FPS Loss

clubberlang12

Lads, i have an A&K SR25 that i decided to put a new spring into as it was firing too close to the limit for my liking. I bought an M85 to put in it. When i first opened it up i noticed that after the initial tooth, the next few teeth had being ground partially down by the sector gear. I replaced with same number teeth piston (full metal teeth). I also changed the o-ring and hop up rubber is new. I have tested cylinder compression with piston and with nozzle on, and its fine. Compression in hop up chamber is fine too. But my fps is down at .5j. I also have a 6.03 tbb installed. With my initial replacing of o-ring and piston now excercised, i am all out of ideas.

Anyone have any other similar experiences or have an idea what might be causing such a drastic fps loss. Thanks in advance.

thermo

is the piston the same width as the old one? i had a loss of power before and it was down to the new piston being .75 mm wider and it didn't travel forward as freely. did you regrease the piston head?
are you sure you re assymbled it correctly?

glicster

i had the same problem with an m85 before and ended up throwing it out after numerous times of swapping it with other springs and checking and i eventually put it down to being a faulty spring.
also what i do when replacing an o ring is to give it a bit of a stretch first(not just one stretch but a good number of times).i have noticed that it makes them a little softer so easyer to expand and make the seal when fireing.

clubberlang12

@Thermo............I'll have to check width when i re-open the mechbox tomorrow. But visually they looked the exact same. Yes, it's greased up and 99% sure it is reassembled properly.

@Glicster...........I tried it with the old spring and was still down to that fps figure. Tried it with an brand new m85 and that also gave a similar fps figure. I'm pretty sure the seal is good as i tested the compression numerous times with the piston and cylinder.

Is there any chance a motor on its way out can affect fps? I would have always presumed it would only affect ROF or the ability to turn the gears but i'm trying to cover every possibility.

glicster

suppose then it could be something with the hop rubber,its the only other thing youve changed.providing its not what thermo has mentioned.
while your at it,you didnt unknowns to yourself loose the little spring from the top of the hop unit?

EDIT:oops,i jumped to the conclusion youve got an m4.read your op again.soz
mmmm,facepalm! same hop unit:o

clubberlang12

I installed a brand new Guarder rubber today just in case the old one (although it was only installed about 2000 rounds beforehand) was causing an air leak. Do you mean the small spring to keep the hop up chamber pressed back against the mechbox? If so, i still have that, although it is a larger spring which goes over the barrel which keeps it pressed on the A&K.

Would the few teeth ground down (on the old piston) cause the piston to short-stroke as such, i.e- gear teeth not engaging piston teeth properly thus not getting a full retraction of the piston on the spring? Maybe the piston wasn't my problem in the first place. It's really starting to bug me out. The only thing i haven't changed is the nozzle, but i did hold the nozzle into the chamber and fire a bb through the chrono and it still fired low.

glicster

im not sure their lad.spose it could be a possability but from here and not being able see/hear the gun i couldnt say.
id presume youd hear the piston teeth grinding on the rest of the gear teeth if that was happenin?

clubberlang12

Sounded to be cycling alright Glic. I'll try and research some more about it and go back at it again another day. Cheers for the suggestions lad.

DeBurca

I assume that the A&K SR25 is a long stroked gearbox 18 toothed piston as opposed to the 15 toothed pistons found in most AEGs and if so it also uses a longer spring
So a M85 spring from a shorter type gearbox will not produce a M85 type velocity when used in a longer gearbox

It is also worth checking to see if the piston is moving freely in the gearbox as mentioned by Thermo
The easiest way to do this is to reassembly the gearbox without the spring and spring guide so that you can push the piston forwards and backwards by inserting a long thin screwdriver through the air-nozzle and hole in the back of the gearbox where the spring guide sits so as to check that it is not binding on the guide rails

If as you say your existing spring is firing to close to the limit then the easiest solution is to snip a coil or two from your existing spring see if that produces the FPS you are after, you have nothing to lose as you are willing to replace the spring anyway

clubberlang12

Cheers Deburca. Should i also remove the anti-latch while testing the piston?

While i have you here i might try and kill two birds with the one stone. I'm looking for a small gear lever spring for my Marui Type 89. It's a spring similar to the spring holding on the tappet plate, except it looks narrower and longer.

http://www.interq.or.jp/tokyo/akishino/factory/ponkotu-inpure/89shiki/89bunkai/03.jpg

In the top left image you can see it to the left of the trigger and connecting up to the gear lever for the 3 burst function.

Would you know where one of these could be found? I've tried to search for one but to no avail.

DeBurca

clubberlang12 wrote: »

Cheers Deburca. Should i also remove the anti-latch while testing the piston?

While i have you here i might try and kill two birds with the one stone. I'm looking for a small gear lever spring for my Marui Type 89. It's a spring similar to the spring holding on the tappet plate, except it looks narrower and longer.

http://www.interq.or.jp/tokyo/akishino/factory/ponkotu-inpure/89shiki/89bunkai/03.jpg

In the top left image you can see it to the left of the trigger and connecting up to the gear lever for the 3 burst function.

Would you know where one of these could be found? I've tried to search for one but to no avail.

You can either remove the Sector gear while testing the free movement of the piston or turn it to a position where the teeth on it do not make contact with the teeth on the piston
During this test the two halfs of the gearbox should be screwed together so as to get as near to the running condition as possible

Sorry cant help you with the type 89 as I have never worked on one of them

clubberlang12

Thanks, will try that out.

DeBurca

Just had a look at the photo of the type 89 and the spring looks simular to the trigger contact return spring found in any V2 or V3 gearbox, might be worth a try to see if it one of them will do

Puding

as DeBurca said the spring you have is not the right length meaning it will get you a lower fps output than it would in a v2/v3 gearbox for example, have a look at psg-1 springs or try a higher rated aeg spring

you should be able to get a spring to work from a pack like this http://shop.ehobbyasia.com/lonex-gearbox-spring-set-for-ver-2-3-aeg.html or a shop might have some around due to replacing the wiring in aegs for people

clubberlang12

Cheers guys. Re-installed old spring and fps went up. Actaully firing hot now but i'll trim some off the spring to bring it under until i get a new longer spring. Was sure i had fired with old spring in but must have been with the old worn piston o-ring, hence the low fps with it.

Cheers once again lads. Problem now solved.

Airsoft5

Hi I don't mean to thread jack here but can anyone link me a 1J spring for an A&K SR-25. Cheers.

DeBurca

Airsoft5 wrote: »

Hi I don't mean to thread jack here but can anyone link me a 1J spring for an A&K SR-25. Cheers.

Mid West Airsoft have a M90 spring on the web site