Huge FPS loss 50-100fps

[LordGeorge]

Hey guys

 

To say the least, i broke the holy law of "If it aint broke, DONT fix it".

While doing a routine checkup on my v2 gearbox i was doing a few things.

Installed a rhop onto the barrel which gave amazing results and a fps gain.

I was at the time firing 350fps consistently, VERY HAPPY.

 

while working on perfecting the firing i noticed a small issue with double feeding, to the point where two would pop out and one usually shattered into many many peices. So for any shotgun lovers, not bad.

Cut a long story short of finding little wrong with the hopup i then focused on the nozzle.

I found that on semi auto the nozzel held back at the end of the stroke as normal.

But long enough to cause a stray couple of bbs to slip in.

 

I then proceeded to open up the case to do a cleanup and check the nozzle and tappet plate.

To find the gears spotless and no clear issues with the entire gearbox, I then put it back together for testing.

 

Took first shot .... not bang or pop of piston.

Looking into the window to find the entire piston had twisted on its side. Being a total dubass but easily done, i had miss alligned the piston from its rails while pushing the sping back into the case.

 

So split the case again and correct the issue, with no damage, twisting or shredding of gears "phew", all but a little mark on the piston rack where the sector had caught it.
Besides that no problems.

Complete the rebuild and test gearbox and all good with smooth complete cycles.

 

I then go to chrono the gun as usual after any work to find quite shockingly a fps drop between 50-100 fps :o

 

Since then i have rulled out as much as possible.

 

Compression on piston and cylinder is great, with no leaks on cylinder head.

No leaks from head to nozzle "no pun inteneded"

Check hopup and find no issues.

No leaking from rhop and flat hopped bucking.

 

I then re-installed the origional hopup bucking and barrel to find roughly the same issue.

 

I did however notice that occasionaly after rebuilding the box/hopup i would get a few shots that were back at the 340fps mark, then followed by a low 230s.

 

I have been working on these AEG's for quite some time and have built many boxes.
I am tottaly mystified what has gone on.

 

The only two defining points where the hopup and the cok up with the piston.

 

I need a fresh pair of eyes..
Any ideas would be brilliant

 

Cheers

George

[Sitting Duck]

Gearbox tightened up pissed to hop

 

As it was tightened up the box tipped backwards

The nozzle is now hitting the hop or not aligned 101%

There is aa gap ciz nozzle not sealing

 

Air pissing out = fps loss ???

 

That is my initial first stab

 

You had a little hop problem before, quite likely it wasn't double feeding but 1 bb not firing correctly

Another bb loads and then you see two fire

Coz 1st one failed to fire previously

 

99% nozzle to hop related unless something worked in box

[LordGeorge]

Gearbox tightened up pissed to hop

 

As it was tightened up the box tipped backwards

The nozzle is now hitting the hop or not aligned 101%

There is aa gap ciz nozzle not sealing

 

Air pissing out = fps loss ???

 

That is my initial first stab

 

You had a little hop problem before, quite likely it wasn't double feeding but 1 bb not firing correctly

Another bb loads and then you see two fire

Coz 1st one failed to fire previously

 

99% nozzle to hop related unless something worked in box

Well i checked any airleak from the bb' feeder in the magwell and found no air what so ever moving. Indicating no air is leaking from the hopup.

The trouble is i have the we mk17 scar, it has very little in the way of holding the gearbox straight besides the pistol grip.

Unfortunately the doublefeeding issue was way before the fps dropped like a stone.

 

I would agree the hopup has something to do with it but i cannot find any evidence to support it.

[Sitting Duck]

If nozzle/tappet is travelling fully back n forth

Correct nozzle length etc...

Nothing out of ordinary or incorrect short stroke

 

Check the tappet plate didn't get bored

Make sure full Max movement back n forth

All silky smooth

And double check tappet and trigger springs fitted correctly

If these are fitted incorrectly they can catch tappet plate sliding back n forth

 

If all OK inside box then it looks to be nozzle not sealing with hop - numerous things, worn out of alignment etc..

