Motoscrubs.com

An extension of my previous post: I have tested all the components of my ignition system to the best of my ability (1965 Mk3 with AC system, no battery 40 watt flywheel mag).

Kicking through, the mag delivers +/- 6VAC to the coil. Ohmage at the coil - 4.5 between the two low voltage poles, +/- 11.4K ohms from HT to LT. Specs are 4 and 9K respectively. Points are good, condenser is not shorted.
I have no spark at the plug.

So - is my coil bad? I do not have a spare AC coil to swap in, and new ones seem to be pricey (if available here in the US) so I'd rather not try replacing it if there is something else I am missing. I do have 6VDC spares, but no AC.

How can I test this coil on the bench to confirm if it's bad? And if I throw up my hands, is there a sensible electronic ignition as an alternative?

To remind of the symptoms, before the fire went out entirely it started in one kick but I was getting missfiring at higher rpms.

I know little about the AC ignition system, but a couple of thoughts....

I think the flywheel needs to be timed so that the maximum voltage is at the right time when the points open on AC systems. Any chance the flywheel has moved on the crank?

Can you test the AC coil with a DC supply temporarily? Either on the bike or on the bench. You can flick the points open and see if you get a spark.

Have you tried a different plug? I have had plugs fail suddenly.

Ian

Thanks, Ranton.
I had the flywheel position marked after removing it years ago, and it does not appear to have moved.
I have tried different spark plugs to no avail.
I tried a DC coil and got nothing.

My analog meter shows 6V supplied to the coil when I spin the crankshaft. But when the wire is actually connected to the coil, I get just the slightest "blip" on the voltmeter at that connection. For whatever that means.

try without condenser, just to make sure it is working as intended.

Yeah, I did, Roland. No joy.

I have now tested the coil on the bench by introducing 6VDC, and I get a spark.
Back to basics; I will check the flywheel position again. My manual says "32 - 36" BTDC but I think I read somewhere that 28 was correct for one system and 38 for another. Can anyone verify the correct position on an AC 40 watt system?

I don't know the correct value, but i think there is something mixed up here.

How is the flywheel located on the crank? Is there a woodruff key for the correct angle relative wheel and crank?
If not, read again 2nd post by Ian.
AC-voltage must be in correct phase at timing point to insure max energy source.

No, there is no woodruff key to locate the flywheel. It's a simple friction fit on the taper. There is a slash mark on the face of the flywheel, and that is supposed to be oriented x degrees off TDC. My question was: what is x? In my manual the spec is 32-36 degrees, but I read elsewhere (don't recall where) that the angle should be 28 or 38 depending on the other specs of the engine.
Lacking further clarification I will install it with the slash mark at +/- 34 degrees to the best of my ability.

To complicate things, I found a thread elsewhere on Motoscrubs from about 15 years ago in which it is said that there seems little or no consistency in the location of that slash mark relative to the magnets in the flywheel, hence location the flywheel to orient the slash mark per specs is a crap shoot.

I have tested all the components in the system, found nothing untoward, and hopefully when I reassemble it today I'll have functioning ignition system. If not...

For engine before 88295 the flywheel should be set at 19-21 degrees and after 32-36. Your flywheel might need to be remagnitized.

Thanks, Mrkprsn. My engine number is 100xxx so I'm going with 32-36.
The magnets in the flywheel seem to be pretty strong. If not quite as strong as new, certainly not weak enough to be a problem. It takes some effort to lift the flywheel off a steel table.