Irregular Low FPS on new stock G&P SR16

volteco · September 19th, 2013, 19:10

My apologies if this was answered earlier, I could not find anything.

I have a completely stock G&P Sr16, the gun is pretty much new, was gamed only once. Bought a year ago and was just plinking with it a few times.
It fires anywhere from 180 to 250 fps. Recently took it apart including mechbox but didn't see any visible damages, apart from the fact that the piston is lose in the cylinder.

Any advice on what could be wrong is much appreciated.

ThunderCactus · September 19th, 2013, 19:17

define "loose" because it's not supposed to be a form fit seal, the O-ring only seals on forward motion.
O-ring may have dried out
Or most likely the nozzle alignment went out, seems to happen on most G&Ps

volteco · September 19th, 2013, 20:27

Thank you for the reply. What I meant is the space between the piston head and the cylinder is half millimeter from each side when inside. O-ring also doesn't sit tight.
Would you recommend replacing the nozzle or the whole cylinder-poston assembly?

ThunderCactus · September 19th, 2013, 21:50

The piston head shouldn't be tight at all, neither should the O-ring. It expands and seals on forward movement.
Your probably don't have to change the nozzle, just make sure your mechbox compresses (youtube compression test) and make sure the nozzles aligned properly

volteco · September 19th, 2013, 21:52

Great! Thank you so much!

lurkingknight · September 20th, 2013, 00:50

hold the cylinder in your hand and close it around your thumb plugging the cylinder head hole... push the piston into the cylinder... it should seal.. if it pushes right down to the end, you need to fix your piston o-ring. Stretch the o-ring, lube it up.. or replace it, try again until it holds air. Awesome compression will either stop the piston mid stroke or pop the cylinder head out. If it pops the cylinder head out, wrap some plumber's teflon tape around the cylinder head and reinsert, or leave as is... if it pops out too easily you could probably use some tape.

Once it holds, put the nozzle and tappet on the cylinder head and hold the tappet so the nozzle is halfway down the cylinder head, not flush against the back... repeat the piston pump. does it leak? this is the sucky point. sometimes you can change just the nozzle, sometimes just the cylinder head...

Best to have a digital caliper to measure the original nozzle... cause you're going to have to find a replacement of the same length or longer. As much as people would like to think m4 nozzles are all the same.. they can all vary in length quite a bit.

Replace the nozzle and/or cylinder head... retest the compression.

Too long of a replacement and you can have feeding issues, too short and you will lose air between nozzle and hop up bucking.

Moving forward... you can wrap dental floss or teflon plumbing tape around the bucking and barrel to snug it up inside the hop up unit... wrap 3 or 4 times in teflon tape, drop some silicone oil on it and reassemble your hopup.

If you did it all right, then the tricky nozzle to bucking seal will show itself when you reassemble and chrono. If it seals well, then you should have your expected fps. If BBs won't feed then your nozzle is too long and you have to very carefully sand it. If your nozzle is too short... well then you'll have a leak.. you can sand the front of the tappet plate, the face that meets the front of the gearbox.. you might be able to get as much as 1/2 a mm with some tappets. fractions of a mm will matter here.

volteco · September 20th, 2013, 09:50

Wow, thank you so much for the great guide! Really appreciate your input!

I tried compression test and it appears that there was no compression in the cylinder at all!

ThunderCactus · September 20th, 2013, 15:23

by "push" he means give it a very quick jerk forward to get the O-ring to seal up

lurkingknight · September 20th, 2013, 15:56

pump it like you mean it.

volteco · September 20th, 2013, 16:25

Thank you, that's what I did. Saw the video on youtube.

Ryoohki360 · September 23rd, 2013, 11:46

I'am not a Tech, tech, but did you check the bucking, maybe it's broken or micro fractured, and check the nozzle too if there's a dent in it

volteco · September 23rd, 2013, 18:21

No, both seem fine... I've put some silicone grease inside the cylinder and stretched o-ring on the piston head a bit. Now it gives me consistent 300fps.
Now, the wierd thing is, on the box it says "for Canadian market only, upgraded to 380fps"

So, still working on it to bring to some 340-esh.

coach · September 23rd, 2013, 18:25

It might just "say" that to get past customs. It's riskier for the retailer but they make more money if they don't have to upgrade before shipping.

ThunderCactus · September 23rd, 2013, 20:21

Best thing to do is to buy a reliable spring of which you know the fps and put it in there. Like a prometheus MS110. Should give you 360-380 depending how good the seal is.

What is your hop set to when you're shooting?

volteco · September 24th, 2013, 02:01

I actually ordered Prometheus MS100 which I hope will give me around 340 fps according to the chart here on ASC. I don't want to go beyond 350fps.
My hopup was set to 'none'. I will try turning it on to see how fps drops...

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September 19th, 2013, 19:10	#1
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	Irregular Low FPS on new stock G&P SR16 My apologies if this was answered earlier, I could not find anything. I have a completely stock G&P Sr16, the gun is pretty much new, was gamed only once. Bought a year ago and was just plinking with it a few times. It fires anywhere from 180 to 250 fps. Recently took it apart including mechbox but didn't see any visible damages, apart from the fact that the piston is lose in the cylinder. Any advice on what could be wrong is much appreciated.

