A Good Ignition is Crucial
Instead of fussing with your used one, why not rebuild it?
Here's an exploded-view drawing of a stock, vacuum-advance, single-points distributor.
The distributor is the centerpoint of your car's ignition system. The basic design remained the same for decades. Electronic versions started to appear in the early 1970s. At first they were prone to breakdowns, but got better over the years and by the mid-90s became standard on every car.
Here's a very simplified explanation of how a distributor works: When the key is on, 12 volts goes to the coil, which converts that to a very high voltage. The distributor has a shaft that is connected to the engine. At the top of the shaft there are six lobes. Next to the shaft are the points, which act as a switch. When the shaft turns, the lobes open and close the points. Every time the points open, high-voltage electricity from the coil is sent to the spark plugs.
When the points are open, that gap must be no smaller that 0.45mm and no larger than 0.55mm. Feeler gauges are used to set the initial gap, and when the engine is running you can attach a "dwell meter" which will give you a much better measurement of the gap and help you fine-tune it for the optimal performance. Needless to say, setting points can sometimes be frustrating even for experienced mechanics.
There are a lot of upgrades available that eliminate points. Electronic ignitions don't have points and are far more accurate, which means more horsepower and better fuel mileage. A common option for that is a 280z distributor, but they're getting harder to find and are typically worn out. Another option is the 123ignition, which not only eliminates the points but allows you to create power curves for various needs. However, the're pricey and sometimes fail (like mine did). Pertronix makes a kit for your existing distributor for a reasonable price, but if the distributor is worn out the Pertronix might not make much of a difference, and they also sometimes fail.
For distributors that are past their prime, you can typically find an inexpensive rebuilt one at your local parts store but the companies that do the restoring do it as cheaply as possible and cut corners. Don't buy one unless you're really desperate or seriously poor. I always say that bad parts at a discount price are no bargain.
There are brand-new distributors on eBay which come with an electronic adapter built-in, but they're junk from in a country known for making junk. Definately don't buy one.
I tried the Pertronix and the 123ignition, but decided to go back to the stock distributor and points. Sure, they'll need to be replaced about every 10,000 miles but they're about as dependable as you can get. At a mere $15 at my local parts house, they're also pretty cheap. I'll keep an extra set in my toolbox just in case anything goes wrong but I'm pretty confident they'll be fine.
This means the only digital thing in my car is the stereo.
I bought a "somewhat recently" rebuilt distributor from Junkyard Jenny (my trusted source for used parts) but the one I chose turned out to be the wrong type for my '71. Several people mentioned Advanced Distributors as the place to go, so I did. Here's their website. Jeff specializes in vintage vehicles and he's very familiar with Datsun engines, having raced his Datsun pickup at the Bonneville Salt Flats.
Jenny sent me the correct-year distributor (which was in horrible condition after sitting outside for years), and I sent it to Jeff. Here's the before & after.
This is how mine looked fully disassembled. Unless you pull it completely apart, you'll never get everything cleaned. It's 50 years old, so naturally it accumulated some dirt and corrosion. The bushings around the shaft were pretty worn out, so when the shaft turned it wobbled all around, which meant the timing couldn't be set correctly.
There are small weights connected to a plate by springs. As the engine RPM increases, the weights are pulled outward by centrifigal force, which moves the position of the points to change the moment when the plugs fire. There's also a vacuum diaphagm connected to the intake manifold, and as the engine RPM increases, the amount of vacuum increases, and the diaphragm also moves the position of the points.
As a distributor ages, the pivot points for the weights can get dirty and wear out, which affects how easily they can move. The springs can get stretched, allowing the weights to move further than needed. The vacuum diaphragm can leak which reduces the amount of force to pull the plate. The linkage between it and the plate can also get dirty and prevent the plate from moving enough. All of that leads to poor performance.
Jeff said the condensor was dead. Not a surprise.
Look at the difference. It's hard to believe they're the same parts.
Here's Jeff's description of the process:
"All the parts are machined as necessary to eliminate play, slop, wear, whatever you want to call it until a precision assembly is fitted back together. This is NOT your typical rebuild where parts are just cleaned and reassembled. The factory tolerances go out the window. Every distributor built here is a hand-assembled masterpiece specifically assembled for its intended vehicle!!!
The last step of a rebuild is choosing appropriate advance springs, tailoring the amount of advance, and testing the distributor on one of our 13 Sun machines. It will be fitted with its fully rebuilt breaker plate, and points will be installed in most cases for tuning.
To "recurve" a distributor is to get the timing exactly correct at every single rpm possible, not just at idle. This allows your engine to rev freely through its RPM range much faster and more efficiently than ever before. This is done in many ways, most frequently by replacing the advance springs with our custom designed springs, and changing the amount of total timing."
Of special interest is this: "I do not sell electronic ignition conversions, as there are numerous side-effects that you are better off without."
Here's how it looks installed with a new rotor, cap and wires. Masterpiece indeed.
The engine runs like when it came from the factory, which is exactly what I wanted. It starts right up, idles fine, and when I squash the pedal it pulls solid up to 6k RPM with a wonderful sound.
Sure, compared to a carefully-engineered modern ignition I might have given up a few horsepower, but those extra ponies are at the top of the power curve and I'm not likely to hit that part too often on the street, which is where the car is going to be 99% of the time.
If your distributor is giving you problems, send it to Jeff.
Tell him Ace sent you.
On a final note, this amazing chart shows all of the distributor part numbers, specifications and timing settings for the entire 240z, 260z and 280z series.
Click on the image to see everything. It's huge.
This is probably far more information than you'll ever need, but it's always good to have a handy reference.