Preshading without clogs

What paint and thinner are you using? At 50/50 ratio there should be no clogging problems at that psi. Unless your paint is bad with lots of particles, it should not be clogging your airbrush.

I am using Model Master enamel paint and airbrush thinner.

And everything is stirred thoroughly? No clumps or particles in your mix in the cup? Enamels are not as prone to tip drying as much as acrylics. 

Have you cleaned your airbrush since encountering the clogging? And if so, did you come up with anything?

I am just trying to eliminate the obvious problems first.

I will try a thorough cleaning and straining the paint through pantyhose and see what happens.

Is it a metallic paint giving you problems? Sometimes those have larger grain pigments.

No, just regular MM flat black.

Just try a little more thinner.  While I start with a 1:1 mix, I often add a little more for any of several reasons, including clogging.  One problem with MM enamel- it settles in the bottle real fast, so depending on how you pour or suck up the paint, your original paint may vary in thickness even with a lot of stirring in the bottle.

I have contemplated a gearmotor driven bottle rotator to keep the MM enamels I am using on a model continously mixed.

I'll definitely bear that in mind next time I preshade, Don. Thanks!