I have \an Air Comp. Model 919.167210 Craftsman

Try carefully lightning the tubing nut to resale.

You either didn't get it back together correctly or you will need a new pressure regulator.

really sounds like a carb problem if it only starts acting up under load

Some of the craftsmen compressors have pistons that are coated on the top of the dome. Once the coating wears off, they have severe loud piston slap. If you continue to run it the way it is, it will snap the connecting rod in half or break the piston and possibly the cylinder.

check air tank drain valve located on bottom of tank, if it is not leaking try adjusting air pres regulator (large black knob close to air hose connection.) the knob when turned in will increase the tank air pressure, if knob wont turn check that lock ring on bottom of knob to make sure it is disengaged. also check for air leaks in all lines and connections

This sounds like it is either the pump is leaking from the piston or more commonly, the rubber seal that seals the outlet tube to the cylinder head blew out like mine. Let me guess, it got a lot louder than it was originally? The seals are readily available from sears parts direct, its like 3 bucks but shipping is around $8. Hope this helped

don't know what model you have. They are all pretty similar tho. Look for the regulator should have a cover on it with a single nut in the center of the cover holding it on.Under the cover is a diaphragm and a spring. tighten the adj. on the spring the compressor will shut off at higher pressure. You don't want to exceed the max rating (ie: if is suppose to shut down at 100psi don't try to go higher.)

This problem is not uncommon with small compressors and the inexpensive plastic regulators they use nowadays. The one surefire way to stop it is ti install in in-line valve before the regulator outlet, even if it involves the double ended & threaded brass soupler so they both fit on. This way you can just turn the handles across the line effectively shutting off air flow. I have two pancake compressors and ended up doing the very thing.

Then when time comes to hook up a hose and tool, just switch the handle back and you have air.

Sirwriter

take the air fiter on pump off and see when pump in running if it has suction if not could have debris under valve keeping it open

Your compressor is most likely made by Campbell Hausfeld. The oily substance could be leaking from the capacitor on the motor. Remove the cover and examine the capacitor for failure. There is a value printed on the cap. (uf) The cap must measure to within 95% of printed value. A failing capacitor will cause the hum that you are experiencing.