Inconsistent Idle

[Crestline]

I've watched the choke/ fast idle pull off move as the engine warms.

The voltage at the terminals on the Pertronix is running at 10.6v (+/- a few tenths due to readout of the digital VOM bouncing a little). Isn't there a dropping resistor on some systems?

 

[Crestline]

To get an RPM reading get a timing light that includes RPM readouts. A 'dial back' timing light is the best choice, you can adjust the flash of the timing light to the current TDC to see your current timing and then set a value of advance that you want and adjust the timing to that value. They are super handy.

16 inches is 'pretty good' vacuum but not as high as I would think you can get with a relatively stock 351C 2v car with a 2bbl carb.

Have you adjusted the idle mixture to 'best vacuum'?

I've started the low idle adjusting needles at being backed out from the seat by 1 turn and went to 2-1/2 turns. At 1-1/4 turns out I got 19" at a fast idle (choke hadn't pulled off yet, when it idled down the reading went to 16". Coil voltage was about 10.7v. 16-17" of vac is the best I've gotten at low idle.

 

[Crestline]

I've watched the choke/ fast idle pull off move as the engine warms.

The voltage at the terminals on the Pertronix is running at 10.6v (+/- a few tenths due to readout of the digital VOM bouncing a little). Isn't there a dropping resistor on some systems?

The 10.6 was with the engine idling, the voltage is a little over 5 with key in the on position, engine off.

 

[Hotstang]

You have to bypass the stock coil wire (red with green stripe) by running a new wire or splicing a new wire in place of the resistor part of the wire. If you trace the wire out of the ignition switch you can tell when the resistor starts because the wire gets fatter. Not easy to see in those tight areas so pack your patients and maybe pull out the drivers seat if you end up spending time on your back.

 

[Crestline]

You have to bypass the stock coil wire (red with green stripe) by running a new wire or splicing a new wire in place of the resistor part of the wire. If you trace the wire out of the ignition switch you can tell when the resistor starts because the wire gets fatter. Not easy to see in those tight areas so pack your patients and maybe pull out the drivers seat if you end up spending time on your back.

Thanks for the explanation. I understand now.

 

[rdj498]

it sounds like power valve,pcv, or vacum leak.

 

[midlife]

I too have an inconsistent idle. When sitting in my favorite recliner, there's always something that needs to be done, ruining my idle timing!

 

[mrgmhale]

I'm measuring 10.3v at both small terminals on the Pertronix. How do I bypass the internal resistor?

If you have a 72 or 73 you can use Circuit #640, which is a Red Wire with a Yellow Hash tracer. It is used to power the Throttle Position Solenoid (TPS), and has enough switched current to power the Pertronix ignition coil.

Do not do this with a stock coil, or any coil that is not clearly made to run at full battery voltage, as it will cause the lower voltage designed coil to overheat (and no doubt eventually fail).

Here is a YouTube video showing where the TPS is located, and the wiring from Circuit #640 to the TPS.



I have never seen a TPS device in a 1971 Mustang engine, nor Circuit# 640 being sent into the engine compartment.

If you want battery voltage switched current, here are two places you ought NOT try to get it:

• On the circuit feeding the electric choke (for carbs with electric choke), as the White Wire with Black stripe tracer is for Circuit #4, and is powered by the alternator stator - which only produces 1/2 battery voltage with AC (not DC) current.
• On the circuit feeding the "I" terminal on the Starter Relay, as the Red wire with Light Green stripe tracer is Circuit #16, and is used to provide full battery voltage during starter motor cranking. In the normal running of the engine the voltage to Circuit #16 is fed by the ignition resistance wire, and runs at about 10 volts or so.
  • An aside, when the engine is cranking the voltage from the battery drops to the 9.6 - 10.4 volt level or so (assuming a charged battery and no starting system issues). Thus, despite folks saying the cranking voltage to the coil is a "full 12 volts" at cranking, that is not correct because the cranking voltage is at the reduced level (9.6 - 10.4v or so). So, during cranking and running through the resistance wire, all the ignition coil is getting is about 10 volts or so (unless a different switched voltage circuit is used to power the ignition coil). It is more correct to say during cranking the ignition coil is getting full battery voltage from the "I" terminal on the Start Relay that bypasses the resistance wire.