Piston Slap: Highway resistance from a misfiring Falcon

Stephen writes:

Sajeev,

Several years ago I wrote you about my ’65 Falcon convertible, asking for options to re-do my suspension. I took your advice and put only upgraded factory stuff on, basically Opentracker A-arms and lower control arms with Bilstein shocks. It’s a very comfortable ride, but now I have a new problem.

When I am driving down the road at 65–70 mph in fifth gear (T-5 swap), the engine loses power and I hear a spitting type sound (fft, fft). If I downshift to fourth, the power seems to come back. This only occurs after the car has been running for 5–10 minutes at 65–70. The car has a 289, Autolite four-barrel carburetor, Mallory distributor, new generic NAPA coil, fresh plugs/wires, and I am using a ballast resistor.

Sajeev answers:

It’s wonderful to hear you chose/enjoy my suggested suspension modifications, as it’s usually in the owner’s best interest to stick with a factory-style suspension, but with upgraded shocks. Aftermarket designs normally include polyurethane bushings and stiff springs for a maximum wow + gotta have it factor, but they are only great for racing or on really smooth roads. Factory stuff with rubber bushings and better shocks are usually the right answer, especially in a not-so-rigid vehicle like your roofless Falcon.

Back to your misfire: I’ve experienced this with my Fox Body Cougar. My source was the 29-year-old ignition coil biting the dust—apparently accelerating at highway speeds without downshifting out of overdrive exposes a coil’s weakness. But maybe your new coil is fine; perhaps it has an internal ballast resistor (like this one) and you therefore don’t need external one?

Ballast resistors are a crucial part of an ignition system; they regulate current much like a fuel pressure regulator does for a fuel system. If there’s too much current, the ballast resistor adds more resistance, and vice versa.  So I’m still concerned you have two of them, and as they heat up they reduce output enough to trigger the misfire on the highway. Look up the part number of the NAPA coil to see if it has an internal resistor, or ask the savvy folks at NAPA.

I’ll admit that older ignition systems aren’t in my wheelhouse, so here’s where our readers’ input becomes crucial: What do you think is causing the misfire in Stephen’s Falcon?