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FX Impact wild erratic fps help

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JamieUK

Member
Nov 14, 2017
127
89
Suffolk, United Kingdom
Hi there,

Really hoping someone can help please have tried searches but drawn a blank....this is going to be a lengthy post so please stick with it....

The fps on my impact has started fluctuating wildly, and I mean up and down by over 200fps (not a case of just loosing power).

Just to give some more detail on my gun-

MK1 .25

700mm barrel

External plenum with clearance hole in valve housing

6.25 titanium VS

Delrin bumper

Modded TP and probe ( probe has not been cut in half as some have done)

Huma gauges (accurate with my fill bottle gauge)

Still on original reg

(Quick thankyou to Edosan and Heavy impact, have followed the excellent info you have both shared)

I power tuned the gun a couple of months ago and had it shooting the NSA 33.5gr at 980fps with an ES of 8fps ( reg at 140b, HS at 4, VS around 3 lines). All was great with the world.

After a couple of weeks the issue began with a random drop of around 100fps every 9 or 10 shots, after a check over I found the probe was loose so increased original grub screw size to 4mm and added a second. Of course at this point I thought it had nicked the seal so replaced it expecting it to solve the problem.....but no joy.... 

Following that ( and after waisting a load of slugs which cost me an arm and a leg to import) I backed off all settings and started using up some jsb 25.4's I had a decent stock of. With a reg setting of 125b and hammer backed right off the fps jumps up and down from anywhere between 880 to 1170fps. The reg gauge stays steady throughout.

As the reg pressure appears to be stable I cant help but think its related to something on the hammer side of things... but ive only stripped and checked areas ive modified so far and fearfull of going deeper in case I mess something else up....

Just to say at this point im an engineer by trade, and although by no means claim to be an expert I am mechanically sympathetic and have not gorged at things with pliers and a hammer!

Apologies for the long post....

Thank you.

Jamie.
 
You’re right to suspect the hammer as one of the most likely culprits. Check first that the trigger overtravel is not set too aggressively because it could be causing some sear drag just as the hammer is released. Then check that the hammer itself and the receiver are smooth and free of any tell-tale rub marks that would indicate high friction. They should be clean and dry if that will allow the hammer to move freely. Otherwise use a dry lube like graphite or molybdenum disulfide burnished into the surfaces, or as a last resort, a very light oil like 3-in-1 oil or pneumatic tool oil. 

Of course there are a lot of other possibilities and I’m sure these guys will step in and help with other suggestions. 
 
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Thanks for your responses.

Ive just completely stripped and rebuilt it again (excuding reg) to check the springs are ok and confirm the valve stem wasnt loose.

I havent adjusted the trigger from new but will check ( although ive had the hammer out a few times now and didnt notice any marks, it moves freely too)

Something that did happen this time upon removal of the bottle- both the pre reg and reg pressure bled at the same time. Ive only had this before when the bottle has gone below set reg pressure, but bottle pressure was at 200b? 

Ive always followed ernest's suggested method of bleeding the reg pressure (after removing the bottle) by increasing the reg pressure a touch. This then bleeds through the bottle valve.

Im now thinking it may be a reg issue afterall? I get 10b creep when its set above 140, but none if set below 140, and although its always been very accurate perhaps its time for a huma...
 
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Thanks for sharing 

Ive had mine for a few weeks now and I feel like I’m still breaking it in but haven’t had any fluctuations as of yet.

i will be curious to what the issue is if you find out



please share results
Click to expand...

I bought mine new around 3 years ago and havent had a single issue with it, until I started tuning! Being a mk1 I probably should have but as long as you read all of the excellent info available on here before messing you should have no trouble either 👍
 
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A simple proveen and reproven recipee that always help on ANY PCP rifle:

Buy a bottle of silicone grase in a diving / squba store.

Take an small portion of silicone grease on your finger.

Remove the magazine of the rifle.

Insert the silicone grease in the barrel in the side by which the pellet gets into.

Reintroduce the magazine.

Make 4 shots.

Periodically leave a few silicone grease into the barrel if you won't use the rifle for a while. The internals will absorve the silicone grease and will keep on fuctioning perfectly.

The grease you inserted with the air of the shots will have two effects:

a) lubricate the barrel improving the accuracy; and

b) lubricate the internal o'rings sealing small leaks and improving the functioning of the internal parts.

Very easy and saving a sending to the gunsmith.
 
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Thank you for your response Emu. I use moly grease where needed and keep my barrel clean/ lubed with bore tech eliminator (same as I use on my rimfire/ centrefire rifles)

Apart from orings I tend not to lube things to death as in my experience it just attracts dirt and causes more issues.

Either way im not sure lubrication or lack of is the cause of my current problem. 

The thing thats niggling at me at the moment is that when im testing it ( in its current erratic state) there is no audible difference with a shot that measures 850fps compared to the next that measures 1170fps, the gun feels the same too and that doesnt seem right. This is the main reason I wanted to try against a different chrony.

