My hitachi nr 90 gc nailer was always slow to fire a nail with about a second delay between pulling the trigger and firing the nail. now however the gun will only fire a nail once in about 20 attempts, and then fires two nails one after the other, but then goes back to not firing at all. i have cleaned and lubricated the o ring, warmed up the gas still got same problem

Most likely stuck trigger valve. Easy to check. Remove nails, remove trigger to expose the firing pin of trigger valve. Connect air and very quickly depress the firing pin all the way in. If the gun fires , adjust the safety to depress the trigger valve completely. If the gun does not fire, replace firing valve, or remove and check /lube o-rings in valve. Good luck

Various reasons, secondary switch faulty. gas out of date, gas is colld, iar leak, faulty spark unit.

Suggest you get it checked out at a repair centre.

Assuming that you are using the correct nails. This will usually happen if you have bump-fire trigger and are trying to place the gun on the work then pull trigger. Replace the safety spring if weak and bump-fire only (spring should be very firm). If you have single fire trigger, replace the trigger and the trigger valve for good operation. If you have always been bump-firing and now double firing, replace the safety spring and trigger orings. Good luck

This is a bottom fire tool. (NV83A2) You need to hold the trigger down and then as you place the nailer to the wood the safety will depress and it will fire once. This is usually done in one, swift action. If you do this in the opposite way by placing the tool to the wood and then pulling the trigger they often will double fire.

Best practice is to bump fire the gun to prevent double fire. Do not place the gun on the work and pull the trigger. This takes practice while using at an angle. Works better if the nose piece has rather sharp points to help nose to stay in postion while you bump. With trigger depressed, practice without nails bumping/hitting wood at different angles untill you can bump hit at the spot that you need the fastener. If the gun double fires while bump-firing, most likely all the safety/trip parts are worn/loose. Not expensive to replace. Good luck.

Start with the basics:
First check that the butane (or whatever gas is used) canister is full. If empty, put in a new one.

Second check to make sure your battery is fully charged to be able to fire the spark to work the nail gun. If not get the battery charged and/or replaced as needed.

Third make sure all of the safeties are working properly and that you have them engaged to the piece of wood you are nailing when you try the trigger.

If beyond that it does not work it could be a bad trigger, a bad safety switch, or even a bad connection from the battery to the device.

the pin pushing your nails is loose and the guides are warned .
try to change the guides and you will see that this problem will will be gone .
one last thing , make sure the coils are in good shape as it may create some feeding problems and push 2 nails at once

The dent in the wood is not caused by the air. It's caused by the set that drives the brad. Are you oiling it consistenly? Are the brads the right manufacturer for your porter cable brad nailer? From the information you give it appears that the brads are not advancing correctly or that the air pressure is not sufficient to dislodge the brad from the brad strip on the first strike - it takes an additional strike to break it free. The standard for adding oil is to add ten drops of air tool oil for every hour of operation. That seems like a lot, but the correct amount of oil is a sure way to prevent most air tool problems.

www.powertoolspares.com sell parts for hitachi nailers.

Take head off of tool. Clean and lube orings on the head valve . Do not use wd40 use air nailer oil.