I think it's a stock duraspark II. I don't know much about wiring. I don't remember doing any wiring on this other than when I moved where the choke power was coming from several months ago.

Yesterday after I hooked the fuel line back up the engine cranked aggressively for about 10 seconds to prime the carb. It started fine every time after that.

 

Where did you hook up the choke power? If you connected it to the ignition that could be the root of the problem?

I have the 1981 EVTM on my web site, and here's a link to the ignition wiring page: Start, Ignition, & Carb Circuits - ???Gary's Garagemahal

 

Looks like in early November I switched the choke from a 12 volt wire to the s terminal wire on the alt .

 

That shouldn't cause any problems then. I would put a volt meter on the battery and see what happens when you pull the lights on. Then do the same on the ignition coil.

 

Where do I put the leads on the coil? How do I test it?

 

That's a really, REALLY good question. I hadn't thought through that part, but am now....

I've included a marked-up shot from the EVTM on my web site. The red circle is power to the ignition module, and you should check there, both with the headlights off and on - just as you do on the battery.

The green circle is where I was thinking you should check on the coil. HOWEVER, that is likely to be a problem since what you see depends on what meter you are using. The issue is that the true voltage there is going to be all over the map. When the ignition module takes the Tach side to ground, simulating the ignition points closing, the voltage on the Battery side of the coil should go to ~7 or 8 volts due to the 1.1 ohm resistance wire in the circuit. And, when the ignition module releases the coil, as if the points open, the voltage should go to battery voltage, which should be about 14 volts. But, most meters won't follow that rapidly changing voltage and what yours gives is a mystery. Plus, there's the ringing voltage from the coil. So, I don't know what to expect, but please run that test also to see what it shows. And, don't be surprised if it is so all over the map that you can't read it.

 

How do I test at the ignition box with out stripping the wires?

I tested the battery with an analog meter. Right around 14 volts when running with and without lights on. No noticeable difference.

I tested at the coil as you showed and I think I'm getting an average value. With the lights off, I got about 11.2 which is right between 8 and 14 as you explained above. With the lights on, it showed slightly lower, right at eleven. I don't know if that's significant.

I did notice once the engine was warm and idling nicely, once the light switch was pulled, the idle would change subtly.

Thanks for all the help

 

You can push a straight pin through the insulation and when you pull it the insulation will reseal.

I just checked on Rusty, and with the key on but the engine not running I had 6.8v at the Bat terminal of the coil. That shows that the resistor is working. Can you do that test? And then pull the lights on. I did that and basically saw only a slight difference, but I have a relay kit on Rusty so all of the current draw for the lights is right off the battery.

 

Yes, poke a straight pin into the insulation, seal it up with RTV or some such (clean it, first).

 

With the key on but the engine off, I had about 6.2 V at the Batt terminal on the coil. With the key on, engine off, lights on, I read about 6.2 V.

With the engine off, key on: Red wire to the ignition module read 11.2 V
With engine running, lights off: red wire to ignition mod read 14.2 V
With engine running, lights on: red wire to ignition mod read 12.9 V

Definite drop with the lights on. Is this expected?

 

The readings at the coil are fine. Maybe a little lower than I would have expected, but different meters and a different resistor/coil combo can explain that.

Further, your voltage at the ignition module with the engine running and lights off shows the alternator is working well. You might check the battery voltage under that condition, but it is probably at 14.4v, which is right where it should be. And, a .2v drop from the battery through the ignition switch and to the module isn't bad.

And, the 12.9v with the headlights on is probably fine since I'll bet the battery voltage came down to about 13.0 then as well. That just says the alternator isn't throwing as much current at idle as the headlights are pulling. Check your battery voltage at idle with the lights on, which will prove my theory.

So, we don't have a smoking gun. But, I'm suspicious that something is pulling the voltage to the coil down when you turn the lights on. Granted, the voltage didn't come down at all when you pulled the lights on but with the engine off. However, if something pulled the voltage down when running that could cause a problem.

You can't put the meter on the coil with the engine running and get a reasonable reading. So, if you want to test it you'll have to run a jumper from the battery to the Battery terminal on the coil. You won't want to do that very long, maybe just a couple of minutes, but if you had the jumper on and then pulled the lights on it would prove whether low voltage to the coil is the problem.

 

When I test at the battery with the engine running I see about 14 volts when the lights are OR off.

I don't understand the jumper test you are talking about.

 

I think you mean "on or off". Right?

The jumper test will be as follows:

1. Start engine
2. Put a jumper wire from the positive battery post to the Batt terminal on the coil
3. Go for a quick drive
4. Pull on the lights to see if the engine sags, surges, or otherwise falters
5. Stop and remove the jumper w/in two or three minutes of adding it to ensure the coil doesn't get too hot
6. Redo test w/o jumper to prove that the engine does sag, surge, or otherwise falter when the lights are turned on

If the engine does falter then we know something with the lights is pulling the voltage to the coil down while the engine is running.

 

You are correct. I meant lights on or off.

So we are hoping to experience no sagging or misfires when the jumper wire is on with a directly supplied voltage from the battery?

 

Yes, and no. We are trying to determine what the problem is by a process of elimination. Placing the jumper on bypasses all of the wiring between the battery post, through the ignition switch and resistor, to the coil. If in this test we still have the problem we know it isn't in that part of the "system".