This makes me lean again towards the reg being the issue, and not the hammer, as the feedback is always apparent after increasing hammer tension....

Really hope someone chimes in soon, even if its to point out something obvious that im doing wrong with the base settings im trying to test/ identify my issue from (I know power tuning can cause the gun to behave quite differently with regards to reg pressure vs reg volume vs hammer weight etc and im still learning) 
 
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Ah, no change in the report! That should prove to be a key piece of information.

Two possibilities come to mind. One is that air is being diverted somewhere other than behind the pellet. Sometimes when an O-ring is beginning to fail, it will behave intermittently like that...the type of O-rings/seals that are responsible for sealing instantaneously upon the pressure pulse of firing. Check things like the transfer port and the barrel O-rings that seal the barrel to the receiver.


Bear in mind it would have to be a seal that diverts air away from the base of the pellet but still allows it into the shroud. If instead it were the breech O-ring, you would hear a loud pop on firing because the high pressure would not be muffled by the shroud. 

The other possibility that comes to mind is clipping. I don’t give this one as high a probability because you probably would have been seeing a more random, erratic range of velocities, not “most shots are fine but 10% are way slower”.
 
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Ah, no change in the report! That should prove to be a key piece of information.

Two possibilities come to mind. One is that air is being diverted somewhere other than behind the pellet. Sometimes when an O-ring is beginning to fail, it will behave intermittently like that...the type of O-rings/seals that are responsible for sealing instantaneously upon the pressure pulse of firing. Check things like the transfer port and the barrel O-rings that seal the barrel to the receiver.


Bear in mind it would have to be a seal that diverts air away from the base of the pellet but still allows it into the shroud. If instead it were the breech O-ring, you would hear a loud pop on firing because the high pressure would not be muffled by the shroud. 

The other possibility that comes to mind is clipping. I don’t give this one as high a probability because you probably would have been seeing a more random, erratic range of velocities, not “most shots are fine but 10% are way slower”.
Click to expand...

We are on the same page sir, the probe oring was my first thought but sadly has not cured the issue.

Although I have inspected the outer orings on the barrel I havent changed them ( I bought the barrel new a month ago). I had assumed the x barrel went straight on the mk1 but perhaps there has been a change of spec in the oring diameters that I wasnt aware of....? and even though it was shooting spot on ive worn them during the tuning/ stripping and rebuilding process....It certainly feels tighter going in than the old barrel ( which ive now sold so cant compare!)

I will check this theory tonight! 
 
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Another alternative that will work for sure is rhe following:

Take the Impact and all the accessories.

Put them in a case. 

Put the case in a closet.

Then buy a Uragan ... The smile will return to your face.
Click to expand...

Sorry Emu only just spotted this.

Affraid my head isnt turned so easily these days, im not saying a fling with an eastern European wouldnt be fun but...My little impact is just asking for some affection thats all, its been good to me for the last 3 years so repairing it is the least I can do to repay the favour.... 😁


 
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Thanks for sharing 

Ive had mine for a few weeks now and I feel like I’m still breaking it in but haven’t had any fluctuations as of yet.

i will be curious to what the issue is if you find out



please share results
Click to expand...

Well to share my results- I changed the 2x1 valve stem o ring along with another full strip down/ regrease and rebuild. Things looked to have improved, with reg at 140 was shooting jsb 33.4's at around 980fps ( with around a 30fps spread, not great but hadnt tuned at all yet).

I then refilled the gun and went back to testing....according to chrony the first shot with JSB 33.4's flew out at 1670 fps 😳

Looked at reg gauge and it was at 250bar! 5 or 6 frantic shots and it settled back down to 140, but if it wasnt toast already it must be now. Looks like its Huma time and fingers crossed that solves the original problem too....
 
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1670FPS Not possible, stock hammer spring and hammer weight isn’t strong enough to fully open 250b reg pressure. I have tested the impact by passing the reg full 250b bottle pressure— most I can get was 250fps.

we had a crony that gives out # that didn’t add up had two of them side by side so we used the other crony of the same brand then we had normal reading. Crony was powered by AC so no battery to Replace even rebooting didn’t cure the issue of random high and low of 100+fps that crony is no longer reliable so she got poop canned. Look like you need to inspect the seating surface on the piston replace is Deep debit, and replace the 2x1 oring closest to the piston any one of those failed the reg tube will equalized with bottle pressure at a given time.

HUMA reg has the same amount of orings and delrin seat in the long run they will fail and need servicing.

Ernest 


 
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Hi Ernest,

Thankyou for taking the time to respond. As you may have read above I questioned my chrony too as I wasnt experiencing the report to match the figures shown. Talked myself into though by testing on a friends but perhaps there was an issue with both...

Either way wish id read your response earlier as just ordered the Huma! Never mind, guess I can rebuild the original and keep as a spare.

Jamie.
 
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