Jumping Tach and Mis-Fire On 1971 240Z Stocker

[rzola]

Had a strange one today. While driving home from work at expressway speed, my tach started jumping between 2000 and 4000 RPM. While the tach was jumping, the engine would have a slight miss or chug. Very minor but noticable. Below 2000 RPM she runs smooth as silk. When I got home, I cheked the points, cap and all wiring to the ignition system. Every thing looked OK. After cooling down, I went for a warm up drive to lash my valves. The car ran fine. After lashing the valves I went for a test drive. The car runs great. My car has 100% stock ignition system. Any of you Z gurus have any ideas about this one? Could it be the tach? Or maybe the coil getting hot? Any advice would be appreciated!:tapemouth

[Ed]

Not too sure but it sounds like it's either the tach the coil or both. Since the tach runs off of the coil it might be the coil. My tach went bad and bounced around for a while, steady at times and then it would go nuts. I couldn't feel any miss in the engine. Cost me about 40 bucks to get it fixed.

Coils are cheap, wouldn't hurt to do some preventative maintanance.

[2ManyZs]

Since it was OK before and after the episode, I would check all the wiring connections around the coil, distributor, and ballast resistor. You might have a loose or corroded connection, of course it could be a bad wire somewhere in that area that has been corroding and causing resistance in the wiring harness.

Might be one has been pinched or crimped one too many times. Also, check the wiring inside the distributor to the condensor and points and make sure you have good connections there too. Wouldn't hurt to use a little dielectric grease after you clean all the connections with some contact cleaner or whatever you have available.

[rzola]

Thanks for the advice guys. I am planning on upgrading to a performance coil. Hopefully that will solve the problem. I do have a spare tach, but it looks like it may be tough to replace!

[Ed]

Replacing the tach is fairly easy IF you can reach the two butterfly nuts on the back side, under the dash, way in there. You have to lay on the floor head resting on the brake pedal. Spray a little WD-40 on the nuts before you get started and let it soak. Once you get the nuts off the tach pulls right out.

[Daniel]

I had a similiar thing once and it was my fuel filter.At time of running large amounts of fuel it SOMETIMES stirred up enough sediment to starve the car.Idle was enough,When the car sat long enough the sediment dispursed until the next time.A fuel tank dip and clean and new tank to engine compartment fuel line resolved mine.I hope your is only an old filter.There are clear filter units at any auto store that allows you to see what your pumping. Have fun!! Daniel.

[tttz]

I had similar symptoms as rzola originally posted, except my 71 240 stocker has a pertronix ignitor and performance coil. Also, slightly different from rzola, my tach would jump only above 4000 rpm. i asked the forum under the thread "bouncing tachometer", here's the link:

http://www.classiczcars.com/forums/showthread.php?s=&threadid=6235

rzola, did upgrading your coil solve the problem?

I'm still having trouble with mine, but i haven't changed the tach yet...

Thanks,

Ty

[EScanlon]

Check the resistor by the coil.

You don't mention how the engine operates ABOVE 4k rpm, and going off ttz's post, I presume that yours does the same.

The resistor is open coil with a ceramic heat holder, it is DESIGNED to burn off excess electricity in the form of heat. That means that this wire can eventually oxidize and break.

Try this, connect the two wires going to the resistor, you'll be sending the full 12v of juice to the points during this time, but you're only going to check things out. Operate the car at the trouble rpm and see if the problem reoccurs, is the same or ?

If the car's problem has changed, you have all or part of your answer.

Then fix, and be sure to restore the resistor back into the circuit.

Although there are various posts regarding removal of the resistor it's only really recommended if you are NOT running points. If you ARE running points you'll pit them out quicker without the resistor than with.

Also check the connection for the coil LOOP at the back of the tach. If the plastic harness that the loop is held by is gone, it can cause the wire to slip, and cause bad or no readings.

2¢