Trouble shooting ignition (MSD 6A)

[XH29N0G]

I think I have a symptom related to the ignition (described below).  I have had something like this happen several times before (in the past 5-7 years) and each time solved it and each time it was ignition-related - twice with new coils and once with a new MSD box, one time it was the key switch.  Not yet doing anything with the distributor.  I am about to embark on the diagnosis and eventual repair, but wanted to list the symptoms in case I am missing something.

The car starts, idles, and usually runs fine.  What I have noticed is the engine seems to miss (and knock - but I think that is unrelated) when I accelerate slowly to moderately while driving along.  Today, I noticed today that the tach was dropping out each time it would miss.  The tach-related-symptom is what makes me think it is ignition rather than fuel or knock. 

First question:  Does this reasoning make sense OR is there something else I might consider.

Second question: What would you suggest testing to trouble shoot?

Third question:  What would be the order of testing?

My inclination is first to check all connections between key and ignition first - in case it is an issue of power getting to the MSD box; second to check the coil resistance (it is only 6 months old);  third to figure out how to check the MSD box, maybe swap in another I have that had some issues, but I think different, and see if the behavior changes.  Beyond that I don't know.  Maybe recheck/rewire the hot lead from the battery.

thanks in advance

[BLK 68 R/T]

Try unhooking the tach and see what happens. I remember reading on here?? Someone else having ignition issues like that and it was the tach itself causing the problem.

[metallicareload99]

What distributor do you have?

... see if the behavior changes.  Beyond that I don't know.  Maybe recheck/rewire the hot lead from the battery.

Definitely check the wiring. I'm assuming you have the distributor wired to the MSD? Check that it is a good connection. Really check everything, rotor in good shape and all the way down, cap on properly, wires/coil wire aren't shorted to ground or burnt and in decent shape, plugs ok? If you have doubts about the wiring maybe rewire the hot. Do this to rule out the obvious and so you can move on to the "harder/more expensive" stuff

Because the MSD has a separate tach output, and because the car runs to some degree, in my mind I doubt the tach is the problem.

Also, if you notice the problem on the tach, I suspect that more than one cylinder is involved in the problem.  One coil fires all cylinders, on the instructions of one ignition box, signaled by one distributor

Assuming all the wiring, connections, and the coil too are all good, I would suspect the pickup in the distributor. If the distributor isn't giving a good signal to the MSD, the MSD won't know to tell the coil to fire and it will tell the tach to show a lower RPM

[XH29N0G]

Thanks both for your replies.  This gives me some things to start with.  I am slow with this usually, so it may take some time for me to sort out, but I have so far been good at sorting it out in the past.

[Back N Black]

The tack is going to drop if its fuel or ignition related. If the engines stumbles and knocks due to a fuel problem the tack will drop because RPM drop during stumble. Start her up put the car in drive with your foot on the brake and load up the engine, see how it reacts.

[68CoronetRT]

I had an odd stumble at cruise and it turned out to the connection from the MSD to the dist. One of the pins had come loose and it was coming and going which was causing my misfire. A quick re wire fixed the issue.

[XH29N0G]

Thanks for the additional input.  I'll keep in mind that I could be fooling myself with the tach.  I have a manual transmission, so I think it should still be spinning at that rate, but don't know if the tach will drop if there is a fuel/detonation issue or something else.  I'll start with electrical and work from there. 

[XH29N0G]

Had a chance to go through the wiring and coil and then took it for a test drive.  It works fine now.  The only obvious thing I noticed was the nut holding the end of the fusible link was loose.  I tightened that.  Otherwise all other connections looked fine.  I either unplugged/plugged them (case of the key harness) or wiggled them (feed through on firewall).  Coil resistance readings looked fine cold.

I am suspecting the fusible link, but need to confirm that is in the circuit that would lead to these symptoms.  If not that, then some other connection I made/remade fixed it.

This let me drive it today and run it.  All seems to be working well.