 

The paper test on magwell is not a sure way of checking for airleaks in my book

I have had paper stay put or move very little but still poor seals/fps

 

Soz for bad grammar - fone is being a nob

[LordGeorge]

If nozzle/tappet is travelling fully back n forth

Correct nozzle length etc...

Nothing out of ordinary or incorrect short stroke

 

Check the tappet plate didn't get bored

Make sure full Max movement back n forth

All silky smooth

And double check tappet and trigger springs fitted correctly

If these are fitted incorrectly they can catch tappet plate sliding back n forth

 

If all OK inside box then it looks to be nozzle not sealing with hop - numerous things, worn out of alignment etc..

 

The paper test on magwell is not a sure way of checking for airleaks in my book

I have had paper stay put or move very little but still poor seals/fps

 

Soz for bad grammar - fone is being a nob

Haha no worries, my grammar is not good at the best of times.

 

All sounds good above, i am 99.9% sure the internals are fine.

Nozzle moves freely with no issues, nothing else was changed so the lengh is also ok.

I do agree but by design, this we is impossible to check any other leaks from nozzle to hop.

 

I am trying any wierd and wonderfull ideas now at almost 1pm, lets hope i dont wake the missus

[LordGeorge]

Just out of interest, as its too late for me to check on another AEG.

When you cycle semi auto shot. The nozzle on mine stops retracted and not extended.

Would it be possible that its pulling away from the hop before it should causing a sudden but slight drop in the fps.

 

Nothing fundermental has changed in the gun so not sure how something like overspin could of occured,

 

what you tink ?

[Sitting Duck]

Perfectly normal

The cycle has slightly over run a smidge
The tappet plate starts to retract before the piston is drawn back and tappet releases in plenty of time with 4 teeth still to go on piston.
Piston is released later when bb is loaded into hop/bucking and away the bb should fly.
Soon afterwards the cutoff lever should kick in.
Often though the cycle will over run a bit but bb is long gone.
Nothing to worry about
but you still have 40% of a revolution of sector as the piston releases before tappet plate pulls back again.
(Long after bb has launched)
DSG's have half that amount of time as everything is going nutz

So I very much doubt if tappet is being jerked away from hop as piston releases.

Check the full smooth movement of tappet and Max Stoke of it fully back to allow bb to feed and Max forward to seal against bucking.

Worse case scenario is where your lips if bucking protrude a little to restrict easy bb entry into hop from feed tube, easy to think nozzle is at fault
But as 2 hop units are having trouble I doubt that is a likely issue atm....

Quite likely some wear has taken place and tappet is not moving the full amount backwards/forwards
Though forwards should still work correctly unless some crap has caught up at front of box stopping the full forward movement.

Sometimes some tappet modding is required to allow Max possible total movement but before you go nutz you need to remove tappet spring and thoroughly check the tappet movement.
Also place hop unit against box with tappet fully forward
Blow down barrel, should seal lovely, pull hop a smidge away and see how much you can withdraw hop unit before seal is lost, hopefully a 1mm or so otherwise you are only just sealing and in use you will lose fps.

 

For the record it would be a very lucky guess if anybody can 101%% accurately diagnose the fault straight away

 

What YOU have to do is check EVERYTHING is operating smoothly and correctly

deciding if a particular is worn or failing

the tappet spring could be too light - but I can tell you to lop off a coil or two or replace

the nozzle could be cracked or not seated correctly

your bucking could be torn - hmm possible but you tried another hop/barrel with same results

 

 

my present gut feeling is nozzle not aligned or sealing perfectly but also possible wear or tappet not moving correctly

 

weird thing is it WAS ok but somehow something has gone right out of whack with a crunch

tappet "may" have got damaged but it was piston skewed off rails, but guess the piston's pick up tooth could of caught

and maybe got close rear of tappet plate ???

