Having found the jet that gives me the best acceleration, my next task was to maximize my highway fuel economy by adjusting the APT.  Leaner fuel mixtures tend to burn slower, which is why many people mistakenly believe that leaner mixtures are hotter.  The slower combustion rate can result in the fuel charge in the combustion chamber still burning when the exhaust valve opens, thereby causing burned exhaust valves.  If combustion occurs at the right time, the pressure peak in the combustion chamber will occur when it can exert the maximum torque on the crankshaft, thereby maximizing power and fuel economy.After fixing the plugged primary circuit problem, I started off with my APT turned in all the way (ie, 0 turns) and found that my fuel economy was poor and I had a lean surge.  To eliminate this surging, with my transmission in Park, I set my idle speed to 2000 RPM so that I would be off the idle circuit and on the primary circuit.  I then turned out my APT 1/2 turn at a time and found that my idle speed kept increasing.  With every APT 1/2 turn, I would reduce my idle speed back to 2000 RPM.  After raising the APT to 5½ turns, I felt that the engine had reached the practical limit of part-throttle enrichment so I used that as my APT adjustment starting point.

When I previously worked on tuning my AFB, I found that my best fuel economy occurred when the economy step on my metering rods was just rich enough to not have a lean surge.  I was a bit surprised to see that my Quadrajet idle speed responded so linearly to APT enrichment.  As I drove my car and reduced my APT turns (1/2 turn at a time), I found that my fuel economy improved as I leaned-out the fuel mixture.  I suspect that this improvement comes from the reduction in pumping losses with increasing throttle opening (lower manifold vacuum).

 

1965 Barracuda APT Tuning Results

 

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