Fitting Dyna Coils under tank?! 1980 750-4

I'd get a multimeter and do some checking. At this point, we don't know if it A.C or D.C, so start at a high A.C. scale and come down. Maybe 200 VAC(Volts, Alternating Current), if no luck, change to 200 VDC(Volts, Direct current). Put the Black probe on either the Negative battery post and the Red probe on the area where the spark was noticed.

Possibly a coil primary wire is touching something under the tank(but no blown fuses?).

Nope, no blown fuses. Like I said, just a little bitty spark. Primary coil wire being the small 14 ga or so wire feeding the coil, right? Wouldn't this cause major issues?

Could it somehow be a leaking current from the coil or wires going to the spark plugs?

Electrical mysteries can be a hair pulling time, as why it happens sometime defies logic. It might be a wire rubbing through the insulation, or an electrical connector has cracked letting other pins touch, energizing the wrong circuits.

To try to isolate it if possible, pull the fuses one at a time, until the spark stops.

I just find it odd that it has enough power to basically energize the head/cylinders, but not enough to cause any running issues.

Once during a ride when the bike was seemingly running along okay, needed to switch over to reserve, and when grasping the petcock lever, received a mild electrical shock -- more of a steady buzz -- coming through the lever.

Am guessing the bike's frame was being "charged" by voltage leaking through old hard cracked deteriorated spark plug wires, and with body in normal riding position being insulated from the frame via rubber handgrips, seat and footpegs.

New Dyna plug wires (together with new Dyna ignition coils) fully resolved the issue, which hasn't since reoccurred.

Am thinking that such high tension leakage does result in a somewhat weaker than normal spark.

Good Fortune!

I'd wish that was the cause. I just got new dyna wires and ignition coils....

The coil isn't touching anything (shouldn't the coil be insulated anyway?) and the wires are touching the cam cover in a few places, but everything looks fine.

If you suspect a leak in the secondary ignition, even with new parts, spray some water around the wires etc. It will jump to ground and the spark would be both audible and visual.
I would do like Mfolks suggested and test for a surface charge on your frame.
I saw a older Chevy pick up leak to the body, the spark would jump 6" after about 5 minutes. The customer found out about it at the drive through, shocked the teenager working at Macca's.

The water thing sounds like a good idea. I just hope the charge is big enough, because it was so tiny the first time and I couldn't get it to do it again. Now, I'm not sure if it matters, but it was my APE cam tensioner that discharged, so it was from the head. I did open my gap on my plugs to .045 per Otto. Not sure if this is related. Hell, this thing might have been doing this longer than just post dyna coils install. Could this problem be what made my stock coils go out?

ok, just hooked my fluke up... black lead on negative post of battery and red lead to the ape chain tensioner (where I saw the spark). bike off, nothing. key on, 0.032 VDC. Nothing AC (obviously). bike running, 0.003 VDC Switch over to AC now it's running and it's 0.2 mVAC. If I rev it up (or it would once in a while do it by itself), I could get 1 mVAC.

So...no smoking gun. damnit.

It might be caused by inductance, as it's a very small amount of current. The ignition coils get pulsating D.C. to fire the sparkplugs. I don't think it's too much to worry about.

Ok, to add to your comment, after I first installed them and cranked it up, there was a small metal shaving sitting on top of the mounting bolt of the coil. While cranking, it aligned (with the magnetic field) and kinda moved around... so, does this sound like anything bad?

Nope, just usual magnetic attraction going on.