 

if you could perhaps take some pics of the internals then others may be able to check for possible issues

that would be a wise thing to consider if still stumped - especially sometimes you can't see wood for the trees

[Colonel Kurtz]

I had these symptoms once after some tinkering, for me it turned out i'd not hooked the piston nozzle on the plate properly. This meant that the nozzle wasn't extending as much as it should and a fair bit of air escaping rather than getting into the hop

[LordGeorge]

I couldnt agree more, i know its going to be the smallest possible thing.

 

While tinkering last night, i found that if i blow down the nozzle "OH GOD NO" air was passing the piston.

I have a very strong feeling this is the problem.

My only argument is while compressing the pison i have no leak.

 

I can say 100% the tappet plate and spring are fine so i may have found the issue.

Need to get home to check it more.

 

If it is pissing air past the piston ring, hopefully a replacemnt O ring will do. Have many spare normal O rings but i have a boreup piston head and cylinder.

 

Will report back later.

 

Cheers for the help dudes.

[Sitting Duck]

It is all around that area - most likely

 

after more screw ups and problems and balls ups and a number of WTF ????

 

I'm still breaking stuff and with each screw up I'm learning more - mainly the more factor

more time

more care

more examination of pieces fitting together and working correctly as they should

 

number of ways to investigate further - it should seal very well against piston inside cylinder

 

but you could open box, remove piston/cylinder assembly

replace tappet+nozzle, close box up for a sec with a few screws, pop finger in cylinder window

push tappet nozzle home against box hop against box n blow - your finger should cover the nozzle so it should seal

 

I mean this may not be the ultimate test but one of many ways to test it seals properly

and this is with hop right up against box btw, if possible with 3rd hand see where you lose the seal by pulling hop forward...

 

Why - coz often when inserting a mag it may pull/push the hop a smidge as mag settles in hop feed tube

so if it shifts forward half a millimetre then the hop may not seal properly in use

 

This is just a passing comment and not a full diagnostic of your possible problem

but demostrates the "joy" of our poxy toys

I feel your pain - we all do, but I know I'm learning more n more with every bastid pile of junk poxy lame ar$e piece of crap gun I break/fix

Still so very much to learn - I'm no expert far far from but have quite a lot of experience in breaking guns

(I just need the time n experience in fixing them properly)

[sp00n]

Silly question - the piston head is properly located in the cylinder?

[LordGeorge]

Sorry for the late reply, gone and got a WE hk416 gbb today for my next project "daytona" and have been working on a problem with what i later found was the bolt carrier sliding too far back due to the magpull pts stock thats too deep.

Huge spacers later.. Fixed

It is all around that area - most likely

 

after more screw ups and problems and balls ups and a number of WTF ????

 

I'm still breaking stuff and with each screw up I'm learning more - mainly the more factor

more time

more care

more examination of pieces fitting together and working correctly as they should

 

number of ways to investigate further - it should seal very well against piston inside cylinder

 

but you could open box, remove piston/cylinder assembly

replace tappet+nozzle, close box up for a sec with a few screws, pop finger in cylinder window

push tappet nozzle home against box hop against box n blow - your finger should cover the nozzle so it should seal

 

I mean this may not be the ultimate test but one of many ways to test it seals properly

and this is with hop right up against box btw, if possible with 3rd hand see where you lose the seal by pulling hop forward...

 

Why - coz often when inserting a mag it may pull/push the hop a smidge as mag settles in hop feed tube

so if it shifts forward half a millimetre then the hop may not seal properly in use

 

This is just a passing comment and not a full diagnostic of your possible problem

but demostrates the "joy" of our poxy toys

I feel your pain - we all do, but I know I'm learning more n more with every bastid pile of junk poxy lame ar$e piece of crap gun I break/fix

Still so very much to learn - I'm no expert far far from but have quite a lot of experience in breaking guns

(I just need the time n experience in fixing them properly)

Ok, so following your post earlier i checked both airseal's between barrel and nozzle.
So assembling receivers with nozzle fully extended and blowing down the end of the barrel and was perfect.

I then tested directly on the nozzle, which i had done before. I only found that i could blow into the nozzle and hear a leak when the piston was all the way back when fired from full auto.
No matter how many times i tried on semi, there was no problem.
Maybe the airloss at the end of the stroke is to blame.

