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Tuning PP750 Tuning - Help request

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R

rash_powder

Member
Sep 25, 2020
23
1
Larimore, North Dakota, United States
I finally had time a couple nights ago to shoot my PP750 over the chrony and get some data so I can start to tune the pistol for max shot count. I'm shooting 5m at paper in my garage and would like to see how many shots I can get from it. Below are my shots, all in one line. They were done in 9 round strings since thats the magazine capacity. The pressure readings are taken from the gauge on the pistol and mark the start and stop of the individual strings. I had a few errors in the mix, but replaced them with a velocity that fit within its neighbors.

Am I wrong to believe that at about shot # 36, 145 bar, is where my regulator setpoint is? Thats where my velocities start falling off. If you graph that data there is a little bump right before, like 2 or 3 shots before, the steady drop after #36.

If 145 bar is my setpoint, and I want more shots per fill, would I be wrong to back my reg off to about 100 bar? Thats assuming I can keep my velocity at approx 400 fps.

Thanks for any help/input!!


 
Hi, it isn't uncommon to have velocity increase as the gun drops off the regulator. As the pressure in the regulator drops the hammer is able to open the poppet valve longer. That can cause the velocity to increase for few shots as the regulator pressure drops off, similar to a unregulated airgun's velocity bell curve. Minor effect in this case, 145 bar is good estimate of the regulator set point. 
As you drop the regulator pressure, you will also need to reduce the hammer spring to match.
The PP750 does not have an externally adjustable regulator. That means disassembling the gun each time you want to adjust the regulator. Each time you back off the regulator, you will have to reduce the hammer spring until the velocity drops off. Then increase the hammer spring back up to where velocity was prior to it dropping off. Too high a hammer spring produces too low a valve opening time, dewel, which increase velocity variation.
Looks like 100bar should be about 420 fps. If your PP750 is 177, then don't go below 100 bar on the regulator.
The bearing surface on a 177 cal is comparably high, so you need 100 bar to break the pellet free consistently.
If you could barrow a regulator tester, you could set the regulator to 100 bar in one step. Also, no need to fill all the way to 250bar, with the reg at or below 145bar just fill to 150bar and shoot from there. Once you have the hammer spring tuned, try one string from 250bar. A PP750 is like having a budget Edgun Leshiy!
 
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Sweet!

What you said verified what i was thinking and filled in a few bits I hadn't thought of. I know a complete disassembly is in order, but it is needed as I lose half a 250 bar fill overnight. thats a lot of pumping for naught! 

I have seen in another thread that Thane polished his belville washers inside the reg to help them seat and wear in more quickly. I may do that as well whilst I am in the pistol; or not. Depends on how bad the fiddle-with-it bug gets me. I find myself causing problems at times because of that.

Thanks!
 
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Can you hear the air leaking out the barrel? My PP750 was an early model and had broken poppet valve. Most likely your doesn't have this issue as it leaks more slowly.

There is a small o-ring underneath a screw on the instead of the cylinder cap (the piece that has the pressure gauge and fill port). That o-ring is the most likely cause of your leak. It will have to be replaced. I don't have a o-ring list for the PP750, but, the PP700 uses a 1.5mm X 4mm Buna-N 70 for the fill port o-ring. You can order o-rings at McMasters, https://www.mcmaster.com/o-rings/. Blead the cylinder down completely before opening it. Be careful with the o-rings that seal the pressure tube. I like to spray down the o-rings with Silicon Lubricant aerosol spray before reassemble. It is easier then using silicone grease. I purchased mine at ACE hardware.

Hope this isn't all info you already know.
 
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No, all good info. Thank you much for the pointers. A while back I found an exploded diagram of the pistol and ordered o-rings. I have something like 20 complete rebuilds worth of o-rings now.


I was going to just replace all the prints, but will definitely inspect that one under the gauge real close before reassembly. I’m thinking i will change prongs after tuning the reg so as to not over stress the new ones.

thanks again!
 
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Just did an .177 8.3 FPE tune using 7.8 grain averaging 692 FPS for 63 shots under 3.75% ES (center 45 shots under 1.5%) reg at 100 BAR. This was after a 4mm plenum port, 3.5 valve port and 3.33 transfer ports on barrel thimble, stock poppet. 

Setting the regulator to make just over desired velocity with high spring force. Then trimming the spring back until the velocity just begins to drop. Yields minimum regulator pressure and spring force for desired energy. Least wasted air, high consistency. You can get more, but you'll have more work to do.
 
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