Huge FPS drop since short stroke and rebuild...

[Antny]

So i recently short stroked my M4 by 2 teeth in an effort to drop the FPS from 370 to around 350 and help with the reset speed. Well i removed 2 from the pickup side, and removed 2 from the release side on the piston, put it back together and test fired. The very first shot was 350 bang on! Awesome!! but the next suddenly took a nose dive to sub 300, even going as low as 250. I thought it might be an air leak with the hop up but i have checked everything and rebuilt the hop/barrel and it all seemed ok. Plus the FPS is very consistent, shot to shot. Only 2 or 3 FPS variance.

So now i'm at loss to what it could be??? Any ideas how it could drop so dramatically?

[theagitater]

Re: Huge FPS drop since short stroke and rebuild...

I thought when short stroking an aeg you took teeth off both piston and sector gear. The drop in fps could be the piston and gear out of sync maybe.

[Antny]

Re: Huge FPS drop since short stroke and rebuild...

Kinda wrote that wrong. Reads like i removed 2 from the pickup and 2 from the release of the piston when it should read, 2 from pickup side of sector gear... my bad.

[theagitater]

Re: Huge FPS drop since short stroke and rebuild...

Shouldn't the teeth removed be from the release side of both sector gear and piston? I can't help but think that removing teeth from the pick up side of the sector gear and from the release side of the piston would throw things out of sync, ie tappet plate.

[Antny]

Re: Huge FPS drop since short stroke and rebuild...

No you remove from the pickup side to keep timing with the tappet plate the same due to where the little clip is on the sector gear and you remove from the release on the piston to stop and pre engagement.

The gun cycles perfectly, it's just this massive fps drop i can't figure out.

[tackle]

Re: Huge FPS drop since short stroke and rebuild...

Surely for a reduced fps simplest solution would have been to nip the spring 5mm-ish ?

[theagitater]

Re: Huge FPS drop since short stroke and rebuild...

I've never short stroked an aeg (as u can prob guess) I normaly change the spring, only other thing I can think of is air seals inside the gearbox.

[Antny]

Re: Huge FPS drop since short stroke and rebuild...

on average 1 tooth equals 10fps, taking coils off is a lot less predictable.

I was thinking about the air seals but surely if it's an air seal issue it wouldn't be consistent??

[theagitater]

Re: Huge FPS drop since short stroke and rebuild...

Yeah normaly in my experience air seal problems can affect fps wildly. Consistent fps is normaly related to good seals but I suppose it could also indicate a constantly bad seal, as you said removing the teeth should have lowered your fps to around 350 but chrono reeding as low as 250 would suggest a seal problem as the drop in fps is quite drastic, check for air escaping from the infeed of the hop unit (in the mag well)

[HunterAndy]

Re: Huge FPS drop since short stroke and rebuild...

Have you got a variable pitch spring, that could also be an issue.

If the majority of the energy is stored in the last say 10-15% of compression of the spring, and the short stroke removes that part of the cycle it would explain it.

[Antny]

Re: Huge FPS drop since short stroke and rebuild...

That's a good point actually. Can't remember 100% if it is or not. Got a suspicion it could be a variable spring.

[Sitting Duck]

Re: Huge FPS drop since short stroke and rebuild...

it is seal related not spring related coz you got 350 first shot then it dropped - but you got 350
(unless 2nd shot was 6 months later and spring lost compression in storage)

what else did you change or was is JUST tooth removal ?

change of tappet, o-ring nozzle etc can cause all sorts of nightmares even if they look exact type/size
some nozzles do not work well with some hops - they can be tight or crooked fit for example

place a bit of card/rubber in front of hop to effectively push the hop back for better seal against box
this will only work though if it can be pushed back firmly - if the nozzle is short or stopping short you won't get that seal anyway

see by a bit of packing you can get a better seal - presume the spring is on front of hop still pushing back against box but still a bit of card in front of hop may provide a better seal

That is my guess on where problem lies - solution might be shaving tappet, new nozzle or another hop
very easy to take for granted when it all works - could be nozzle a tiny bit crooked and snagging in hop
but quite sure it is seal related.


btw 370 to 350 could of been done by doing AOE if not already done - then leave spring parked precocked for say 24/48 hours

[dave38x]

Re: Huge FPS drop since short stroke and rebuild...

How ported is your cylinder and how long is your barrel?

You could have reduced the amount of air drawn into the barrel enough that the volume of air being injected into the barrel is less than the volume of useful air in the cylinder - worth checking.

2 teeth can take 5 fps off at a time, it can take off 30 at a time depending on the spring, cylinder, barrel etc - it's always best to do one at a time. (What spring does it have by the way?).

@Sitting Duck - In my experience I've never seen more than a 5fps drop from correcting AoE with a sorbo pad apart from when I've eaten up too much cylinder volume - again it's probably heavily dependent on the gun.

[Antny]

Re: Huge FPS drop since short stroke and rebuild...

To the best of my memory the cylinder isnt ported and it has a 275mm madbull 6.03tbb. Has a Ultimate double o ring cylinder head with matching Ultimate piston head.
I have even tried the original 6.05 barrel and the fps is the same.

Guess I'm just going to have to open her up and check all internal seals.

[Sitting Duck]

Re: Huge FPS drop since short stroke and rebuild...

Correcting AOE can require anything from 2mm to even 6mm I have seen on really bad guns

That amount of sorbo to correct AOE varies massively depending on head & piston used
Ultimate Slient piston & cylinder heads need virtually nothing they are that much bigger than most
Also double O-ring piston heads are longer than POM heads so sorbo will vary

Also worth stating that normally when correcting AOE for first time you will lose aprox 15fps I have found
but reality is that this normally goes hand in hand with other stuff like better seal on piston, cylinder head and fitting O-ring nozzle
so this would cancel out or claw back what fps lost by doing usual 3mm or 4mm AOE
if adding a bearing spring guide this will compress spring aprox 5mm more and will put add I would say 15fps - perhaps more

This is what I have found but each gun is different - even same gun model/box can yeild different results

here's the maths bit - max stroke = 50mm once AOE done 16 x 3mm teeth + say another 1/2 tooth for the "slip" to release piston
2 teeth off has only reduced that 50 by 6 - so OP has got 44mm stroke - plenty for his barrel

Short Stroking is highly recommended especially in hi-speed setups and Airlabs recommend this

OP's problem is seal related - as I say the nozzle is not making good seal with bucking is highly likely the main culprit
no offense to OP but just coz it fired at 370 before you opened it doesn't mean it all went back 101% as before

the nozzle is not sealing properly against bucking - could be tappet plate related need shaving or snagging etc....
there is such a fine fine line sometimes and if nozzle is just 0.5mm 0.25mm short of sealing perfectly = loss of pressure

275mm barrel & the cylinder should be ported - somewhere - even underneath - it should be ported
the end of port before it shows full cylinder should be aprox 45mm to 50mm back from front of cylinder
so you could be over voluming but before with full teeth/stroke you would of been seriously over voluming
this might have been to over compensate for not 101% seals like many manufactures over volume a little bit
Not normally full cylinder - that is AK's with 455mm up to say 500mm