Lower FPS after a delay

[scottg0814]

I'm shooting 25.39gn JSB's in an FX Maverick Compact

I just got a new FX Chrony today and started messing around and I've noticed something that makes me wonder; Do I have a leak somewhere or what other phenomenon am I experiencing?

I cock the gun, get set and fire. Several seconds pass between cocking the gun and pulling the trigger for my first shot. First shot in the string was always in the 880 FPS range. But now that I'm situated and firing more quickly, my FPS is in the 920 range.
I notice that if I cock the gun and fire immediately, I'm shooting 25.39gn JSB's at around 920ish FPS. However, If I cock the gun and then pause for a few seconds, my FPS drops to the 870 to 890 FPS range. This happens every time, the quicker I fire the FPS is in the 920 range. Every time I hesitate, the FPS drops below 900. It's consistent.

Any thoughts?

 

[Sqwirl57]

What happens if you cock it and let it sit for 5 minutes?

 

[Emu]

After filling a regulated rifle first two shots always change in regard of third and following shots.

 

[nervoustrig]

Sounds like the recovery time of the regulator is somewhat slow. After a shot, the pressure should recover to a stable value within 3-5 seconds. Try firing and then quickly check the pressure gauge and see where it is after 5sec, 30 sec, and 1 minute and report back.

 

[scottg0814]

What happens if you cock it and let it sit for 5 minutes?

Just went outside and did a quick 3 shots.
Cocked gun, loaded mag, took first shot so just a few seconds) 865 FPS
2nd shot ( immediate cock / fire ) 905
3rd shot (cocked, waited 5 mins ) 870

Maybe I need to find my guns timing. Do I need to wait X seconds before firing to maintain a consistent velocity?!?!
Is that normal ?

 

[scottg0814]

Sounds like the recovery time of the regulator is somewhat slow. After a shot, the pressure should recover to a stable value within 3-5 seconds. Try firing and then quickly check the pressure gauge and see where it is after 5sec, 30 sec, and 1 minute and report back.

My 2nd reg gauge isn't very detailed but I can give it a try

 

[scottg0814]

I have another Maverick with a 700mm barrel. ( That I plan on selling ) I'll throw the chrony on that tmrw and see how the shot string looks.

 

[nervoustrig]

Maybe I need to find my guns timing. Do I need to wait X seconds before firing to maintain a consistent velocity?!?!
Is that normal ?

Referring back to the recovery time I mentioned above...a regulator needs a short time after each shot to replenish the pressure in the plenum. Ordinarily this recovery time is less than 5 seconds and therefore isn't noticeable for most types of shooting...by the time you chamber another round, aim, and squeeze the trigger, more than enough time has elapsed.

The recovery time may be abnormally long if one or more of the following is true:
1. the regulator's valve seat is worn or damaged (typically referred to as regulator creep)
2. there is some debris trapped in the orifice or in the threads of a metering screw
3. the setpoint is adjusted too high

Note also that the hammer spring tension (HST) has a large influence over how sensitive the gun will be...meaning how much the velocity will fluctuate as a result of the pressure. Best practice is to adjust the HST to somewhere in the ballpark of 95 - 97% of peak velocity. To do that, gradually dial up the HST until the velocity no longer increases. Let's say it tops out at 980fps. Back off the HST until the velocity comes down to somewhere between 930 - 950fps.

 

[scottg0814]

Referring back to the recovery time I mentioned above...a regulator needs a short time after each shot to replenish the pressure in the plenum. Ordinarily this recovery time is less than 5 seconds and therefore isn't noticeable for most types of shooting...by the time you chamber another round, aim, and squeeze the trigger, more than enough time has elapsed.

The recovery time may be abnormally long if one or more of the following is true:
1. the regulator's valve seat is worn or damaged (typically referred to as regulator creep)
2. there is some debris trapped in the orifice or in the threads of a metering screw
3. the setpoint is adjusted too high

Note also that the hammer spring tension (HST) has a large influence over how sensitive the gun will be...meaning how much the velocity will fluctuate as a result of the pressure. Best practice is to adjust the HST to somewhere in the ballpark of 95 - 97% of peak velocity. To do that, gradually dial up the HST until the velocity no longer increases. Let's say it tops out at 980fps. Back off the HST until the velocity comes down to somewhere between 930 - 950fps.

