bar oil leaks a lot of oil only when saw is running

[GIBSAW]

I have an older Stihl 011 ATV chainsaw. I recently removed the plastic cover, blade & chain to clean it, something I've done many times in the past.
This time, after I put everything back and started it up. A short time later, I noticed that a lot of chain oil was coming out from the bottom part of the cover. What could be causing this?

 

[Bob Hedgecutter]

At a guess- if you used compressed air and managed to direct a blast down down through the hole in the bar pad the oil comes out of- you may have torn the diaphragm in the impulse pump, or damaged the spring under the diaphragm, or blown the foam filter off the pick up side.