JT 3.5e Info from a beginner.
1. O-Rings - The 3 orings on the bolt should be checked especially if the gun is shooting hard and loud like my was when I got it 2nd hand. Was shooting 450 PFS and taking limbs off trees. I replaced the 3 on the bolt and now it shoots about 230 max with 3000 PSI Air Tank. Much safer.
2. Dissasembly - Use the guide for the JT 3/4 on page one if this thread. Many thanks to those who posted it
3. Common Problems - Won't fire or stops fireing, bolt is stuck or sticking, will not cock or dry fire - Solution. Take apart your gun and clean your bolt, polish and then oil well. Ensure orings are in good shape (usually break one of mine removing from bolt before polishing). Remember this bolt is exposed to the elements and dirt, paint, moisture etc get's in easy.
I had cleaned my bolt and re-installed (without oil) and gun was fine. At a scenario I put about 300 rounds through it and the bolt seased up almost solid from friction/heat and lack of oil. I removed it, oiled it, and installed it again and then it worked good the rest of the day.
4. Battery Life - A Duracell E2 lasts me the day usually 2000+ shots, I only have semi-auto.
5 Gun won't fire when trigger pulled - Check your grip and make sure you didn't turn your electronics off with the palm of your hand. This happens to me all the time and it sucks!
6 Gravity fed Hopper Problems - The viewloader that came with mine didn't work well for me, I found that it started to seperate at the bottom, I would see a lot of stuck/broke balls and was in-consistant especially when shooting fast. I had an extreeme rage in stock so I took it apart, cleaned it (watch the spring don't fly away) and it works well. I haven't drilled any holes in my feed neck yet and I can fire almost as fast as I can pull the trigger now (with good paint).
Stock barrel seems to be dead on within about 20-25 feet. After about 30 starts to become a tricky shot. Try to chrono this gun where it wants to be, not maxed out. Go for consistant pressure even if it's only 240 if it's steady and not in-consistant it's worth not having 280 FPS on your first shot only.