Coil Issues

[sixt9x33rs]

I wanted to share a problem that I had and the end result to fix. I had a misfire at 4500 rpm seemed like a lean problem. So I checked the carb, and ended up rebuilding it which was not the problem. Then changed the fuel pump which resulted in me removing the smog pump to get to the fuel pump. Still not the problem. So I changed out the old 293 coil with a Mallory coil that I have had for 24 years. Bingo bad coil. Since I did not have the correct number coil, I ordered a repo coil with the "298" embossed on the part, correct for a 69 Z28. I thought new coil should take care of the problem. Nope, same misfire at 4500 rpm. Put the old Mallory coil ran great. I thought maybe my plug wires had aged so put in good relatively new wires with the new 298 coil...misfire. So this told me the coil was bad for sure. So put the Mallory coil back in the car, and then put the crappy "correct" date coded Packard wires back on. Car runs great now. But previously I had the old 293 coil on the car with the Packard wires and the car ran fine. Just over the time has this problem surfaced. So I am wondering if I still have a problem masked by the better high output coil?

[KevinW]

I had a similar issue, random misfires. Changed a lot, but they went away when I put in a Pertronix flamethrower coil (the black one and I removed the label).  works much better now.  The new coil is a tad bigger in diameter, so I had to get a longer clamp bolt

[L78 steve]

Its probably the coil. If you test a stock dist. with points on a Sun machine the points will start to bounce at around 4500rpm.
This causes a weak waste spark ATDC and a weak coil cannot recover from this.
With a strong coil this waste spark has no effect on performance.
This can be clearly seen on the Sun Dist. scope.

[sixt9x33rs]

Its probably the coil. If you test a stock dist. with points on a Sun machine the points will start to bounce at around 4500rpm.
This causes a weak waste spark ATDC and a weak coil cannot recover from this.
With a strong coil this waste spark has no effect on performance.
This can be clearly seen on the Sun Dist. scope.

Great info thanks. I bet the points are bouncing. No idea how old they are.

[69z27z87]

I also had issues with a 293 repro coil.... once the engine had some heat in it, if I put it in gear (auto trans), the engine died. Tried fiddling with the carb settings but nothing made any difference. Put the AC Delco service replacement coil back in... problem goes away....

Guess the 293 coil is just for show.....

[Kelley W King]

In the early 70,s we only had points and crappy wires. If you drove your car on the street a while the misfire around 4500 started. We called it (busting out). Fresh plugs, and I mean fresh would straighten it out. We carried a new set for money street races. Someone had a spark plug sandblaster that helped a lot, small unit with a grommet hole for the plug. Then came Accel points and the big yellow Accel super coil the hung on the firewall. You also have to remember the tune up intervals when cars had points. When HEI started the selling point on new cars was no tune up for 20,000 miles. I think some guys might have weak coils but some just have original equipment old cars.

[169INDY]

FWIW; I have experienced perfectly good LOOKING plug wires that were breaking down with a increase of temperature, Thermal breakdown, the OHM readings were within spec on a cold motor, but after some heat development and soaked in,  the wires were the problem, fresh set and off we went. 18 y/o Restoration wire may look nice but silently failing.

JIM