cjsdad

Lumpy idle as in surging? Missing? Is the vacuum advance ported or manifold source? If on a carb port, is it getting minor vacuum fluctuations from the carb? Is the mechanical advance jumping because of a loose spring? Is there a vacuum leak at the base of the carb? Lots of little gremlins can cause an erratic idle. How is the timing chain? Is it still the factory one? Could be disintegrating as you drive it. How old are the plugs? Since you put in a new (to you at least) HEI I would hope new wires already. Have you checked the PCV valve? Pull it from the valve cover and put your thumb over it and see if the idle changes. You're moving in the right direction so far, but sounds like you still have a problem to solve. Keep at it, it will be worth it in the end.

Chris98Buffalo

Thank you. Not surging. Missing. Vac advance is ported vacuum and hooked up with the solenoid, per spec. Fuel delivery, vacuum system (PCV) and ignition (no loose springs) are all totally new.
Timing chain and balancer mark verified finding TDC. Timing chain is tight.

cjsdad

Have you tried pulling each spark plug cable one at a time? If the plugs are good, the idle will drop as you pull the wire. If the plug or cylinder is bad for some reason the idle will not change when the wire is pulled. That will narrow your diagnostic window if you find a plug/cylinder that is not firing all the time. Does the miss persist if you disconnect the vacuum advance?

Chris98Buffalo

Since everything is new, I assumed the wires and such were good. But based on recent experience, my next move was going to be an awl and a ground wire...easier way to do what you suggest.

oldcutlass

Since you went to an HEI, "the book" is N/A. HEI has a shorter advance curve than points. 18 initial would be a good starting point with a goal of 36 total. The vacuum advance will be a stumbling point as most often they will provide too much. Limiting it to 10* is a common cure to keep your total advance with vacuum around 50ish.

ROCKETMAN269V

Couple of thoughts here:
1) Did you change the spark plugs to an HEI plug. I use AC R46SX or R46SZ, gapped at 0.060". A points type plug will not work well with HEI.
2) There is the possibility of a vacuum leak on the UNDERSIDE of the intake manifold. To check for that, attach a vacuum gauge to manifold vacuum, unplug the PCV from the valve cover and spray some carb cleaner into the hole for the PCV valve. If the engine smooths out and the vacuum picks up, that means that the intake is drawing in carb cleaner from the underside.
3) I know that you said that you had your carburetor "blueprinted"; but be sure that there is no vacuum leaks at the throttle shafts.
I hope this helps. Bob.

Chris98Buffalo

I didn't change the plugs, interesting observation. Perhaps that and more timing as suggested may cure it. The carb shafts were rebushed as needed; this guy makes em better than when they came from Rochester.

I thought of the intake leak also, but with a strong vacuum number now I kinda put it low on the list.. thanks!

oldcutlass

Imo, the spark plug type (thread length and seat) required are not based on ignition system but what your particular head requires. A good starting point is the stock plug. Heat range is determined by reading the plugs.

Although your carb has been rebuilt, look down in the throat as the car is idling and make sure the fuel is not dribbling as that will give you a surge and erratic idle but still provide decent vacuum numbers.

MDchanic

Actually, I believe that the plugs intended for use with the mid-'70s HEI cars are slightly different in that they have a ground electrode that sits up taller, allowing for a more-or-less 90° electrode angle at the extreme spark plug gaps that were specified in those years. If you set your gap to 0.035"-0.045" you don't need them, but if you run them at 0.060" or 0.070", then the other plugs would be a good idea.

- Eric

jRoofgunner

I read all the posts regarding your problem. I just thought I'd mention that during the build of my engine by CutlassFi he used an MSC "all in one" distributor. On the dyno it was jumping timing way up. So he replaced the module. and it did the same thing. On another unit, where the module was mounted to the firewall there was some sort of failure. I think MSD admitted problems so he put a Petronix in mine.