gf1800 hard start

[jfox]

I have a mid 90's gf1800 that is hard to start. It starts and is rough running for a few seconds with gray/black smoke. It looks to me the glow plugs are not doing thier job. The injectors were recently reset and cleaned and I was told they were ok but now are good. It didn't change anything.
The glow plugs check good individually. They do get 12 volts when the key is turned to glow position.
any advice?