 

As far as it goes with the mag causing the problem, its not possible on the we mk17 as in the magwell is a feeder that feeds directly to the bottom of the hopup.
And very true, up untill now i have been able to fix AEG issues no problem. Always learning

 

Silly question - the piston head is properly located in the cylinder?

 

Not silly at all, fortunately yet My piston is a very tight fit and if not exactly straight in the cylinder causes it to get stuck "only a issue when out of the case, when under a m110 it dear not" . Always been there and never a problem

[sp00n]

Doh, i meant cylinder head to cylinder lol "too much blood in my alcohol system"

[LordGeorge]

Doh, i meant cylinder head to cylinder lol "too much blood in my alcohol system"

Haha, im on the earl grey tonight. Very intoxicating lol

Yeah dud the cylinder and head are fine

[Sitting Duck]

Oh crap, it is a weird hop indeed

http://www.airsoft-forums.co.uk/index.php/topic/24153-we-mk17-scar-h/

[LordGeorge]

Oh crap, it is a weird hop indeed

http://www.airsoft-forums.co.uk/index.php/topic/24153-we-mk17-scar-h/

Haha yes, it works well but not ideal.

 

Update.

Still no luck, not had a huge amount of time to test but still not progressing.

The only thing i have found is when blowing down the nozzle, on occasion i can blow and hear air passing the head. Once i blow a bit harder the seal kicks in and back to normal.

I actually re-shimmed the gears while at it, re lubed and sealed everything with no change. Slight rps increase.

 

My only argument for my piston is if the piston is not 100% and i mean 100% straight with the cylinder, it suffers from allot of resistance and sticking.

I could not hold the cylinder and drop the piston in, but this has been a problem since i had the parts and never used to be a problem.

The piston simply wouldn't fall straight enough to slide down. But when it is straight in the cylinder it has very little problem moving.

I am wondering if some bias in the spring or even the piston is causing the piston head to catch on the walls of the cylinder too much.

 

This may explain why its been acting a goat since the piston got jammed. Most likely indicate a slightly bent piston.

As well as the slow rps for a lonex A1 supreme.

Though nothing is visually damaged, bent or wearing .

 

Hmmm

[Sitting Duck]

Haha yes, it works well but not ideal.

 

Update.

Still no luck, not had a huge amount of time to test but still not progressing.

The only thing i have found is when blowing down the nozzle, on occasion i can blow and hear air passing the head. Once i blow a bit harder the seal kicks in and back to normal.

I actually re-shimmed the gears while at it, re lubed and sealed everything with no change. Slight rps increase.

 

My only argument for my piston is if the piston is not 100% and i mean 100% straight with the cylinder, it suffers from allot of resistance and sticking.

I could not hold the cylinder and drop the piston in, but this has been a problem since i had the parts and never used to be a problem.

The piston simply wouldn't fall straight enough to slide down. But when it is straight in the cylinder it has very little problem moving.

I am wondering if some bias in the spring or even the piston is causing the piston head to catch on the walls of the cylinder too much.

 

This may explain why its been acting a goat since the piston got jammed. Most likely indicate a slightly bent piston.

As well as the slow rps for a lonex A1 supreme.

Though nothing is visually damaged, bent or wearing .

 

Hmmm

 

 

First the piston itself......

 

remove cylinder/tappet plop piston (with piston head on and either remove sector or ensure no teeth engaging piston teeth)

The piston MUST flop back n forth with box done up - it should NOT snag or bind or stick

it should fall down up/up depending on how you tilt box - but should of course remain on the runners

(not so loose or worn it could come off runner under compression)

 

If piston sticks or binds you will have a slower running gearbox and a slower returning spring = lower fps

 

Piston head itself should be sitting perfectly in line

 

Cylinder - free of any scores - check in case the jam damaged the internal surface of cylinder

polish cylinder using silicone oil - not a lot but a few drops or so on soft tissue and then polish with a soft dry rag

[LordGeorge]

Thanks dude will certainly check to ensure the piston gets stuck or not.

