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Make sure there's enough oil in it. Read the manual. The base screws off, you remove a large piston, then fill the center hole with oil to the top. I'm not saying fill the whole 4" diameter cavity - that's where the piston rides. I'm talking the ~1/2" hole in the center. Like I said, read the manual - it will describe it, or at least it *should*!
Additionally, most of these guns, regardless of source, are sensitive to air pressure. Some folks like to pull at 40 lbs, others 70 or more. Experiment to see what works best for your application.
Make sure you have the proper size jaws installed. Some pneumatic riveters require you to change jaws for different size rivets. It sound like you could have a jaw that is for a larger size, and is not gripping the stem adequately. Check the size it is intended for in the manual (or on the box) and check to see if additional size jaws are included the the package.....
4 to 6 drops of oil in the air inlet should provide enough lubrication to operate the tool properly, (regular air tool oil or automatic transmission fluid), more than that merely blows the excess out the exaust and makes a mess. I normally run my guns about 45 to 50 pounds for A4 rivets and 55-60 for A5's. I went thru 2 of the Harbor freight guns before I switched to the Neiko guns. Some have had good luck with the HF but I was not one of them. Here is the gun I use https://amzn.to/31X4QUo I run one for A4 and one for A5 and have had no problems with either.
Also be sure that the previous rivet stem is dropping clear, if the stem remains in the jaw area it will not allow the current rivet to pull in many instances.
Sounds like the seals or tolerances are bad and is not retaining enough air in the chamber when pulled. As Carl said adding lots of oil may seal it, but that is a temperary solution as it will leak out and the same problem is back. that is the problem I had with the 2 HF guns I tried. They would last a while then I had to tear them apart and fill with oil. I dumped them and have not had any problems since.
one other thing you might check before you give up on it is make sure you have big enough air hose and fittings to supply enough air. If you are using a long 1/4" hose or small ID connectors it may not be supplying enough volume?
Try this: There are two-three HF guns. This is my second airplane, so I've gone thru 'em. As they say, it may be cheap, but if it gets you thru to the end of construction, it was a bargain!
When I started my Cruzer build, I went back to HF, cuz, of course, my first gun was toast. I noticed they had 2 green ones, and a black one that was a bit more pricey - like $65-70. I bought it. THIS THING IS PERFECT. That's just my opinion, though. After 500 build hours, I've only had to add oil to the piston area once - when it was new. One stroke perfect rivet pulls. The seals are better, too. My old gun would spray oil out of the vent under the trigger such that I always had to have a rag wrapped around it. This gun doesn't leak, doesn't leave oil rings when you set it down, etc. Pulls great from 40-70 lbs (haven't tried other ranges). I would never buy a green HF gun again.
Get a Milwalkee rivet tool. About $250 Electric. no cords. Fast. I threw all the junk air tools away. I wish I had this when I started. Model 2550-20
Depends on what you want and what your work flow is. I have been going to get a milwaukee just to try it out and I think it has it's place, but as a scratch builder and my other projects I have in the shop air tools are a large part of my work flow. It alows me to have multiple small lightweight air drills dedicated to the common size drill bits on the Zenith. 1/4" CP drills at about $120 each allows me to have 2 for every 1 cordless Milwaukee (if you consider extra batteries) and are lighter. Same thing with the Neiko riveters I have in the shop. $50.00 each, one for A4 and one for A5 less than half the cost of one milwaukee, and lighter. Yes the cordless tools are convenient for a quick job and I grab cordless drills and drivers constantly to go and do a quick job, but when I have already dedicated a workspace for the plane I don't need the portability of cordless.
with the Milwaukee cordless, do you have to get the custom machined heads for it from Zenith? Does one transfer their old custom heads from previous riveters? How does that work?