Digital BW, The Print

I have been using the MIS Hexton VM system for a about a 
year...averaging printing about once to twice a wekk. I do not get 
print head clogs at all. I consistently get a significant clog in 
the tube that feeds from the darkest black ink. I removed this block 
twice so far and it has returned again. The other inks have never 
clogged.

I have not seen clogging in tube addressed in my search of the 
archives. 

Cleaning the clog is messy and a pain n the butt. I would like to 
solve it if possible.

Is it the nature of this ink? A tube/ink reaction if it sits in the 
tube for a week or so? Humidity? Bad batch?

Any experience and advice in this regard would be appreciated. 

Cheers,

Murray

I used this printer for the black and white component of my digital 
imaging and photography couse, so reliability is essential.