Distributors are often overlooked in routine maintenance. Seems everyone knows to replace the points, condenser, rotor and cap, but not to check and grease a few other items related to the centrifugal advance and vacuum advance.
The air/fuel mixture takes a certain amount of time to burn. Igniting the mixture too early in the cycle cause problems, as will too late will waste fuel, reduce power, and cause drivability problems.
Maximum total amount of advance for most 1600 and 2000's should be around 35 degrees at the crankshaft. The initial setting of timing should be 35 less the maximum centrifugal advance.
The 68 and later roadsters came with a distributor that allowed 17.5 degrees centrifugal advance, (distributor rotates once, for every two rotations of the crankshaft, hence 'crankshaft degrees' are twice the amount of 'distributor degrees'). The 17.5 at the distributor will translate to 35 degrees at the crankshaft.
So the '68, and later, roadsters with an ORIGINAL distributor should be set to TDC (Top Dead Center). While the engine will idle with this setting, it isn't producing as much power at the lower engine speeds to get moving from a standing start. You either to rev the engine into a higher power band, and slip the clutch. While this will get you moving, it won't help your clutch much! The TDC timing allowed NISSAN to pass smog test (tested at idle only during the '68-'70 model years), but caused drivability problems.
By advancing the distributor to, say, 10 degrees BTDC (Before Top Dead Center) you'll notice the improvement in drivability, especially getting off the line. HOWEVER, you'll have way too much total advance if you're using a stock "smog" distributor, leading to all sort of nasty problems in the engine.
After reading the above, why not do away with all the mess of the weights, springs, cams and just WELD everything together, and be done with it! That is fine for engine speeds above 4,000 rpm, and is commonly done with racing engines, but not ideal for a street driven car. If you try to start the engine with too much advance, the piston is moving slowly enough that the burning mixture starts producing pressure before the piston makes it past TDC! This will rotate the engine in the wrong direction! Great way to blow the lid off the air-cleaner, or mess up your timing chains!
So to give a reasonable low rpm power , and not exceed to total maximum spark advance will require less centrifugal advance in the distributor. Thankfully this is what the '66-67 1600's were running! They have a maximum DISTRIBUTOR advance of 7.5 degrees, for a total of 15 Crank degrees. So with an initial setting of 16 BTDC, and an advance of 15 degrees, gives us a total of 31 degrees.
The astute reader will note I earlier said the engines like a maximum of 35 degrees total advance, but this will vary with the grade of fuel, compression ratio, engine temperature, camshaft installed, and so on. The number 35 is an approximate, not absolute, number.
'68 and later roadsters (both 1600 and 2000) can benefit from a simple rework of the distributor.
To convert your '68 and later distributors the earlier specs, use:
cam part number: 22132-14605 (for METRIC engines!)
light spring number*: 22110-14600 --> I May have swapped
heavy spring number: 22110-25600 --> these two numbers!
weights, if needed : 22109-71300
*Part number has TWO SPRINGS in it, one light and one VERY HEAVY.
New weights were used, as 20 years of rubbing between the springs and weights had resulted in the spring cutting a slot into the weight mounting ear... the weights could flop around at will! Made for a very interesting timing problem.
On the SU equipped engines, I recommend keeping the vacuum advance unit functional. Under cruising conditions, the engine is "throttled" by the butterfly, hence the engine is not purging the cylinder as much as it would under WOT (Wide Open Throttle) conditions. The resulting fuel/air mixture is "diluted" with exhaust gas, needing a little more time to fully burn. The vacuum advance will provide for a longer burn time under this lightly loaded condition.
The Solex carb's were designed with racing applications in mind, and do not have a vacuum advance port. DO not use the manifold vacuum, but just leave the vacuum advance can open. [Due to the difference in 'cylinder filling' the Solex equipped U-20's are timed at 20 BTDC].
If you buy a NEW distributor:
For a 2000 engine, the ideal way would be to order a brand new '67 model distributor. But they have been NLA for some time. You could order a '66 1600 model one, which will have the correct advance, but comes with a different vacuum unit attached to it.
Also for 1600's: If you own a '68 - '70, I recommend using the earlier 'non-smog' distributor from '66 - '67. Note that year will be using SAE screws, the '67.5 model distributor used metric screws but was not available when I check a few years ago.
If you have noticed the timing jumping all over the place when trying to adjust the timing, it may indicate either worn bushings, or worn advance springs, in either case it is worth while to inspect further before condemning the distributor.
Rebuilding is straight forward. I recommend leaving the distributor cap, with the plug wires attached, in the car! Seems odd, especially if you always have 1-3-4-2 (the firing order) memorized, but saves times when you reinstall the distributor.
Pop off the cap. Note the orientation the rotor. Make yourself a little sketch of the direction. Carefully remove the distributor, now look at the bottom of the distributor, note the of location the "offset drive"
To disassemble the distributor, start with removing the vacuum advance unit. Remove the two screws, and wiggle it to make it pop lose of the little tang (on the contact plate). It is almost best to have either an egg carton to hold the screws, or thread them back into the correct holes, so you know which ones went were later (they are different length).
Tip: I usually clamp the distributor on my workbench vise, using two piece of wood between the distributor and vise jaws. To remove stubborn screws, I firmly place a well fitting phillips screw driver into the screw, and "wack" the back of the screw driver with a rawhide mallet. If you use a metal hammer, you may split the screw driver handle.
Note on the screws: The earlier 1600's ('66-67) used standard fine pitch SAE screws. The "half year" (67.5) used metric screws, but the threading maybe unique. Do NOT loose those oddball screws, as you can not replace them. The later metric screws that complied to ISO standards have a little do on the head. These are easily replaced, if required. While metric and SAE screws may seem to initially fit the wrong hole, you'll most likely strip the hole if the screw is installed to far.
Check the smoothness of the breaker plate pivot. The little tab, for the vacuum unit, should move back and forth smoothly. A common problem is too long of a hold down screw holding the points down, and the plate doesn't slide!
Next slide the little plastic block out of the way. Inspect the wires going to the points, and the little ground wire.
Remove the two screws holding the breaker plate [Assembly - breaker in the parts book]. Lift it up and out of the way. There as some three little ball bearings held by a retainer on the bottom, also BETWEEN the sliding plates are three miniature ball bearings. While you can disassembly the sliding plate, it is extremely easy to overlook those three -- near miniature -- ball bearings. Usually if the breaker plate does not slide, for the vacuum advance, it is best to replace is as a unit.
You now can observe the weights, and springs. If you hold the bottom pivot, and twist the top you should see the operation of the weights move out. Let go, and they should snap back into place.
You should disassembly everything, clean, inspect, and grease everything lightly to make sure it does the "twist and snap back when released." White lithium grease is what to use here, not heavy wheel bearing grease.
Remove the advance, clean off the old grease, regrease the post on which it rotates, and reassemble.
Now you'll be glad you made notes, as you could have easily put the top cam back 180 degrees off. If so, the rotor will be pointing at the wrong place, and one reassembled, you'll spend more time trying to figure out what went wrong. (It happens!)
Reassemble, noting the correct screws in the correct holes.
Even if you buy a BRAND NEW distributor, I still recommend disassembling it, and regreasing everything. I did just that, the grease was 25 years old, and dried rock solid!