September 19th, 2013, 20:27	#3
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	Thank you for the reply. What I meant is the space between the piston head and the cylinder is half millimeter from each side when inside. O-ring also doesn't sit tight. Would you recommend replacing the nozzle or the whole cylinder-poston assembly? Last edited by volteco; September 19th, 2013 at 20:32..

September 20th, 2013, 00:50	#6
lurkingknight "bb bukakke" KING! Trader rating Join Date: Jul 2011 Location: Ottawa	hold the cylinder in your hand and close it around your thumb plugging the cylinder head hole... push the piston into the cylinder... it should seal.. if it pushes right down to the end, you need to fix your piston o-ring. Stretch the o-ring, lube it up.. or replace it, try again until it holds air. Awesome compression will either stop the piston mid stroke or pop the cylinder head out. If it pops the cylinder head out, wrap some plumber's teflon tape around the cylinder head and reinsert, or leave as is... if it pops out too easily you could probably use some tape. Once it holds, put the nozzle and tappet on the cylinder head and hold the tappet so the nozzle is halfway down the cylinder head, not flush against the back... repeat the piston pump. does it leak? this is the sucky point. sometimes you can change just the nozzle, sometimes just the cylinder head... Best to have a digital caliper to measure the original nozzle... cause you're going to have to find a replacement of the same length or longer. As much as people would like to think m4 nozzles are all the same.. they can all vary in length quite a bit. Replace the nozzle and/or cylinder head... retest the compression. Too long of a replacement and you can have feeding issues, too short and you will lose air between nozzle and hop up bucking. Moving forward... you can wrap dental floss or teflon plumbing tape around the bucking and barrel to snug it up inside the hop up unit... wrap 3 or 4 times in teflon tape, drop some silicone oil on it and reassemble your hopup. If you did it all right, then the tricky nozzle to bucking seal will show itself when you reassemble and chrono. If it seals well, then you should have your expected fps. If BBs won't feed then your nozzle is too long and you have to very carefully sand it. If your nozzle is too short... well then you'll have a leak.. you can sand the front of the tappet plate, the face that meets the front of the gearbox.. you might be able to get as much as 1/2 a mm with some tappets. fractions of a mm will matter here. __________________ I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.

September 19th, 2013, 19:17	#2
ThunderCactus Not Eye Safe, Pretty Boy Maximus on the field take his picture! Trader rating Join Date: Feb 2007	define "loose" because it's not supposed to be a form fit seal, the O-ring only seals on forward motion. O-ring may have dried out Or most likely the nozzle alignment went out, seems to happen on most G&Ps

September 19th, 2013, 21:50	#4
ThunderCactus Not Eye Safe, Pretty Boy Maximus on the field take his picture! Trader rating Join Date: Feb 2007	The piston head shouldn't be tight at all, neither should the O-ring. It expands and seals on forward movement. Your probably don't have to change the nozzle, just make sure your mechbox compresses (youtube compression test) and make sure the nozzles aligned properly

September 19th, 2013, 21:52	#5
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	Great! Thank you so much!

September 20th, 2013, 09:50	#7
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	Wow, thank you so much for the great guide! Really appreciate your input! I tried compression test and it appears that there was no compression in the cylinder at all!

September 20th, 2013, 15:23	#8
ThunderCactus Not Eye Safe, Pretty Boy Maximus on the field take his picture! Trader rating Join Date: Feb 2007	by "push" he means give it a very quick jerk forward to get the O-ring to seal up

September 20th, 2013, 15:56	#9
lurkingknight "bb bukakke" KING! Trader rating Join Date: Jul 2011 Location: Ottawa	pump it like you mean it. __________________ I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.

September 20th, 2013, 16:25	#10
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	Thank you, that's what I did. Saw the video on youtube.

September 23rd, 2013, 11:46	#11
Ryoohki360 Trader rating Join Date: Oct 2011	I'am not a Tech, tech, but did you check the bucking, maybe it's broken or micro fractured, and check the nozzle too if there's a dent in it

September 23rd, 2013, 18:21	#12
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	No, both seem fine... I've put some silicone grease inside the cylinder and stretched o-ring on the piston head a bit. Now it gives me consistent 300fps. Now, the wierd thing is, on the box it says "for Canadian market only, upgraded to 380fps" So, still working on it to bring to some 340-esh. Last edited by volteco; September 23rd, 2013 at 19:11..

September 23rd, 2013, 18:25	#13
coach aka coachster Trader rating Join Date: Mar 2008 Location: T dot	It might just "say" that to get past customs. It's riskier for the retailer but they make more money if they don't have to upgrade before shipping.

September 23rd, 2013, 20:21	#14
ThunderCactus Not Eye Safe, Pretty Boy Maximus on the field take his picture! Trader rating Join Date: Feb 2007	Best thing to do is to buy a reliable spring of which you know the fps and put it in there. Like a prometheus MS110. Should give you 360-380 depending how good the seal is. What is your hop set to when you're shooting?

September 24th, 2013, 02:01	#15
volteco Trader rating Join Date: Jul 2012 Location: Etobicoke	I actually ordered Prometheus MS100 which I hope will give me around 340 fps according to the chart here on ASC. I don't want to go beyond 350fps. My hopup was set to 'none'. I will try turning it on to see how fps drops... Last edited by volteco; September 24th, 2013 at 02:30..

Irregular Low FPS on new stock G&P SR16

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