@nervoustrig Thanks for this detailed response.

Step 1 above sounds like a rebuild of the reg. (no idea how to approach that)
Step 2 could just be some maintenance
Step 3 What do you mean by "setpoint" ?

Here is my current tune:
JSB 25.39 Reg-1 170bar, Reg-2 150bar, Wheel- 2, IHS is fully clockwise
I'm definitely open to thoughts on this tune!!

Let me say that I am new to PCP tuning. In my mind your description sounds opposite of my experience. If the reg is taking too long to recover then wouldn't my FPS be lower if I shoot too quickly? When I shoot quickly, my FPS is higher. Is my newbness showing? LOL

 

[nervoustrig]

Step 3 What do you mean by "setpoint" ?

Setpoint is just another word for the pressure to which the regulator has been adjusted.

JSB 25.39 Reg-1 170bar, Reg-2 150bar, Wheel- 2, IHS is fully clockwise
I'm definitely open to thoughts on this tune!!

I don't see where you give the caliber. If .25 cal, my guess is 150 bar is too high for the somewhat lightweight 25.4gr...and the HST is probably dialed back too much to get the velocity down to an acceptable level, and the gun is operating at a state of partial valve lock. Thus it is very sensitive to slight differences in pressure.

If it's a .22 cal, the 25.4gr is pretty heavy for the caliber and 150 bar seems more reasonable.

Let me say that I am new to PCP tuning. In my mind your description sounds opposite of my experience. If the reg is taking too long to recover then wouldn't my FPS be lower if I shoot too quickly? When I shoot quickly, my FPS is higher. Is my newbness showing? LOL

Yeah it's a little counterintuitive at first. The outcome depends on how the hammer spring tension is adjusted. If it's on the weak side, as the plenum refills and the pressure rises, the hammer strike has an ever more difficult task of knocking the valve open...the partial valve lock I mentioned above. It's having to overcome high pressure holding the valve closed. As a result, the valve barely gets knocked open and the velocity is diminished.

Conversely, if the HST is set really high, you'll get an ever increasing velocity as the pressure rises.

 

[HogKiller]

Persoally I think you have the regs set too high. I've got a Maverick VP .25, Front, after a rebuild, easy to do, set to 105 - 110, rear at 78, had set the hammer adjuster out to .6" but screwed it in on #4 to achieve about 900fps, getting a SD of 2.1 ish. Please note, I didn't take any notes and the ONLY that are are accurate is the rear reg,( checked it) and the hammer screw and speed. IF you do decide to rebuild the regs, please be aware that the drawings for the Maverick are wrong, it only shows one reg, Capt. O-ring only gives o-rings for one reg, he's been informed of this. I bought bags of the most necessary o-rings. Way cheaper.

 

[scottg0814]

As I mentioned earlier, I have 2 Mavericks both .25 cal:
Mav1 is the main shooter. Currently set up as a compact. ( I also have a 700mm barrel which is what I've been shooting 99% of the time, I only recently added the 500mm to check it out ): Shooting JSB 25.39 Regs- 170,150 Wheel-2, IHS 0, with slug power kit

FWIW: The reason I went with this Mav1 tune was I saw a post from someone ( don't recall who ) who I think shot in the top 10 at RMAC. He mentioned that this was his tune and that he was shooting the 25.39 JSB's on Wheel 2 and then move the wheel to 7 to shoot the 33.95 JSB's. This seemed like a great way to go since these are the 2 weights that I'd be shooting the most and air is not an issue for me. Plus I added the 700cc bottle when shooting the 700mm barrel

Mav2 is my night rig. Don't shoot it much. JSB 25.39 Regs- 170(?),150 Wheel-6, IHS 3/4 turn. This is the way it came to me. I don't really know for sure what the reg1 pressure is but I have to assume it's 170-180.

