1986 740 estate, 4 cyl. petrol, carb. Began missing at speed (60-70mph), now missing at much lower speeds, under load-acceleration, and uphill. Goes like a brick downhill. Miss is erratic, therefore thinking it must be electrical- where to look; coil, ignition module or other?
Justin

The connection at the bottom of the distributor is a fairly common failure point on these cars. I assume you've got the same or at least very similar distributor to our fuel injected models. Had to replace the distributor on my '88 740 due to intermittent failures.
Rob Bareiss, New London, CT

When you load an engine by opening the throttle, the pressure in the combustion chanbers increases. This increases the resistance at the plug gaps. Any marginal component in the high voltage side can fail, causing the stumbling. Most likely it is a plug, a wire, or the cap.

Perform the "midnight test."

Be sure the underhood light is switched off, and switch off your interior light. Find the darkest location possible on the darkest night. Turn all lights off, and allow your eyes to dark-adapt for 10-15 minutes. (This allows you to see the dimmest glimmer of light.)

With the engine idling, open the hood and stare at the ignition components as you manually rev up the engine (turn the throttle spool). Everytime you open the throttle, it increases the combustion pressure. Rev the engine repeatedly.

Any breakdown of the ignition will be (faintly) visible as arcing. You can mist the ignition components with water to make the situation worse -- just a mist, not a flood.
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Don Foster (near Cape Cod, MA)

It's had plugs, leads, gas line anti-freeze/fuel dryer and an air filter. I am starting to think it must be on the carb/fuel supply side, as the miss is eratic, but there is no back-fire. I figure that if the spark were eratic, when it did fire again, there would be several charges of mixture in the chamber, leading to a tremendous bang...and there isn't...therefore, it must be that the mixture must be insufficient, not the lack of a spark. Is my thinking right?
Justin

A really good diagnostic is to attach an ignition scope -- the type that's powered by 12-volts (or use an inverter) -- and monitor the ignition as you drive under various condition. Most importantly, you can watch the quality of the ignition while the misfire occurs to see if it's reflected in the spark pulses. A good scope will show a "parade" of each individual spark pulse, so you can tell if it's just one bad cylinder (and which it is) or if it occurs randomly among them all.

You'll need to snoop around to find a shop or a mechanic who has an ignition scope of this type. (I have one that I built as a kit some years ago, and it can be a lifesaver in solving these problems.)
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Don Foster (near Cape Cod, MA)

Don't know how you're doing your diagnostics, but the usual suspects are MAF sensor, and the fuel charging system (esp. fuel pump relay, though it's not an acute problem in 700's as it is in 200's). Also look for intake vacuum leaks (and pressure leaks if you have a turbo).

Also inspect your distributor and rotor....I had to replace mine twice in the 275K miles I drove my 745T as the rotor contacts get worn.