Ignition system issue

[Tom L.]

Just got my 72 BB back mfrom the shop after a year and it looks great but runs like POO. To rule out the simple things I added a fresh tank of gas, air filter, plugs, cap, Wires have less than 100 miles and finally points and condenser. Upon removal of the points I noticed that they were quite used up. I decided to check the voltage to the coil to be sure that was not the cause. My findings were strange, so here they are:

All voltage measurements taken at the + side of the coil.
1. Ignition on, engine not running- 5.8v
2. Engine running- 12.6v
3. Cranking the engine (coil wire removed to prevent start-up) 8.9v.
4. With the wire that supplies battery voltage to the coil during starting removed, no change

Also, there does not appear to be any extra or missing wires on the coil, on the + side of the coil there is the wire from the ignition switch and wire from the starter. On the - side there is only the wirethat leads to the points in the distributor.

I have 2 questions- Is the resistor wire known to fail in a way that provides an increase in voltage when current is being drawn during engine operatoion? And.. Any other ideas???

By the way, I'm new to the forum...hello to all and have a happy Thanksgiving!

[Timothy B.]

Re: Ignition system issue

Lynn,

Your problem may be with the carburetor if the car sat for some time. If you have new points etc. the ignition is probably OK. Are you sure the plug wires are in the proper position on the cap and firing order is correct.

I don't think a resistance wire will fail unless it's broken by some means.

[Jim T.]

Re: Ignition system issue

The resistor wire is a specific length from the coil and through the wiring harness to the fuse block (engine side). 12 volts connects to the resistor wire in the fuse block.
The only experience I have with the resistor wire is it getting to hot and melting the insulation. Do not remember all the details, happened about 32 years ago, just replaced the resistor wire from the fuse block to the coil, no other problems since then.
Tim has the right solution IMO. The fuel inlet filter could be restricted, build-up inside the carb. A rebuild kit and cleaning of the QJET might fix the problem.

[Tom L.]

Re: Ignition system issue

Thanks for your input. however I think that I did not clearly define the focus of my question. When I described the way that the car ran I did not mention that the problem is intermittent. Also, I forgot to add that i also replaced the fuel filter in the carb.

The focus of my question was really about the voltage isuue. Why would I have 12 volts at the coil while the engine runs but only 5.8 not running?

[Timothy B.]

Re: Ignition system issue

Lynn,

With the engine running the alternator is charging the battery at approx 14 volts so the reading at the coil is higher.

[Jack H.]

Re: Ignition system issue

Excellent point, Lynn! The ignition system is NOT static in terms of B+ voltage. The battery charging system DOES raise battery voltage once the regulator goes active and engages the alternator.

Also, when measuring from the coil's + terminal to ground, you're looking at the COMPOSITE circuit response of the coil primary, ignition points & ignition condensor.... That means you're dealing with a SWITCHED DC circuit (non-linear) having both passive and active (energy storage) elements.

When the engine is running, you've got a dynamic, non-linear, circuit that's solving a second order differential equation producing a VERY complex voltage wave form.

Staring at the display of a multimeter in DC voltage measuring mode is ridiculous! You need an oscilloscope to properly analyze this voltage waveform since its voltage value is complex, changing in the time domain as the points open/close...

[Duke W.]

Re: Ignition system issue

Just got my 72 BB back mfrom the shop after a year and it looks great but runs like POO.

If you could provide just a little more specific information, Jack can solve the second order diffy Q, which will reveal your problem.

Duke

[Jack H.]

Re: Ignition system issue

Thanks for the holiday laugh, Duke! The last time I cranked out a 2nd/3rd order diffy Q solution was 1988... But, it was a LOT easier than doing it by hand as I was forced to do in the 60's & 70's!!!!

That PC math software package (Math CAD) was GREAT! Just drag and drop functional components onto the screen, input their static chararcteristics, interconnect them, apply power and say 'RUN'. Out popped the current & voltage waveforms to any degree of resolution you desired!!!!

Seriously, the 'knock' was an attempt to tell the poster that he was working with a time domain issue trying to force it into a time-invariant (static) issue...he was mixing apples and oranges.

Want to verify and eliminate the spark system as a source of performance troubles? Take the car to a competent garage, have them put the ignition up on their engine analyzer, and you're done in 30-minutes flat....

[Tom L.]

Re: Ignition system issue

Interesting... As I stared at the multimeter in DC mode I did suspect that I was getting a false reading. I even went to see my engine builder about it but as time has gone by he flushed most of his "points' knowlege in favor of electronic ignition systems knowlege.

So if I have this right what you are saying is that the reading I get is peak DC voltage as the condenser discharges Not average voltage in the way that an RMS meter interprets an AC voltage reading.

If this is the case I'll fell better about running the car down the road as it should be. By the way, thanks for leaving out any additional diffy Q, my head may have exploded!

Thanks!

[Jack H.]

Re: Ignition system issue

It depends on your multi-meter... Some of the advanced models provide a true RMS mode (root mean squared) which gives you the actual DC equivalent value of the complex, fluctuating waveform you're measuring.

The 'ordinary' meters simply have a fixed integration period where they measure and report the 'average' DC value they saw in the last measure/report interval.

An analog meter will display some level of needle 'bounce' to indicate the waveform is dynamic and the degree of 'bounce' varies with that meter's damping characteristics.

Bottom line, it's not so easy to say what you saw without knowing what you were using to measure with. But, what I'm saying is you're using a square peg to fill a round hole when you try to use a voltmeter to analyze the ignition system's primary circuit characteristics and infer go/no-go health on a running engine...