Oscilloscope Traces

[Jetaway]

Greetings,

Background:

Inspiration: A couple of weeks ago I posted a plea for help in the Engine & Drivetrains forum under "Plug Reads?" Much help was given and since then I have replaced the plugs, wires, distributor cap and rotor. Running much smoother since then. But because a big part of my troubles was due, I think, to the plug wires wearing out, not because of age, but because of physical handling, I wondered if there was an inexpensive, non-destructive way of getting a handle on the health of my ignition system. Which leads to:

Knowledge of Oscilloscopes: While not 100% accurate, my knowledge concerning the use of oscilloscopes is well approximated by Sgt. Schultz of Hogan's Heros fame: "I know nothing, nothing!"

I mean, I know they plot voltage (vertical) against time (horizontal), that the scale is adjustable by turning knobs and that each square of the grid represents the voltage and time displayed on the bottom of the screen. Beyond that, not much beyond the on/off button seems to turn it on and off.

The scope is a Tektronix 2465 that I picked up at a garage sale for between $35 and $50 a few years ago. It has not been calibrated or indeed has had any sort of servicing done since I bought it. It does, however, appear accurate in the time dimension when tested with an audio generator. I tested voltage with a D-cell (new and tested with a multimeter) and the voltage, at least up to 1.6 volts, is correct.

The pickup I'm using for these traces is an open reel head made by no idea and of unknown vintage. As a guess -- cheap and mid 1960s. It is held in place on the plug wires with a small C-clamp, which now that I think of it, may be a perfectly serviceable inductor in it's own right.

The Car: 1972 240Z, but has an early ZX distributor (E12-80 ignitor model) which has been further modified by replacing the dying E12-80 with a GM HEI style ignitor. A cheap one at that, cheapest Carquest model at the time. Replaced the existing Nissan coil with an Accel coil, mainly because Nissan's coil - terminal was physically decaying. That and yellow is such a groovy color.

Questions:

General:

The big one is am I simply wasting my time with this oscilloscope? The traces I've gotten aren't anything like the neat, precise lines I've seen on this site and in references from here to outside resources. I'm guessing (see Knowledge of Oscilloscopes above) that part of the problem is that newer, digital scopes can separate the display from the measurement units in a way that an old analog scope cannot.

If the scope is indeed useful, am I out of my mind using a tape deck head as an inductor?

Specific:

I've attached 3 pictures taken of the scope's screen. In the lower right hand corner of the scope I labeled which wire was being tested. C= Distributor to coil wire and the other two should be self-explanatory. I would have taken more, but the pictures got worse, that is, only the 'flash' could be seen as the sun continued moving to the west. Memo to self: Start earlier in the day.

I know that the voltages won't mean a thing by themselves without calibration (not going to happen) and consistent positioning (working on it), but I am hoping that a comparison between wires will be of use at spotting potential misfires.

Tried to keep the engine at 1000 rpm, but it does drift a bit.

Preface all statements with: I think, as far as I know, etc.

The trace Z...-2 is of the distributor to coil wire. I'm pretty sure that what is seen is two firings of the ignition coil. The oscillations after the peak voltage are normal, as is upside down U right before the peak voltage.

I'm puzzled by two things: The first is the difference between the peak voltages. Looks to be about 1 and 1/2 grids, or 30% to 40%. The second is the secondary peak between the peak fires. I'm pretty sure that its not an artifact of the photo, as I saw it consistently, and, looking at Z...-14 which is a trace from the distributor to the #1 cylinder shows a similar secondary peak.

Z...-14 also brings up another puzzler, with the measured voltage about 4 times as large as the coil to distributor's voltage. Could be inconsistent placement of the pickup, though it still seems large. OTOH, the overall shape and timing of deviations from zero appear very similar to the coil / dizz trace.

I deleted one attachment and substituted a second of the distributor to coil trace. It's not a close-up of a fire, rather it was taken with scope taking twice as many measures per unit of time (twice the speed? Half the speed? Don't know the vernacular) than in the first picture. Whatever, it more clearly shows the mysterious (to me, at least) secondary peak.

Any comments, interpretations, or suggestions, including those concerning my measurement regime, are appreciated.

Chris