Pertronix question

[Zedyone_kenobi]

I have a 1971 Z that has me plaguing the boards with this question and that, and to this boards credit, the amount I have learned is amazing. I recently got happy with the timing of my car in both base timing and max advance.

On my test drive though the car does something particularly strange.

At anything below 5500 to 6000 rpm the tach is dead steady. I mean new car steady. Very nice! However, when I rev it to this weird range the tach goes up smoothly and linearly. When it hits this band the needle swings back down to 3000 rpm and moves around strangely. The car still feels fine but acceleration seems to have slowed or leveled off.

What would make it run flawlessly below this limit and then go electrically crazy above 5500 rpm.

[bigoak]

Your dilithium crystal in the warp drive is all messed up Captain Kirk...

Other than that, I have precious little constructive...

[Zedyone_kenobi]

Drat, I told scotty to fix that!

[soundmasterg]

Thats downright weird. I'm running the Pertronix in my '72 Zed and my tach works fine. I did have a problem with intermittant cutout above 5000 rpm but it was a bad Pertronix unit. The new one has worked just fine since I exchanged it under warranty a couple weeks ago.

I'd guess the problem is somewhere with the tach or it's associated wiring perhaps?

Greg

[d240zx2]

You possibly could have a ballast resister problem. I say possibly because typically it either works or it doesn't work. Your '71 may be different than my '70 in that regard, as I have a 4-wire tach and yours is a 3-wire.

As the BRs are pretty cheap, you might try replacing it if you run out of other otpions.

Frank

[Zedyone_kenobi]

is that the resistor connected to my coil? NOt sure

Still its not a nagging concern to be honest. Car runs great. I don't often find myself revving it that hard, its just nice to know its operating correctly within spec.

[EScanlon]

Taking a WAG between the BR and the Tach.... I'll say the Tach.

If the BR were intermittent,whole ignition circuit would be disrupted ... as in no juice to the coil, let alone through the tach for it to sense RPM's.

The Tach, over time is known to fail just from the age of the components ... and poor wiring. If the coil loop that the tach uses to sense the RPMs is loose, with the vibration it is quite possible to "mis-read". But that's a very long shot. More than likely the Tach's internal circuitry (resistor, capacitor, etc.) is beginning to fail and at 5500 / 6000 RPM it's starting to "overheat" and no longer senses accurately.

FWIW

E

[Arne]

I agree with E. If the resistor or other ignition component were causing the problem, the car would run just as erratically as the tach. Sounds very much lice a dying tach to me.

BTW, it's normal for the power to taper off a bit at that point, unless the engine is thoroughly modified.

[xray]

When I had some issues resurrecting my engine from its 2-year sleep, I had some bouncing tach issues as well. Everything seemed in spec, using the Pertronix with the recommended 3-ohm coil and no BR. Still, it lost power above 5k and caused the tach to act very dodgy--I was thinking the tach was bad....Then I bought a new 1.5 ohm coil and repositioned the BR in the circuit and the problem was fixed. Still not entirely sure how that fixed it, except to assume the prior coil was going bad. Now it revs to nearly 7k with no notable loss of power (no internal engine mods, F54/P79 with 2.5" exhaust).

How new is the distributor? Any shaft play? If the air gap on the Pertronix fluctuates it may cause intermittent spark/ignition symptoms.

Good luck,

[Andydando]

Xray

When we were racing we found that the Pertronix operated best with 12 volts directly to the unit and the ballast resistor bypassed.I would also recommend replacement of the coil with a pertronix unit.Use should check out the play in your distributor shaft as this would cause an air gap problem(only if real bad).Make sure you don't use cheesy connections to the Pertronix and check your plug wires,particularly the boots around the coil terminal as it could short causing all these problems (check for little white marks on the boot or any cracks).

[xray]

Andydando,

I no longer have issues with my ignition system, but thanks for your input. I could've bought a new 3 ohm coil and gone without the BR, but chose not to.

[Zedyone_kenobi]

This is good stuff. I am running the Bosch coil which I believe is sold through MSA or black dragon (or both, I don't know where I saw that). I have no idea if its a 1.5 ohm coil or a 3 ohm coil. I suppose I could disconnect everything and measure the resistance through the coil easy enough, that would tell me something. Okay, I have lots to work with here, I will check a few things out when I get home. I do most certainly have a resistor hooked up to one of the terminals on the coil, but I need to figure out what size. Its very simple. Is a wire from my coil to a resistor. It does not go anywhere else.

I may need to snap a picture.

More to follow.