Yesterday I said I would shoot Mav2 to compare with Mav1 to see if the velocity changes when shooting fast or slow. I shot Mav2, both quickly and slowly. It shot consistently:
11 Shots, Hi 981, Low 970, Avg. 977, Spread 11, SD 3.4

I then shot Mav1 and it was still behaving as before. Then for kicks, I adjusted the IHS on Mav1 to match Mav2. 3/4 turn CCW.
Shot another string with Mav1. Shooting both fast and slow and got this result:
Shots 16, Hi 935, Low 909, Avg 927, Spread 26, SD 8.6. I seem to see an improved consistency in velocity since making the 3/4 turn on the IHS
I shot a 2nd string as a sanity check:
Shots 16 , Hi 895, Low 847, Avg 880, Spread 48, SD 12.4 Not as good as the last string but still no wild changes in velocity!!

Then figured I should compare apples to apples so I put the 700mm barrel back on Mav1 and shot this string:
Shots 15, Hi 944, Low 888, Avg 920, Spread 56, SD 12.9
15 shots cuz I forgot to put the chrony on .The low 888 was the 12th shot. Seemed odd as my other lowest was a 907. The string would have looked better without that 888.

I know this is long and a lot to take in. Did the 3/4 turn IHS make the gun happier? Do I need to investigate tweaking the IHS + or - ?

Appreciate your inputs and insights. I'd be lost without this community!!

 

[HogKiller]

As I mentioned earlier, I have 2 Mavericks both .25 cal:
Mav1 is the main shooter. Currently set up as a compact. ( I also have a 700mm barrel which is what I've been shooting 99% of the time, I only recently added the 500mm to check it out ): Shooting JSB 25.39 Regs- 170,150 Wheel-2, IHS 0, with slug power kit

FWIW: The reason I went with this Mav1 tune was I saw a post from someone ( don't recall who ) who I think shot in the top 10 at RMAC. He mentioned that this was his tune and that he was shooting the 25.39 JSB's on Wheel 2 and then move the wheel to 7 to shoot the 33.95 JSB's. This seemed like a great way to go since these are the 2 weights that I'd be shooting the most and air is not an issue for me. Plus I added the 700cc bottle when shooting the 700mm barrel

Mav2 is my night rig. Don't shoot it much. JSB 25.39 Regs- 170(?),150 Wheel-6, IHS 3/4 turn. This is the way it came to me. I don't really know for sure what the reg1 pressure is but I have to assume it's 170-180.

Yesterday I said I would shoot Mav2 to compare with Mav1 to see if the velocity changes when shooting fast or slow. I shot Mav2, both quickly and slowly. It shot consistently:
11 Shots, Hi 981, Low 970, Avg. 977, Spread 11, SD 3.4

I then shot Mav1 and it was still behaving as before. Then for kicks, I adjusted the IHS on Mav1 to match Mav2. 3/4 turn CCW.
Shot another string with Mav1. Shooting both fast and slow and got this result:
Shots 16, Hi 935, Low 909, Avg 927, Spread 26, SD 8.6. I seem to see an improved consistency in velocity since making the 3/4 turn on the IHS
I shot a 2nd string as a sanity check:
Shots 16 , Hi 895, Low 847, Avg 880, Spread 48, SD 12.4 Not as good as the last string but still no wild changes in velocity!!

Then figured I should compare apples to apples so I put the 700mm barrel back on Mav1 and shot this string:
Shots 15, Hi 944, Low 888, Avg 920, Spread 56, SD 12.9
15 shots cuz I forgot to put the chrony on .The low 888 was the 12th shot. Seemed odd as my other lowest was a 907. The string would have looked better without that 888.

I know this is long and a lot to take in. Did the 3/4 turn IHS make the gun happier? Do I need to investigate tweaking the IHS + or - ?

Appreciate your inputs and insights. I'd be lost without this community!!

My IHS was at the recommended distance out by Moose, I did that and set it to 4 and it was HOT 950ish running the same pellets you are and my lower reg pressures. Turned the IHS in several turns and not at 900, pellets seems to like them with a 600 barrel.Personally I'd make sure the reg pressures are good, then start turning in the IHS. I did that and and getting good 16 round single holes at 25 yards.

 

[nervoustrig]

I know this is long and a lot to take in. Did the 3/4 turn IHS make the gun happier? Do I need to investigate tweaking the IHS + or - ?

It’s hard to tell. Rather likely the modest 3/4 turn of the IHS is insufficient to get it anywhere near the velocity plateau. With such a high regulator pressure, I wouldn’t be surprised if it tops out somewhere north of 1050fps…in which case it is unsurprising that the extreme spread is terrible at the moment. Partial valve lock acts like that.

