With car running at warm idle, unscrew the oil filler cap on valve cover. If the motor stumbles and/or dies, your vacuum advance system is tight. If there is no change, you have a vacuum leak somewhere.
I think the previous responses didn't understand the question ... so I'll ramble a bit.
The vacuum advance on the distributor helps the engine rev more quickly as engine load increases (and intake manifold vacuum surges). At this point the ignition timing is quickly advanced. Since the diaphragm is pourous and is a moving part then it wears out. Most of our beloved cars have vacuum leaks including this part, but what is acceptable? Some would tell you to hook up a spare vacuum hose (you have some right?) to the advance and suck on it. If it's totally dead you might notice that you're sucking fumes. Otherwise you wouldn't notice how bad it is. Using a "mighty-vac" you can draw a vacuum and watch the gauge drop showing you how bad the leak is. On my '76 I've done this test with depressing results, I have a leak, but it's not too bad yet. Yes this allows the engine to inhale unfiltered air, this is one reason why so many ZXs have blown engines and our Zs still run strong. All those ZX vacuum do-hick-ee's broke long ago and no one bothered to fix them (hence all the "No Defrost" topics.)
What sucks out of all this is that you'll spend lots to get just a new vacuum advance. And while pulling the dist to replace it you'll probably break some of the sensor connectors on the thermostat housing ... O what fun!
The oil cap trick works with every modern engine I've ever tried it on. It should actually work even when you pull out the dipstick. To expand upon Conor's post, sometimes the vacuum advance in the distributor won't work even when you have 'good' vacuum (your tongue sticks to the test hose until you free it). That's because of the little ball-bearing carriage under the breaker plate in the distributor. The ball bearings were carried in this ridiculous plastic 'keeper' that after 25 years has given up the ghost in most cars. I've seen websites where people have tried to replace these, but apparently Nissan no longer sells just this part. Your only option then is to get a new or rebuilt dizzy or comb the boneyards till you find a good one.
This is applicable to the Hitachi Distributor model D6K8, D6K9, D6K80 and D6K81 part number 22100-P9101 found on a 1979 N/A L28E.
Inside the distributor there is a part called a Breaker Plate Assembly, part number 22136-Q1700. It is two metal plates that has a triangular plastic plate with three ball bearing, one at each point of the triangle, sandwiched between the two metal plates. The ball bearings keep the two plates apart. NOTE: Part 22136-Q1700 is superseded by 22136-H9501.
The plastic triangle acts as a race to keep the ball bearings in place. The bottom plate is fastened to the distributor housing. The top plate is connected to the rod from the vacuum advance, part number 22301-N4700 (this number is for reference ONLY, there are 12 different vacuum advance modules).
The Pickup Kit, part number 22229-Q1700, is fastened to the top plate of the Breaker Plate Assembly. The Stator, part number 22163-Q1700, is placed on top of the Magnet, part number 22158-S6700, and both units are fastened to the Pickup Kit.
Finally, the Reluctor, part number 22115-Q1700, sits on the distributor shaft and is held solidly in place with a pin. It is located mid-way between the Rotor and the Mechanical Advance Mechanism.
There are six teeth on the Reluctor and six teeth on the Stator. Each time all the teeth line up, the magnetic field, that is always being produced by the magnet, is altered.
Each time the magnetic field collapses and expands it acts as an on / off switch for the current flowing through the Pickup Kit.
The distance between the teeth on the Reluctor and the Stator is called the Air Gap and should be between .006 and .016 inch. This should be measured with a brass feeler gauge because of the magnet.
The Pickup Kit is the part that sends pulses to the Transistor Ignition Unit on the outside of the distributor.
The operation of the Transistor Ignition Unit is outside the scope of what I am trying to explain at the moment but it is basically an electronic filter and a high current switch. The filter part takes the input from the Pickup Kit and makes it into nice, clean, square waves. These square waves are sent to the high current switch part which is connected to the (-) side of the ignition coil.
The Ignition Module for 1979 to 1981 is Nissan part # 22020-S6702 (aka E12-80).
The Ignition Module for 1982 and 1983 280ZX N/A (E12-92 Nissan # 22020-W3100 or E12-93 Nissan # 22020-P9700)
Each time the high current switch closes there is current flow between the battery and ground through the '+' and '-' terminals on the ignition coil. Current flow in the Primary Side of the ignition coil induces a higher voltage in the Secondary Side of the ignition coil.
The Secondary Side of the ignition coil now contains around 50,000 volts. When the high current switch opens there is no current flow in the Primary Side of the ignition coil and the Primary field collapses. The voltage stored in the Secondary Side of the ignition coil now has to go somewhere.
When the rotor lines up with a tower in the Distributor Cap the voltage goes out the high tension lead of the coil to the distributor, down the rotor, JUMPS to the NEAREST spark plug wire post inside the distributor cap, down that wire to the spark plug and then BOOM, an explosion in a cylinder.
Z'ed should go vrooom.
Way back, up in the beginning. I mentioned the Mechanical Advance Mechanism.
There are two key components of Mechanical Advance.
First is the AMOUNT of Mechanical Advance.
For the Hitachi distributors, this is completely controlled by the length of the advance slot in the Cam Set (part number 22132-P7100). If you don't know where these are, open up your distributor, take off the rotor, and look under the Breaker Plate Assembly. Now force the weights out and look for where you see the pin moving in the slot. Once you have seen this, it is pretty easy to understand how to change the amount of advance. To get less, braze the slot so it is shorter. If you want more advance, then lengthen the slot.
