I have a John Deere 318 mower that has a weird problem that I can't figure out. The engine runs fine until I engage the electric clutch. Once engaged it will idle fine but miss badly once revved up. I will run fine at high RPM with clutch disengaged. I thought bad clutch and swapped with a known good clutch but it still acts the same. I then tested coil voltages to see if this caused the miss. With deck disengaged, + voltage on coil runs about 13.5, the same as battery voltage. - terminal on coil runs about 10.5 volts. Once I engage the clutch, the positive terminal on the coil will run the same 13.5 volts. With engine idling the - terminal will be about 10 volts. Once you rev engine up with clutch engaged, + coil voltage remains at 13.5 volts but the - coil terminal will fall to about 6 volts and the engine will begin to miss badly and backfire through the carb. What could be causing this? I'm thinking it almost has to be the ignition module but yet everything is fine with the clutch disengaged.