Tuning Newbie here: Why completely degas before lowering reg setting?

Yes you can do that. You can run the pressure down below the regulator’s setpoint, making the valve seat open up and remain open, then adjust the setpoint downward to where you want it. This sequence avoids grinding the valve seat so long as your new reduced setpoint is something higher than the current system pressure. Naturally you won’t be able to observe the new setpoint until you add pressure to the system, which is why some prefer to do the incremental adjustment technique with the system fully pressurized.

I am the sort that would rather take no risk of damaging the seat. Meaning I prefer the approach you are asking about, or degassing fully.

When you rcvd the gun, the reservoir had already leaked down to 1000 psi. The 1700 psi Regulator gage showed you it had a FIXED volume of regulated air that leaked down from 2600 psi to 1700 psi during transport to you. You can never increase the regulated pressure beyond what the available source pressure is, in this case 1000 psi. The regulators used in this rifle is a very inexpensive knife edge seat design that can easily be damaged if trying to increase setting before degas. Not sure if this helps you but I hope it does.

Oops, yes increase ok, lower not ok.

If you lower the reservoir to 1000psi, the Regulator gage will also read 1000 psi if it is set for 2600 psi now. Once your lower in charge vs regulator setting you have an unregulated gun. The regulator adjustment is not precise nor is the gage used. The effort to pump to 1000 psi is minimal. 2000 to 3000 far more exhausting. I suspect the regulated gage reading 1700 psi ( gage on right hand side) was stuck. I just now degased my rifle from 3000 psi to 1200 psi. Regulator was set for 2200 psi. Once the reservoir gage drops past 2200 psi BOTH gages tracked down to 1200 psi as I continued the degas. This is normal. 

Thank you nervoustrig for the info about the regulator valve seat not being seated and should be free to move. I received the gun with 1000 psi in the reservoir and the regulator was reading 1700 psi. Hand pumped the reservoir to 3400 psi which moved the regulator needle to 2600 psi. Thinking of shooting the res down to about 1000 and then closing down the regulator 1/4 turn at a time while cocking and firing until I get down to the 1600 psi range on the regulator dial. Have the gun dialed in pretty well with 10.5 gr (.177) pellets but have a mess of lighter pellets that don't do well with the higher pressure/velocities.

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A quarter turn adjustment is A LOT on that gun. Better go slower.

I lowered the regulator pressure on my 22 Avanger.

I did no degas completely but shot blanks until the air pressure in the tank was around 700-800 PSI.

Then I rotated the regulator screw clockwise about 1/4.

The main thing is to have the pressure in the tank lower than the regulator pressure when you adjust the regulator.

If you’ve never sourced repair parts for a reg and had to remove it from the gun to do a full rebuild or outright replace a broken one I wouldn’t chance adjusting for lower pressure with air in the gun. That there is a task for seasoned folks, folks that can easily rebuild a reg in their sleep.

ive rebuilt a few regs now and to date am very familiar in their function and design, and I still always degas the gun, close off the regulator up to 80%, refill gun with air and work up to my new desired pressure by slowly opening the reg adjustment screw.

Some reg parts are not easily acquired and some regs can be tricky to get to and rebuild. Not to mention I don’t feel like throwing away $100 to $150 towards a new replacement reg cause I was trying to take a shortcut, so I don’t chance it by skipping the simple task of first de gassing the gun.

the pictures are an actual regulator from my Avenger. This is a very inexpensive design very short stroke on the spool. They used a series of arranged Bellville washers for adjustment spring. The delron material you see on the tip is the critical area where the knife edge, internal to the block, seals. If this area is marked or damaged, it is junk and won't hold or track properly. Very crude, but, for a $350 gun regulator it works fine if you adhere to directions. 

Thank you 454. " knife edge seat design that can easily be damaged if trying to increase setting before degas". Thinking you mean decrease setting as the manual says I can crank up the reg pressure anytime, just can't go lower without degas. So you think my plan to lower the reservoir to 1000 followed by small adjustments to the reg will most likely cause damage?

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Yes, lowering pressure causes valve cone to move INTO contact with the seat, raising it moves the cone away from the seat, while no pressure has the valve full open so no valve contact unless a really large lowering adjustment is made.