On the bar with the slot in it will be a number stamped into the metal. It will be 8.5 or 9. These numbers correspond to the number of degrees that the rotor will advance relative to the base of the shaft. Since the crankshaft rotates twice each time the distributor rotates once simply multiply the number by two to get the total mechanical advance.
The second component of Mechanical Advance is the Advance Curve.
The Advance Curve is controlled by the shape and weight of the weights, shape of the cam (in between the weights) and the tension of the springs. If you install lighter springs, the weights will be able to extend out at a lower RPM. If you install heavier springs, it will take a higher RPM for the weights to overcome the force of the springs.
Since the Stator is fixed to the Breaker Plate Assembly, which in turn is fixed to the distributor body, and the Reluctor is fixed to the Cam Set, as the RPM changes, the weights move, and changes the relationship of the Reluctor to the Stator.
To check the mechanical advance:
Disconnect the vacuum advance hose and plug it with something.
Set the engine speed as close to 600 RPM as you can and note the timing. Now increase the RPM to 2,500 and your timing should advance 17 degrees (if the Cam Set has 8.5 stamped on it). I.E. If base timing is 10 degrees BTDC at 600 RPM it should be 27 degrees BTDC at 2,500 RPM. (See Note at end)
Vacuum Advance is used to advance the spark as manifold vacuum is increased. It should be noted that Vacuum Advance is primarily for Fuel Economy, and has no affect on your performance at Wide Open Throttle (WOT), since you have almost no vacuum present.
As engine vacuum changes the rod attached to the Breaker Plate Assembly moves the top plate and changes the timing by moving the Stator.
To test if the mechanical part of the vacuum advance is working do the following:
Remove the Distributor Cap.
Disconnect the Vacuum line at the Vacuum Advance unit.
Connect a vacuum pump to the Vacuum Advance Unit. As you apply vacuum the rod should retract and the Stator should move. At around 10 - 15 inches of vacuum the rod should be fully retracted and the vacuum should not change.
If the vacuum goes down there is a leak in the diaphragm and the Vacuum Advance Unit will have to be changed. You have also just discovered the vacuum leak that makes the engine run rich.
If the vacuum does not change, and the rod does not move, the Breaker Plate Assembly needs service.
If you can see ball bearings in the bottom of the distributor you need a Breaker Plate Assembly anyway.
When the loose bearings end up wedged in the counter - weights for the mechanical advance that will stop working also.
To check the vacuum advance with the engine running:
Hook a vacuum pump to the vacuum advance unit. Slowly increase the vacuum to 6 inches and at this point engine RPM should start to increase and the timing should start to advance. At 10 inches of vacuum you should have more than 10 degrees of vacuum advance. (See Note at end)
Just remember that as RPM increases when testing the vacuum advance the mechanical advance will be kicking in also.
Now hook the vacuum line back up to the vacuum advance unit and increase the RPM to above 2,500 RPM.
If you started off at 10 degrees you should now have 27 PLUS whatever you got when you tested the vacuum advance. I.E. 10 + 17 + 10 = 37 degrees @ 2,500 RPM.
The manner in which Nissan chose to list the Service Data Specifications for the Mechanical / Vacuum Advance need explaining.
[Distributor Degree / Distributor RPM]
If you have access to a Distributor Analyzer this means that at 600 distributor shaft RPM you should have 0 degrees of mechanical advance, and at 1,250 distributor shaft RPM you should have 8.5 degrees of mechanical advance. All of this is measured comparing the relative position of the rotor to the distributor cap tower. Since it is not likely that we would have a $40,000.00 Distributor Analyzer in the closet we need a more practical method to test this and that would be a timing light. For each complete rotation of the distributor shaft the Crankshaft has to turn twice.
So the S.D.S. has to be translated into Crankshaft relative information so that it becomes:
[Crankshaft Degree / Crankshaft RPM]
You simply take the original numbers and multiply each one by 2 and now you use the timing mark on the Damper Pulley.
This means that at 1,200 Crankshaft RPM you should have 0 degrees of mechanical advance, and at 2,500 Crankshaft RPM you should have 17 degrees of mechanical advance.
[Distributor Degrees / Distributor mm Hg (in Hg)]
This one has to be converted into:
[Crankshaft Degrees / Distributor mm Hg (in Hg)]
Vacuum is measured in mm of Mercury or (inches of Mercury) with a vacuum gauge. Vacuum is applied to the Vacuum Advance with a hand held vacuum pump. In the above example when 200mm (7.9 inches) of vacuum is applied you should have 0 degrees of advance and when 350mm (13.8 inches) of vacuum is applied you should have 15 degrees of advance.
The vacuum advance mechanism can be checked with the distributor out of the engine also. Simply cut a hole in a cheap protractor and slide the hole over the distributor shaft and fasten the protractor to the distributor body with tape. Re-install the rotor and apply vacuum and watch how many degrees the rotor moves. In this case you would use the "[Distributor Degrees / Distributor mm Hg (in Hg)]" specification.
Excellent description! Now we have a place to send everyone with dizzy problems. I'm constantly amazed at how many people will screw around with timing and plugs and mixture without checking out both mechanical and vacuum advance systems. Stuck advance mechs will cause all sorts of various problems, and then if someone resets the timing while the plate is still stuck then you'll introduce a whole new list of problems.
Anyway, just glad we have a "go read this first" post now. Any movement on the FAQ that was started awhile ago?
well, here's one you should try it on, after it made no difference on my 77 280, i tried it on my 2001 dodge neon, still no difference. oh yeah, it has no vacuum leaks, it has 800 miles on it and is well maintained and runs perfect. so i am questioning that oil cap trick. i don't even see how that would work.