Tracking a miss - check my work

[02Pilot]

Recently developed a slight, intermittent miss at light, steady throttle on my 74 2002. Idle OK, WOT OK, so naturally I'm looking at ignition first. I haven't found anything that seems overtly wrong, but I could use some extra eyes on it. Have a look and see if I've missed anything.


Centrifugal advance distributor, Pertronix, stock cap & rotor, correct NGK plugs, Bosch wires & red coil, resistance wire intact, no ballast

- Plug appearance consistent, gaps good.
- Plug wires resistance consistent.
- Cap & rotor clean, no measured resistance.

- (Meter resistance 0.4 ohm - values below are corrected)
- Resistance wire 1.4 ohm
- Coil primary resistance 1.4 ohm
- Coil secondary resistance 10k ohm
- System operating voltage 13.8v
- Voltage at coil + at idle 10.5v
- Voltage at coil + running 11.8v

Nothing seems wrong to me. The coil is not overheating, no wires melting. The one thing I cannot pin down is the resistance spec for the Bosch coil (p/n 0221119050). I've found listings for the 030 red coil at 1.6-1.8 ohm primary resistance, but nothing for the 050. Nothing has been changed in the ignition for several years and thousands of miles while the miss is new in the last few weeks, so it's clearly something failing rather than incompatibility. Any thoughts?

[bmwdirtracer]

First, a disclaimer: I'm quite unfamiliar with carbureted, pre-Motronic cars.

But, my friend, I get hunches, even on the old ones. My hunch says carburetion, not ignition.

That said, the only suggestion I could possibly offer for fixing it involves a computer, an O2 sensor and 4 injectors.

[02Pilot]

In my coal-black heart, I'm an unapologetic Luddite.

I have not ruled out the possibility that I have some sort of debris clogging a progression hole in one of the carb throats (dual Weber 40 DCOEs, so one throat per cylinder). The miss is subtle enough that it could just be one cylinder that's causing it. I figured I'd chase down the ignition stuff first, as I have plans to pull and rebuild both carbs (plus port-match the intake) this winter. There's still a couple months of driving season left, so I was hoping not to have to rip everything apart sooner than planned.

[bmwdirtracer]

Oh, I too, am a confirmed Luddite, my friend.....it's just that back when I should have been learning about carburetors and points ignition, I was too busy just driving stuff until I broke it. I became a mechanic much later in life, after having been a merchant banker, restaurant owner, whitewater river guide, professional racing instructor, and a few others. By the time I learned what I was doing, carburetors and points had gone away. (Thank God)

And hell, didn't I see "Pertronix" up there somewhere? Ahem, Mr Luddite. (Mr. Luddite who owns a 2009, I might add)

[02Pilot]

How dare you contradict my overly-general declaration by using evidence! And evidence I provided, no less! Of all the unmitigated gall!

But, yes, well, technology has its place. I use it, and I even understand it a little bit, but I don't like it.

[ross1]

I also suspect carburation. I'm not familiar with your Webers so cannot be any more specific.
How do the plugs read?

[02Pilot]

Uniformly a bit rich, which is pretty typical for sidedraft Webers in my experience. The only time they seemed to clean up fully was after an autox or track event.

[ross1]

Uniformly a bit rich, which is pretty typical for sidedraft Webers in my experience. The only time they seemed to clean up fully was after an autox or track event.

Too fat can cause a misfire too. Hotter plug perhaps...??

[02Pilot]

OK, update. After dip-tank cleaning all the bits, I rebuilt my spare 002 distributor (same as the one in the car) to dead-on .005 axial play. Installed with new cap, rotor, points, condenser, and BP5ES; all the same as what was there except they sent me a 1K rotor instead of the 5K that was in there. Red coil is only a few years old, as are the wires.

Power actually seems a little better (probably just the placebo effect), but the miss (or perhaps better described as hesitation) is still there under the same conditions. At this point I'm calling it a fuel problem. I'll suck it up for a month or two, then yank the carbs and rebuild them over the winter. I'm guessing one of the progression holes is plugged.