[EricVanWyk]

You can attempt to duty cycle modulate the solenoid. Unfortunately, you might break it, and your resolution would be horrible.

Turn the solenoid on and off very quickly, and you might be able to get a quasi-open state. In an ideal world, if you blink the solenoid once per cycle, you would get half the air through-put.

For a slew of reasons, this won't happen perfectly. For one, you need to toggle it very quickly. Neither the spike nor the solenoid are rated for this.

That being said, I have seen it done and I have done similar things on different hardware. If all you need to do is slow it down, go for it. Just keep spare parts on hand.