Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Idle and Perf Issues
RMA:
Model: 7040251
1970 Cutlass SX 455 TH400. Rear is probably a 2.56 or 2.78. Complete stock rebuild last summer (200 miles) except 0.03 overbore and comp cam hyd. flat #42-114-3 (so called factory muscle). LSA 113, Intake/exhaust dur. 308/308, Int/Exh Duration at .050”: 233/233, Intake/Exh. Lift: 0.296/0.296.
After rebuild Poor performance, idled in park ok, idle dropped in gear & continued to stall.
My carb guy purchased Cliff’s book and parts.
Changes made - Parts from Ruggles:
Idle Tube .039
Idle Down Channel drill .059
Upper and Lower Air Bleed .070
Idle Bypass Air drilled .110
Mixture screw through hole drilled .090
Main Air bleeds plug & drill .040
Primary jets .070
Primary rods .026, .048, .061 (was .026, .038, .062)
Secondary rods .045, .084, .132 (was .053, .102, .134)
Secondary air valve ¼ turn from zero
Power valve unstuck and new “red” spring
High flow needle & seat, new cam on secondary air valve
After above changes very slight improvement. It will idle in gear if idle set to 850-900 rpm in park. Initial timing = 12 BTDC (tried 8-14, 12 seems most stable). Running rich, black soot in pipes, burns your eyes. Manifold vacuum at idle erratic 10-14 inHg (advance plugged). Checked for vacuum leaks multiple times w/ carb cleaner, none found. A/F mixture screws very little to no effect. Secondaries will not come on (due to lack of vacuum?). Also as rich as I’m convinced it is, it still seems to gain a few rpm if I hold hand over the carb. PS – it ran smooth as silk/great acceleration (broke tires loose 1-2 shift) prior to the engine rebuild/carb work! Any advice is much appreciated. Thank You.
Cliff Ruggles:
What is the static compression ratio of the engine?
RMA:
Engine builder estimate is 9 to 9.5:1, running 93 octane. Thanks
77cruiser:
Is the cam degreed?
RMA:
Yes.
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