Technical help please; low speed runs rough and tach needle bounces

In Clarence. Crate 350 with 3K miles. Last time I drove it on the way home the tachometer needle started to bounce erratically and when I slowed to a stop it ran rough. At highway speeds I didn't notice a miss.

I and my guru friends all agreed it must be a coil issue, even though the (Pertronix) coil had only about 1K miles on it.

Today I replaced the coil with a new one (Edelbrock, fwiw) but symptoms remain.

Ignition is Pertronix electronic btw. Both coils are 1.5 ohms.

I can replace just about any part on a car but seldom know which part to replace/fix.

Anyone got any ideas?

 

"Assuming" an electronic tachometer, the signal driving it is the output pulses (ON-OFF voltage fluctuations) from the distributor (This is the wire between the distributor and the coil). If the tachometer is erratic at low RPM, I suspect that the distributor output is erratic at low RPM.

At this hour, I don't have any definite answers... Some thoughts.

1. In the distributor, check the gap between the Pertronix module and the rotating magnet. It been a while - I seem to remember a warning from Pertronix to use a non-magnetic feeler gauge. If the gap to too large, the magnetic pulse seen by the Pertronix is week at low RPM.

2. Are the fasteners holding the Pertronix to the distributor base plate tight?

3. Visually inspect the wires for broken connections or bare spots hitting the case.

Russ

 

What was the condition of the distributor when converted? Have you checked for shaft wobble

 

Not exactly but kind of relevant but when my factory Corvair tach started acting up, the points needed adjusting and probably cleaning.
No clue how that relates to Pertronics, but I suspect it does.

I'll try to remember to Google Pertonics for tech info using dwell meters and oscilloscopes.

 

How old are the plug wires? I have seen something similar on OT Chevy truck. It was the secondary (HV) coil wire. The conductor had developed a break inside the wire and was arcing. I would also check the distributor cap terminals inside and out including the spring loaded rotor button. Never been a big fan of those Pertronix.

 

You’ve not said how your tacho picks up a signal. If it’s wired to the coil, disconnect it and see if it makes a difference. You don’t need to change anything to rule that out.

 

I'm assuming the breaker plate the Pertronix is fastened to has been checked for wear. If not, wear may cause the gap to change as vacuum is applied or diminishes according to throttle position.

 

I would start here. Over the years, I had a few tach failures that acted just like there was an ignition problem. By disconnecting the tach wire from the coil, it eliminates the tach as the possible problem. One wire removal, easy test. If the problem is still there, I would look into a distributer problem. If its gone, the tach is the problem. You can reconnect the tach wire after you solve the other problem.

 

I'd be very suspicious of the Pertronics module. All the checks suggested above are a good plan, but if they fail to locate the trouble the module is where I'd be heading.

 

I'm just now back at my computer for the first time since I posted last night.

THANK YOU one and all for reading and responding. I now have a laundry list of items to be checking on. I'll let you know what turns up.

 

How about a new vacuum leak?

 

UPDATE:

Got another thousand miles on it.

When we got ready to leave Chicago to come home Monday it wouldn't start. Long story short; it took a rollback ride to a hotrod shop and a new HEI was installed.

Not only is it running it's running better than it had been recently.

Bonus: we are at the daughter's and not in Timbuktu paying a hotel bill.

Thank you all for your time and advice.

Btw, the problem was the Pertronix unit that had 4K miles on it.

 

Yeah, Pertronics can be finicky, and the HEI (especially if genuine Delco) is quite reliable. Not beautiful, but stone reliable. There are folks with LOTS of miles on Pertronics without trouble, and I applaud them. For every one of those folks I know, there are five with less than satisfactory experience.
But what do I know...I still like points.

 

I dumped my Pertronix and went back to points. My car wouldn’t idle anymore till I got points back in there. Glad it worked out for you.