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A ballast resistor will lower the current flow to the primary side if the ignition coil which will then reduce the high voltage produced by the coil going to the spark plugs.  Not sure if that will help with hot starting unless your present ballast resistor, assuming there is one, is changing resistance when hot or worse, becomes an open circuit when hot. A few voltage checks with a good voltmeter are needed when the hot start problems occurs.

 Today a digital voltmeter capable of making these measurements can be had for not a lot of money and would be worthwhile to carry along in your car should you experience the problem on the road.

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I thought the whole point of a ballast resistor was to reduce the voltage across the points when running (not starting) so as to maximize point life.  Assuming the coil can take the current, the car will run fine with no ballast resistor but point life from arcing (and subsequent pitting) will be shortened (significantly, from what I've read).  I would have thought the Petronix-type of electronic ignition would allow you to run the full voltage since there are no points to worry about and higher primary voltage gives higher secondary voltage (to plugs).  I always thought that was a good thing.  Unless you're using the car as a daily driver, it just doesn't make much economic sense to me to go with e-ignition.  If you can determine the value of the resistor wire, you may be able to select a lower resistance ballast to get back some of the higher primary (without going over the unit's limit).

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1 hour ago, TerryB said:

A few voltage checks with a good voltmeter are needed when the hot start problems occurs.


Yes that would have been my first thought. Next time it has a problem starting when hot, use the voltmeter to check that a full 12-plus volts is going to the + terminal on the coil. Must be several things you could measure, but that is the first that came to mind. Maybe a bad ignition switch could be compromising voltage or current going to the coil.

 

 

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Apparently Petronix systems want you to use full voltage to the coil.  My guess is their electronic switch circuit has some residual resistance to ground as opposed to points that are zero resistance assuming good electrical connections exist.  If there is a ballast resistor in the circuit too then the coil output can be weak and make the car hard to start.

There are several types of coils, some of which are designed to be used with an external resistor and some who have it built in to the coil.   The designer of the circuit must have made choices based on spark level and coil / points durability for the car that uses a particular setup.  

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I am using this electronic distributor. It has a magnetic switch as the trigger instead of the points. Yes there is potential for damaging it if too much current goes though it. So they require you to use the type of coil that has at least 3 ohms resistance built into it (the "primary" resistance). So even if it handles more current than points, it still needs to be limited, and I suppose you could add a ballast if you wanted. Just make sure you have 3 to 4 ohms.

 

Note : It is important that you measure the resistance of the coil your going to use so as not to damage the new module.

coil resistance should be 3-4 ohm’s across the primary side of your coil….  do this with NO wires connected to the coil.

 

http://rmlalfa.com/rml/rml-performance-ignition

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5 hours ago, Buick35 said:

I have a 74 MGB that sometimes has a starting issue when hot.It has points. Some guy said I should change the ballast resistor,he has one on his 74 Triumph but I don't think I have one on my MG. I'm wondering if I should add one. I've changed coil,points,condenser and fuel pump. Thinking of getting a "sport coil".

Actually, what you should do is perform dark, arcane rituals and incantations to Lucas the Prince of Darkness.😈

 

 

I'm trying to remember if a high school buddy's MG had a ballast. I know it had individual fuses for just about every electrical component on it, as it usually fell to me to operate stuff while he was busy changing those little ceramic fuses and swearing at it, saying things a little Baptist boy oughtn't to...

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1 hour ago, TerryB said:

Apparently Petronix systems want you to use full voltage to the coil.

 

Pertronix Igniter says to leave the ballast resistor in the circuit. 

 

Pertronix Igniter II says to remove the ballast resistor. 

 

The Crane/Allison/Fast/Fireball XR-700 requires the ballast resistor. The XR-300 version wants the ballast resistor removed. 

 

Then there is coil resistance. 

 

Then there is the crazy maximum voltage of the Hot Spark instructions. Of course one can not measure the voltage on the + terminal of the coil unless the car is running (because the ballast resistor can only accomplish voltage drop when current is flowing through it), and if the voltage is too high, you may already have damaged the unit.🤔

 

As always, a prospective buyer needs to do research before buying. 👍

 

 

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I never heard of Petronix II before, but accidently came across this info that they require, or recommend a special coil with very low primary resistance.
So I suppose that system handles high current and produces a hot spark

So many variables. Also the coils secondary resistance. Seems you need to use the coil recommended for your "triggering system."

 Pertronix Flame-Thrower II Coil, Epoxy Filled (45, 000 Volts, 0.6 Ohm), Black

 

Flame-Thrower II coils feature a super low resistance which make them the ideal coil for Ignitor II ignition systems as well as many other high energy ignitions. Low resistance and improved turns ratio help to produce up to 45,000 volts. The higher voltage enables you to run larger spark plug gaps which Pertronix claims adds power and better fuel economy.

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