If you re-read Kerrys post he says that if one is bad then none will work on the series type. I have the same ones in my '77 240 and I am getting hard starts too. It is accecptable to give light throttle to start cold.
I just hate waiting so long to glow.
Try checking ALL of the connections on each glow plug AND the fuse strip.
Getting each connection clean and tight can make a difference.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.