From what I remember. It doesn't slide around but does move a little.

My only trouble is the fact is nothing else's changed, the gun has fired 100fps better with the same piston. Either way I will check.

Will update tomorrow !!

[Ian_Gere]

Bear in mind that, when you blow down the nozzle towards the piston head, the main thing which will determine how much air you can blow is the attitude of the piston head O-ring - it is not static: it only seals completely when the piston is moving forwards so that friction against the cylinder wall drags the O-ring back so that it seals behind the ports which lead to the holes in the piston head face; when the piston is moving backwards, friction drags the O-ring forwards so that the ports allow air from behind the cylinder head to fill the cylinder through those holes in the piston head face. If this did not happen, until the piston head reached any cylinder window, the only way air could fill the cylinder would be through the nozzle which, since it is restricted, would create negative pressure inside the cylinder, adding more load to the motor compressing the spring.

 

If the O-ring is nearly all the way back and there's a decent amount of grease, then blowing down the nozzle may well be enough to push the O-ring back so that it does form a seal, as if the cylinder was moving forwards. But if not, blowing may actually push the O-ring forwards, as compressed air trying to get behind it expands in the space between the ports and the cylinder wall.

 

As for your problem, I dunno - 1st thoughts were the interface between nozzle and rubber, but the problem should not be replicated when you switched to another rubber unless there's something wrong with either the nozzle itself or its travel. Even if you've accidentally lost an O-ring from inside the air seal nozzle, it shouldn't result in such dramatic FPS loss.

 

And that is the clincher really: 50-100FPS is dramatic, thus it must be inside the cylinder. How much grease do you have on/around your piston head O-ring and coating the cylinder wall? If not much, try some more (preferably CT-2 Teflon grease). I don't think grease alone could account for 100FPS, but CT-2 is money well spent anyway and if you see a 20-30FPS increase from it you'll know that the rest is probably a mechanical problem in the same area.

[LordGeorge]

Thanks for your input ian.

I can clarify that both o-ring and cylinder are fine. And while there is no air leaks between the piston/cylinder/head/nozzle and hop i seemed to hit a road block.

I wanted to look more into the issue with the sticky piston issue, which AF-UK pointed out I should have a freely moving piston.

 

My piston head was certainly a problem, while it does not leak. The piston head was far to large to fit freely inside the cylinder.

As soon as any side to side play is put on the piston as it travels, the sides of the head would catch the cylinder wall and slow it down, or stick if pushed by hand.

 

I foruntnately had a spare pom piston head that fit a hell of a lot better, shame my AO is now out .

Not by much though so no worries.

 

Unfortunately while working on the shell, one of the bearings for the sector gear popped little BB babies everyhwhere, ok no worries i got spares... ah wait i have 6/7/9 mm bearings but no 8s ! Typical.

Should have bearing by tuesday. Untill then i cant really test anymore.

Everything else looks good, so once parts come i will find out how this gets on.

 

As for lubricant, i have been using 100% silicone jell by webbs for air seal parts and ptfe for guides and tappet plate to case.

I know the lube cant be my issue here as bulding many guns and fixing a whole lot more,i have never had any issues.

I will certainly get some of that though .

 

After all these issues, im bulding a HPA kit to be finished the end of the year. Lets hope i have less issues with that .

 

report back tues/wed

 

Cheers

George

[LordGeorge]

Issue resolved !! :D :D

 

Thanks guys for your help.

Lowe and behold the issue in the end was the piston head!

After rulling everything else out, my only conclusion was to replace the piston head.

Though the origional worked and was sealing like a boss, it was a very tight fit.

So in goes pom asg piston head, build back up. Carefully check piston on rails .. al good.

Pop the xcortech, shot 1 355 shot 2 353.... .

Many happy shots later and its consistently back up to the 350fps mark i was so used to.

Also due to the piston fitting happily, the rps has also increased .

 

All back together and ready to go for stormforce on sunday !!

 

Cheers guys.

 

George