Just wanted to share the results of some recent testing, hoping this may help others some time down the road.

So with no other changes, I dropped 2 heat ranges(85s to AR73 @0.035" gap) & made a primary jet change from 74s to 76s as suggested. btw...74/84 jets are what was in the carb when I purchased it.

The plugs are now black, and the surge at 30-40 mph cruise speeds got worse.

So based on these results, I decided to take a step back, re-evaluate all settings, then implemented the following single step change/test approach...with very positive results.

- Cleaned and reinstalled the AR73s plugs gapped at 0.035". Once tuning is complete, I plan to evaluate 74s at various gaps between 0.035-.042" to see if they remain clean during current driving conditions.
- Set initial timing from 15* to 22*...improved idle vacuum.
- Set total advance timing & curve from 37* @3k rpm to 35* by 2k rpm... in case there was inaudible detonation taking place with a heavy car, 93 octane & auto trans.
- Adjusted carb primary throttle position (approx. 1.5 turns from closed) to expose 0.020" idle transfer slot, then set idle rpm using secondary idle screw under base & set idle mixture screws (approx. 1.75 turns) for highest idle in gear.
From these simple ignition/carb changes alone, I now have much more idle mixture control, increased manifold vacuum, more consistent idle in/out of gear, and the overall throttle response & drivability has improved significantly.

Still experiencing a 30-40 mph surge, which I determined to be right at the point where fuel just starts flowing from the main circuit. Based on this observation & the test results from the above changes;
- Changed primary PV from 4.5 to 3.5, determined by selecting a PV "2" below the lowest manifold vacuum reading observed in gear to assure it was closed during cruise & only opened under heavy load. I plan to revisit this value once jets are determined. Still experiencing a 30-40 mph surge.

- Returned Holley 4778-2 (700 cfm) prim/sec jets back to original listed spec of 66/76. Low and behold, the 30-40 mph surge for the most part was eliminated by this single change alone(76 to 66), confirming primary circuit was far too rich, & not lean as originally suspected. See attached plug photos for AR73 and 85 at current carb settings.


Will continue to jet primaries down until a lean surge is experienced & plugs show signs of the condition, then back to the prior settings, then determine secondary jet sizes based on primary, which is currently 1.15 times greater than the primary.
Then zero in again on the power valve for best loaded performance
Once I'm happy with the carb performance, I'll refocus again on plug heat range and gaps.

Hope someone finds this info helpful.