Wanted to share a story related to the ignition system of my 5-speed converted '72 automatic 240z:  Shortly after I purchased my car, I converted from automatic to 5 speed.  Left the dual points distributor alone except for changing the points, condenser, and cap.  Car ran ok, but I had to dial in a lot of timing advance to keep the engine from backfiring through the carbs when cold.  Car ran seemingly fine all winter with an occasional misfire condition in warmer weather with warmer engine temp...never could figure out why.

Fast forward many months and I decided it was time to get rid of the dual point distributor.  So, I found a used single point distributor on Ebay and did a complete cleaning/refurbish.   My vacuum advance from the dual point setup seemed to operate better, so I used it.  Dropped in the distributor, and engine ran horribly with severe missing, rough idle, low power, knocking noise, etc.  Sometimes the symptoms went away and the engine ran smooth with a higher idle.   Uh oh, what did I do/what's wrong?  Messed with it for a few days before the light came on...I grabbed the distributor while the engine was running and wiggled it.  Yep, when I wiggled it, it would change from running poorly to running well.

Pulled the distributor and cleaned/sanded all the mounting bolt points.  That did the trick.  No more running like pooh, no more missing, no more knocking noise...Just smooth running.  And the best part...cold start drivability is so much better.

Just wanted to share in case others have similar issues.

Edited June 7, 20186 yr by David F
Grammar

Grounds, the oftenoverlooked part of the circuit...

There's supposed to be a ground wire that runs from the distributor mounting bolt to the body.  Yours must have got lost over time.

Yep, no wire.  Thanks

Finally was able to drive the car significantly since the distributor change out.  New problem is severe missing at part throttle cruising...even the tack jumps around.  Disconnecting the vacuum advance solved the problem.  I used the dual points advance and am now thinking I need to use the single point advance.

You're having odd problems, with odder solutions.  Fixing a tach problem by disconnecting vacuum advance is weird.  Seems like the breaker plate moving might have been working a loose connection.  You might recheck the mounting of the points themselves.  The screws and wires and contact areas.