Tracking a miss - check my work

Recently developed a slight, intermittent miss at light, steady throttle on my 74 BMW 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?

How old is the distributer cap? Any moisture in the cap at all? Woops, just saw where you say the cap is clean. No fine cracks?

Nope, none. No carbon traces either. Just the usual light marks on the interior contacts.

Points clean, not worn, gapped right? Condenser? Due for an Italian tune up? I'm all out of ideas :)

No points - they've been replaced with the Pertronix ignitor, which is a magnetic trigger setup. It's not impossible that this is starting to fail somehow, but I don't think there's a way to test it. I carry spare points and condenser in the tool box just in case, but it's worked flawlessly for many years.

Voltage at coil low ?

My understanding - and it may be imperfect - is that the coil doesn't need (or want) full system voltage, hence the resistance wire. I do not, however, have a spec for operating voltage measured at the coil, but the resistance wire that delivers voltage to it is within spec.

Hairline rotor cap crack?

Well, one simple thing you could do to give you a better idea if it is in fact an ignition problem is to temporarily make all the plug gaps tiny and see if the symptom goes away. 

Not that i think this is the problem area or highly relevant but voltage drops proportional to resistance in a circuit, so if you have two 1.4 ohm or otherwise equivalent resistances in a series circuit, they will each drop half the voltage, so you'd see 6 volts in between them on a 12v circuit. That would be hard to check on a coil circuit though because it's technically an open circuit most of the time and would show 12v everywhere up to the ignitor except for the milliseconds of dwell time that the coil primary circuit is actually complete in each cycle. Probably need a scope to measure that voltage while running. Luckily, i don't think it's important!

Pull into a dark garage, pop the hood, start it up and hit it with a spray bottle.  If you get a light show.....

 

Pull into a dark garage, pop the hood, start it up and hit it with a spray bottle.  If you get a light show.....

Good idear. Also reminds me, another thing you can do is take a long screwdriver or metal rod and jumper it to your battery negative. Now you have a grounded probey thing. You can run the tip of it along all the plug wires and if you ever get a spark out to it, or are able to cause a miss, then you found a problem with your high voltage insulation (plug wires). 

partial blockage of main jet in carb?  idles OK because main jet doesn't feed idle.   WOTs OK because power enrichment circuit and accelerator pump OK.  stumbles at constant part throttle because main jet.

Pulled the jets out of the carbs (a five-minute job on DCOEs) and made sure they were clear (probe and blow-through). Didn't see any visible blockage. I'll drive it around and see if the miss reoccurs.

In reply to 02Pilot :

back when carbs were a thing, i'd hear people talk about how complicated webers were.   i, OTOH, didn't understand how carbs worked until i read a weber tuning manual.

I've heard people complain about the Pertronix and replacing them. Then there are the people that don't seem to every have a problem with them.

I have a Pertronix 1843 sitting in my tool box that no longer has a home. PM me if you're interested in it at a discounted price since I tried to install it and realized I didn't have the Bosch dizzy.

What about a pressure test on teh cooling system?  What if you are getting slight amounts of water in the engine?  It might not show up at idle due to lower load, and not at WOT if it is not enough to really mess with the AFRs.

Just a random outside the box thought....

I'm most familiar with sidedraft Webers, and while it took a bit to understand how all the parts work together, they really are very logical. Tuning requires some time, as everything can be changed - right down to the diameter of the throats - but it also means you can get them pretty close to perfect for almost any setup.

Thanks for the offer on the Pertronix, Ross. I'd have to look and see which model my car uses.

I don't think it's a head gasket issue. I'm pretty sure the plug in any cylinder getting coolant in it would be shiny clean compared with the others, and they are all uniform.

The spray bottle trick is worth trying as is the grounded probe (I use a 12v test light for a probe) but I've lost track of how many running issues I've fixed by removing Pertronix pickups.

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.

I'm no carb expert but would this be one of the times where revving the engine up and then slapping your hand over the carb inlet would help? I've seen it done many times, never done it myself! Seems freaky! But the progression slot/hole thing seems likely. And now you have a spare known-good distrib setup!