Hi Boys and Girls.  I'm having a problem with my old 72 350.  About 8 years ago I installed a HEI dist to replace the original.  Bought  it from Mid America and got the one with gear drive tach. Also replaced the plug wires and tach cable.  Installed it and everything went well.  Car ran better than ever.  About 2 years ago it developed a tapping noise, thought it was a lifter but we determined the noise was coming from the dist.  So we replaced it with another dist including the tach drive.  No noise, problem solved.  Drove it for a year (keep in mind driving for a year is only 500 +/- miles) until I it drove to Effingham IL this past Sep .  Put 800 miles in 4 days.  Ran great,  never missed a beat until we got 5 miles from home.  Negan miss firing, coughing,could not accelerate. Also backfiring LOUDLY!  Nursed it home.  Thankful to make it.  Started it the next morning and it ran great.  I put about 200 miles on it before putting it away for the winter.  This Spring it started right up and has run great for about 100 miles.  Although it started the same tapping noise as the last HEI.  However the tach stopped working and the noise is gone.  Answers that question. Driving it last week and it started miss firing backfiring like it did last fall.  nursed it home again.  Started it again yesterday and it ran great.  Any thoughts?  Has anyone else put a HEI dist on a pre 75?  I would try and find a NOS 75 dist but that would have an electronic tach.  I'm afraid to drive it.

Dean, you are describing the classic symptoms of the HEI ignition module taking a crap. I would suggest you replace the ignition module, coil, cap, and rotor. As far as the gear drive for the tach, the gear(s) have prolly also taken a doo-doo. One suggestion for a distributor would be to use the regular/original distributor, and get an aftermarket electronic conversion for it, such as the Pertronics unit. It fits in the original dist with no problems, and gives you the electronics for better reliability. A lot of Corvette guys use it because it is not noticeable...it looks completely stock, but works soooo much better. Just another option for ya. 👍

Thanks for the quick reply Joel.  I have heard of an electronic conversion kit for the original but not until after I installed the first HEI.  I think I can find the original but it will have to be rebuilt.  That is why I changed dist. in the first place.  Thanks again Joel.  

        I installed a Pertronix unit in my 72. works well and reliable.

Just an up date on the old 72.  We replaced the Ignition control module and every thing is working well.  Except the tach.  That is a project for another day.  Thanks for the help Joel and Mark!!

Good deal, Dean! 👍  Experience has shown, that usually when the ign module gives up the ghost, it is the coil that causes it to fry....I would still recommend replacing the coil also, if possible. 🤠

Thanks Joel.  I'll check it out.

Just nosing thru old post did u solve your problem, I had a similar issue and found out it was the plugs not open far enough, standard gap is .035" open up another .010-.015" and that might solve ur problem of missing, it did mine.

I'm pretty sure when the HEI first came out, the recommended plug gap was around .070 to .075.  I believe they also later reduced that gap due to the fact it was not being handled by the plug wires very well.

Great recommendation Joel..I have had a Pertronix I in my '68 Camaro since 1997 and has not missed a beat..I would only suggest that he uses the Pertronix coil slated for the Pertronix he has install i.e. Ingniter I coil,Igniter II coil,and Igniter III,3 different coil with different ohms for the Igniter..

Sal C                                    nosal