MSD unwanted spark
 

911 2.0L.
I have MSD and petronix ignition. If I turn the key on and off I get a spark. EG. Sometimes I turn the key on and if the engine is in the correct spot it kicks.
Ive checked the wiring and believe its correct.
Lyndon

It's most likely a function of the Pertronix. Because the spark boxes fire on a rising signal (e.g. 0 -> 12V), whenever you switch the ignition on, the Pertronix has no option but to output 12V and the ignition box sees this as a firing event.

With the original points system, the points are either closed (grounded to 0V) or they are open. The difference is that the points do not apply 12V so you don't have the problem.

It's actually quite a difficult problem to solve with electronics unless you had an output that truly was a digital switch with a true open circuit. Unfortunately devices in this category are generally quite slow, so they are ill suited to time critical applications.

How about feeding the small gauge red wire on the MSD with a relay. The slight delay from the relay might be enough. Fred

I guess delaying the power up of the MSD could be an option.

I generally turn the ignition on for a few seconds for the fuel to prime before starting so I'm guessing a relay would not work.. I need to power the petronix only on crank and run. If that is the problem part.
Lyndon

I disagree, The times you are getting a spurious spark will be when you cycle the ignition switch quickly. Not to say the relay will solve the problem as I have never tested such a thing. Fred

It will kick ( and backwards ) with just the key turned on. Not a common occurrence. But the spark is always there.
Lyndon

Right, So when the MSD is powered up, and there is a trigger signal to it at the same time due to the voltage rise at the pertronix, the MSD will fire. So, delay the powering up of the trigger circuit in the CDI long enough that it doesn't see that spurious trigger signal. Cycling the ignition switch on and off quickly will do the same thing even with points on most CDIs. This is not uncommon with CDIs. What is uncommon is the spurious trigger when power is first applied to the CDI. With most CDIs this will not create a spark either due to inherent time lag or by design. Fred