A Good Ignition is Crucial
Instead of fussing with your used distibutor, rebuild it.
Here's an exploded-view drawing of a stock distributor.
It's the centerpoint of your car's ignition system. The basic design has remained the same for decades. Electronic versions started to appear in the early 1970s and at first they were prone to breakdowns, but they 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 itself has a shaft that's connected directly to the engine. At the top of the shaft there are six lobes. Next to the shaft are "points", which act as a switch. As the shaft turns, the lobes push the points open, and that open podition is called "the gap". Every time the points open, high-voltage electricity from the coil is sent to the spark plugs.
When the points are open, the gap can't be smaller than 0.45mm and no larger than 0.55mm. Feeler gauges can be used to set the gap, but when the engine is running you can attach a "dwell meter" which will give you a much better measurement and help you fine-tune the gap for the optimal performance. Needless to say, setting points can sometimes be a little 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 you'll see a little more horsepower and better fuel mileage. A common option is a 280z distributor, but they're getting harder to find. Another option is the 123ignition, which not only eliminates the points but also allows you to create various power curves. However, they're pricey. 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.
You can find inexpensive rebuilt distibutors at your local parts store but the companies which do the restoring cut corners everywhere possible. Don't buy one unless you're really desperate or seriously poor. Bad parts at a discount price are no bargain.
There are new distributors on eBay which come with an electronic adapter built-in, but they're junk from in a country known for making junk. Don't buy one.
I tried the Pertronix and the 123ignition, but decided to go back to the stock distributor. Sure, the points will 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 pretty cheap. I'll keep an extra set in my toolbox just in case anything goes wrong on the road, but I'm pretty confident they'll be fine.
This means the only digital thing in my car is the stereo.
I bought a very used distributor from Junkyard Jenny (my trusted source for used parts) and several people had mentioned Advanced Distributors as the place to go for a quality rebuild. Here's the website. Jeff, the owner, is very familiar with Datsun engines, having raced his Datsun pickup at the Bonneville Salt Flats.
This is how mine looked when it was disassembled. Unless you pull it completely apart, you'll never get everything cleaned. Mine was 50 years old, so naturally it had accumulated a lot of dirt and corrosion. The bushings around the shaft were pretty worn out, so when it turned it wobbled badly, which meant the timing could never be set correctly.
There are small weights connected to a plate via springs. As the engine RPM increases, the weights are pulled outward by centrifigal force, which moves the plate and changes the moment when the plugs fire. There's also a vacuum diaphragm connected to the intake manifold, and as the engine RPM increases, the amount of vacuum increases and moves the plate and also changes when the points fires.
As a distributor ages, the pivot points for the weights can get dirty and wear out, which affects how easily they can move the plate. The vacuum diaphragm can turn brittle and leak, which also reduces the amount of force to move the plate. Both of those problems will lead to poor performance.
Jeff said all of that was worn out. Not a surprise.
Here's how it looked after being cleaned. It's hard to believe they're the same parts.
This is 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.