If the efi is getting the tach signal from the HEI, then that is where I would start. LOTS of electrical "noise" at that terminal, which can cause the ECU to think that the actual RPM of the engine is bouncing up and down. The ECU will chase the false signals, causing the system to run poorly. Remember, the ecu is making a bunch of calculations every second.
At the least, use the HEI to trigger a CD box, and let the CD box provide a tach signal to the EFI. Even better is to use the distributor designed for this system. With the ECU controlling fuel & spark, AND providing a very accurate tach signal, the system should perform significantly better.
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'73 T/A (clone). Low budget stock headed 8.3:1 455, 222/242 116lsa .443/.435 cam. FAST Sportsman EFI, 315rwhp/385rwtq on 87 octane. 13.12 @103.2, 1.91 60'.
'67 Firebird [sold], ; 11.27 @ 119.61, 7.167 @ 96.07, with UD 280/280 (108LSA/ 109 ICL)solid cam. [1.537, 7.233 @93.61, 11.46 @ 115.4 w/ old UD 288/296 108 hydraulic cam] Feb '05 HPP, home-ported "16" D-ports, dished pistons (pump gas only), 3.42 gears, 275/60 DR's, 750DP, T2, full exhaust
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