I think I may need a rebuild, but.

My Dart with 3.55 rear, Clifford headers & intake, Holley 4160 vacuum secondaries, and mild cam in June returned 21 mpg. on that great $4.45/ gallon gas, Now it's using the top third of a tank to go 45 to 50 miles on the cheep $2.15 gas. Dollars per mile remained constant, but fuel consumption seems to have become a bit glutinous. I suspect the secondaries are cracking open too soon, but what do I know.

Changes performed since June: installed missing long spring on 11-R mechanical advance (7-R vacuum can); set valve lash .002" over standard .010" & .020"; and reset timing to 12 deg advance at idle. These changes increased manifold vacuum, smoothed out the loping idle, and perhaps increased performance marginally as judged by standard SOP test method (seat of pants).

There is no bog on take off, looks to be a good strong accelerator pump squirt, but there is a nice flat spot when wide open throttle is applied at road speed say 45-70 mph. The engine has always had this flat spot, but it seems to be more pronounced now.

There is no smell of gas when parked, no gas leaks in and around the carburetor, no back fire up through the carb, the choke pulls fully off when warmed up, no spark knock, and no black smoke out the back.

Do I need to change out the secondary's vacuum spring, or perform a total rebuild? Any ideas.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC