Your Megasquirt ECU is getting its tachometer signal from the wrong source. Since you're running fuel only, the ECU is looking for simple 50%DC square wave pulses representing ignition events. This signal is output from the stock ECU on pin 73, a white wire. The wire you're currently using is from the crankshaft position sensor; it carries a signal that Megasquirt doesn't know how to use without the proper 420A modifications and settings.
