, I know about using a tester and I will once I have the starter motor on and the flywheel secure. This was a preliminary test.

Original spec plugs are J4J and replacements are J4C. Only difference I can see is the J4J has a cut back ground electrode. Can not the J4C be effectivly turned into J4J by simply cutting a hair off the ground electrode? Is it better? Maybe because the shape of the piston top effects mixture flow so a cut back electrode exposes the spark better? - Pete

 

[lindy46]

I've always used the "C" plugs as-is and they work fine. I used J6C's in my 1957 Lark as I did a lot of trolling and they were less apt to foul.

 

[F_R]

The purpose of the short electrode was two-fold It helped prevent fouling, not really a problem with today's oils. Also, a spark will more easily jump to a sharp edge than a flat one. That is the reason lightining rods are pointed. The spark "sees" that sharp end of the electrode.

 

[jimmbo]

I've always used the "C" plugs as-is and they work fine. I used J6C's in my 1957 Lark as I did a lot of trolling and they were less apt to foul.

A J6J(now J6C) was the speced plug for a 57 Lark. The J4J(now J4C) came into spec in 1960 when the Lark became a 40hp

 

[Cadillac-ack-ack]

I forgot to mention, what was in the engine when I started on it was a pair of L77JC4's. Though they are too long, piston contact was never made and according Champion's coding, the heat range of 77 is for industrial and special purposes. I have no idea how that compares to automotive heat ranges (what we are using), but can't be 71 numbers hotter! 3+ maybe? I think in the day, dad or a parts guy got confused with the JC4 part of the L77JC4 number and figured it meant it was probably the same as a J4C. - Pete

 

[F_R]

That plug is an extremely "cold" plug. About as cold as you can get without going to surface gaps. It is for Capacitor Discharge ignition systems. I'd be surprised if your 40hp with magneto will run very long on them before they foul. You simply don't have the available spark voltage and short rise time. It was a life saver to us guys that had to make the 1970-71 60hp motors run.

 

[oldboat1]

The purpose of the short electrode was two-fold It helped prevent fouling, not really a problem with today's oils. Also, a spark will more easily jump to a sharp edge than a flat one. That is the reason lightining rods are pointed. The spark "sees" that sharp end of the electrode.

Didn’t know about the sparking difference (or don’t remember knowing….) Suggests the old Js are a good choice when available.

 

[Cadillac-ack-ack]

That plug is an extremely "cold" plug. About as cold as you can get without going to surface gaps. It is for Capacitor Discharge ignition systems. I'd be surprised if your 40hp with magneto will run very long on them before they foul. You simply don't have the available spark voltage and short rise time. It was a life saver to us guys that had to make the 1970-71 60hp motors run.

Well that's interesting. Having a closer look INTO them I see the body is reduced greatly about 1/8" down, almost closing right up against the insulator. - Pete