I'm not surprised, I didn't expect retarding the timing would correct cold misfiring. retarding it further probably wouldn't help either and 14 degrees is not enough for smooth full throttle operation. you can easily check the glow plugs, simply remove the metal bar connecting them and test them with a testlight. put the clip from the testlight on battery positive and touch each glowplug with the testlight probe the light should illuminate for each one.