I searched and read a lot of talk stating you should ream glow plug holes but never could find exactly what this entails? I stuck a drill bit in the hole and twisted. Do I actually want to scrape hard around the sides to get carbon off. Where/what is this carbon building up on to make my car not start? I thought that the glow plug tips protruded into open space where they would help ignite fuel air mix so i do not understand the carbon build up problem.

thanks,

p.s. It was in 40s the last weeks so 81 300D car started fine. I had replaced all glow plugs and adjusted valves. It ran great with tightened valves. I was waiting for cold to see if i would need to take further action This morning at about 28F , I had the same old problem -- took 45 minutes of cranking before it would start.. Now to the further action -- 1) ream glow plug holes 2) Watch plug get red hot despite apparent function of relay (voltage at all pugs fo 45 seconds)
3)fuel star of delivery. 4) starter (it really is unhappy) . Can anyone suggest anything else?