The firing pin is only retracting about half way. dont know why its doing this.
You have damaged o-rings in the gun. Remove the head cap & valve assembly, remove the ring on top of the cylinder, remove the cylinder & piston. The cylinder is in tight so work it out carefully. Check the condition of the following: a split flat ring with a rubber o-ring under it on the top of the piston and two o-rings on the outside of the cylinder. If these are damaged air will leak around the piston instead of pushing it back up. The inside of the gun should be clean with a light coat of oil on everything if you are oiling it properly before use. Dirt in the gun means the gun is being laid on the ground without the air hose attached. Dry innards means no or not enough oil is being put in the gun before use. Oil pooled in the bottom of the gun means too much oil is being used. Senco doesn't make a rebuild kit for this gun, you'll have to go to the Senco website http://www.senco.com/ or other website like http://www.ereplacementparts.com to get a parts breakdown of your gun and order the o-rings (any part beginning with LB) individually. If you replace one, replace all of them and grease them up liberally when reassembling the gun.
What air pressure are you using? Every gun is different but somewhere around 70psi the pressure will not be enough to return the driver. Most operate best at the 90-100psi range. You also may have a bent driver which could be hanging up when trying to retract.
SOURCE: Senco SN4 misfiring intermittently
This is a discontinued tool by Senco but there are still parts available for it. It sound to me that the o rings and possibly the firing valve need to be replaced. I would recommend going to Senco.com and you will find a complete parts diagram in their discontinued tool section. They also have a toll free number if needed and you can talk to their air pro. I have rebuilt quite a few of these and they are not that hard just make sure to clean all the parts and use lube when replacing O rings.
SOURCE: senco finish nailer not firing nails
firing pin called the driver is probably broken or needs new seals max air pressure 125 psi senco sells repair kits
SOURCE: The Senco SFN-1 gun will drive one nail and then not retract
sounds like time for a rebuild i do this once a year on all of my guns i buy a seal kit and replace them at that time and oil the interior oparts and check for worn or broken parts
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