My new (used once) hitachi roofing nailer wont feed.when I hand feed it,it has no power.Like theres no air pressure.....Yes the compressor is on. any ideas?

Nailer power is directly related to the air line pressure. To reduce the power, rotate the knob on the regulator counterclockwise while observing the pressure gauge. The regulator is mounted on the compressor.

If your using refills that have wire holding the nails together.
Check to see if bits of wire, debris is collecting in around the striking hammer. I switched to paper and never looked back. Must have gone through 5k worth of nails and rarely jams. Other things to check,
1. Check the striking hammer, make sure it is not bent or damaged.
2. Check air pressure, Low pressure can cause jamming, misfires.
3. If the tool needs pneumatic oil, 2-3 drops before you start good rule of thumb.
4. Check spring return. Sufficient pressure must be maintained or the nails will not feed properly. Misfires, incorrect feed, jamming etc.
I had a Hitachi brad nailer and it was horrible. Fire 1 brad nail would jam or misfire. Did everything I could think of nothing worked. Put it up, got a Rigid brad nailer and never looked at the Hitachi again. :)
Sometimes you just get a lemon! (PS I understand you have a framing nail gun. Brad nail gun above is used for an example)
Hope this helps

make sure there is not any tar or black cement makeing the driver stick to the inside of nose

Hitachi coil roofing nailer will skip nails if the piston oring is worn, the feed spring is weak or lower bumper is cracked. Easy to check. Remove nails and air supply. With side door open point nose of gun up then down again. If the piston moves up and down in the cylinder, replace the piston oring. If piston does not move and is in the up position, check the feed spring for tension. With air supply removed and door open, slide feeder back and press on feeder from door side to check tension. It should spring back to position against rails.If is feels loose, replace spring and retainer pin if the pin is worn. If piston oring is good, and the feeder has good spring tension, suspect lower bumper and the plastic part below the bumper called the "sheet". Remove head and pull the cylinder to examine lower bumper. Parts are available at www.toolpartsdirect.com. Good luck with your repair

you need to thoroughly clean and replace all the o rings also check toe condition of the bumper for wear and debri in the small hole that feeds the advance piston mechanism

check for the proper nail collation and then check the feed pawl to make sure it is working properly, also check to make sure driver is fully retracting,

Hitachi nailers are built on very close tolerences. Dirt /grit will prevent cylinder from resetting causing air to leak out of top of gun. You can force a reset by striking the gun head first against a rubber mat. Only one or two hits because you could damage the head. If the gun does not reset you will have to take apart and clean out grit and replace orings as needed. Good Luck and post again if you have other questions.

Yes. Roofing nailers all use about the same size and spacing on coil nails. You will find that Harbor Freight is about 1/2 the price of Home Depot or Lowes for those nails.
-Jim

If the driver moves on it's own then it's o-ring is shot,replace it. Pop the feeder cap and clean all the roof crud that's built up underneath it.Now you can see the feeder spring,check to see if broke or old and replace.The feeder plunger is air assisted so hook the air up and depress it, it should depress hard and pop right back up. If not it needs to be cleaned and new o-rings. If this doesn't work then it's a new feeder plate you need. good luck.
5run7

The roofers that frequent my shop want their nailers to work like machine guns. Here is what I do to make the hitachi 45 nail fast. Remove the cylinder and sand the fine scratches where orings meet cylinder. I use a rubber slug with a small bolt through the center. The rubber slug fits snuggly into the cylinder and the bolt into chuck of drill. Turn the cylinder with the drill while holding fine sandpaper against the scratches untill smooth. remove the nose from body and replace the sheet (part # 877125) and the bumper. If the sheet fits loose on the driver and the bumper has cuts or missing pieces, the driver will not return with full force. also the oring on the piston should not be too tight. Just snug enough to hold the weight of the piston. While nose piece is off check the air passage to feed piston for trash, use a small wire and push through to make sure. Make sure the orings on the feed piston fit snug. Actually push the feed piston by hand in the nose piece to make sure the three orings fit snugly. A loose feed piston will reduce nailing speed. Finally replace the boot on the firing pin. If the boot is blown off its seat with use replace the firing pin. A loose firing pin will waste air and lower pressure reducing nailing speed. Good luck