My advice is to make coarse adjustments until the velocity no longer increases. Shoot just 2 or 3 pellets to get an idea of the velocity, then make another coarse adjustment and repeat. Pretty soon you will have found the baseline amount of hammer spring tension needed for the 150 bar setpoint.

 

[Bigragu]

@nervoustrig Jason- I have to applaud you cause you’re always Johnny on the spot when it comes to any form of reg problems. I think AGN should have a Sticky just for you on the advice ive seen you give over the years. This time i noticed you worded things differently but in the end it made perfect sense and came together like it always does.

You guys on here receiving his assistance, applaud and give a big thank you to this man cause I can tell you for sure you’re not getting any bogus info, and for as many times as he’s explained this regulator troubleshooting info, he’s very patient.

 

[scottg0814]

@nervoustrig Jason- I have to applaud you cause you’re always Johnny on the spot when it comes to any form of reg problems. I think AGN should have a Sticky just for you on the advice ive seen you give over the years. This time i noticed you worded things differently but in the end it made perfect sense and came together like it always does.

You guys on here receiving his assistance, applaud and give a big thank you to this man cause I can tell you for sure you’re not getting any bogus info, and for as many times as he’s explained this regulator troubleshooting info, he’s very patient.

@Bigragu - I'm with you 1000%. My words of gratitude never feel like enough. These guns are complicated to me. It's like the old camera days, do you want a simple point and shoot or do you want to learn photography and master a camera that's totally manual.
I have other guns that are point and shoot but I really wanted to learn how these higher end guns work and WOW, they can be and are complicated. HUGE KUDO's to those who have mastered the platform and take the time to help out those of us still trying to get a grip. I couldn't enjoy this hobby as much as I do without communities like AGN. I really do appreciate the kindness and patients that people like Jason provide.

 

[scottg0814]

It’s hard to tell. Rather likely the modest 3/4 turn of the IHS is insufficient to get it anywhere near the velocity plateau. With such a high regulator pressure, I wouldn’t be surprised if it tops out somewhere north of 1050fps…in which case it is unsurprising that the extreme spread is terrible at the moment. Partial valve lock acts like that.

My advice is to make coarse adjustments until the velocity no longer increases. Shoot just 2 or 3 pellets to get an idea of the velocity, then make another coarse adjustment and repeat. Pretty soon you will have found the baseline amount of hammer spring tension needed for the 150 bar setpoint.

Awesome!! Thank you again for all the advice and walking me thru this.
I'll shoot from the 7 position on the power wheel as I'm making these IHS changes. Once I find max velocity then I'll dial down on the wheel to get to a velocity that the diabolo pellet likes. Somewhere in the 850-900FPS range.

 

[scottg0814]

My IHS was at the recommended distance out by Moose, I did that and set it to 4 and it was HOT 950ish running the same pellets you are and my lower reg pressures. Turned the IHS in several turns and not at 900, pellets seems to like them with a 600 barrel.Personally I'd make sure the reg pressures are good, then start turning in the IHS. I did that and and getting good 16 round single holes at 25 yards.

Thanks @HogKiller Looks like I need to get the IHS correct for the reg pressure and go from there. I have other tunes from AEAC that I can try also.

 

[nervoustrig]

Augie, thank you sir for your kind words. That really lifted my spirits.

 

[nervoustrig]

I'll shoot from the 7 position on the power wheel as I'm making these IHS changes.

That's the right idea...finding the right setting for the internal hammer spring adjuster so the power wheel on it's highest setting will deliver something close to max velocity at whatever regulator setpoint you're using. That will give you the broadest adjustment range for the power wheel.

Once I find max velocity then I'll dial down on the wheel to get to a velocity that the diabolo pellet likes. Somewhere in the 850-900FPS range.

Just bear in mind if partial valve lock is indeed the source of (or at least a significant contributor to) the wide extreme spread, dialing back the power wheel to the same 850-900fps range will put you right back where you started. There is only one combination of HST and regulator pressure that will yield the very best consistency so if you plan to do a lot of shooting with the 25.4gr, you may want to consider changing the regulator pressure to something more suitable for that particular pellet.