Huge Loss of FPS :(

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Willyg1981

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Hi , 

  So Iv installed the new inner barrel (Prometheus tight bore ) on my Cyma m4 cm068.B which is 100mm longer than stock and it gave a reading of 315-319 FPS (which is great) then I went on to exchange the cylinder head to the Prometheus aluminium aero cylinder head , and a guarder sp1100 spring put it all back together and I unfortunately misaligned the piston on the guide rail on reassembly and  that caused damage to the piston head , so I frantically ordered a replacement piston (dytac full steel teeth Pom aeg ) and reassembled it again and it’s just still not working 128 FPS !! Miss firing , firing 2-3 bbs at a time , it’s starting to get on my nerves now I must of taken it apart and put it back together 3 times now , I have just watched a Airsoft tech video about stretching the o ring and considering taking it back apart this minute and trying that as I don’t like being beating by something like this lol . Any help or tips would be appreciated . 

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You have used silicon grease to lube the o ring right? If the Oring is dry then it is useless.

 
Pistons backwards :lol: (just messin'...)

You only need to do that Oring trick if the seal is poor. It's a good little trick though.

 
Take it apart and check there is nothing obvious. Sounds like it could be a tappet plate issue, did you put the little spring back on the tappet plate correctly. Also while it’s apart check compression and fix it if it needs it ?

 
Asomodai said:
You have used silicon grease to lube the o ring right? If the Oring is dry then it is useless.
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Iv used some silicon OIL but I think you’ve hit the nail of the head right there it feels useless and not performing correctly at all I’ll take it apart again and try that , 

i did take it apart again last night and try that stretching trick but that didn’t work .

So something so simple may be the key !!!! 

Thank you Asomodai ?

 
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Use a thick silicon on the oring and some lithium grease on the pistons wings/rails.

 
Sorry I'm not familiar with those specific products so I don't want to give you the wrong advice as you're already in a pickle. ?

I'm sure someone else here will know and say soon enough.

 
Did you attempt to shim the gearbox? Or get the shims in the wrong order?

Did you check the air seals on the piston and compatibility between the new cylinder head and the air seal nozzle?

I think you are looking at a compounded set of problems.


The first thing to do is start from scratch. Strip it fully.

Build the cylinder and piston arrangement, check it for airleaks with a push test.

Then start to look at the gears and motor height. I suspect you have overshimmed the sector gear and it's grabbing the tappet plate.



From your original post, I suspect that the tappet arrangement is not moving fully. This is either allowing multiple BB's to feed for one cycle, or the gun is cycling but not shooting. Either way, it's the tappet that is causing it, It may, however, be caused indirectly through the tappet, If the Airseal nozzle isn't fully compatible it could be binding on the cylinder head and stopping movement, The gears been badly shimmed could cause it, Or a spring failure.

The lack of FPS is caused by the tappet plate in this instance. Because the tappet isn't fully forwards it's not making any attempt at an air seal.


Since you spooned the piston as well I'll need to see pictures of the piston arrangement and the o-ring. You may have done some damage you haven't noticed, Like crushing the rear, or chipping the sealing face.

 
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Iceni said:
Did you attempt to shim the gearbox? Or get the shims in the wrong order?

Did you check the air seals on the piston and compatibility between the new cylinder head and the air seal nozzle?

I think you are looking at a compounded set of problems.


The first thing to do is start from scratch. Strip it fully.

Build the cylinder and piston arrangement, check it for airleaks with a push test.

Then start to look at the gears and motor height. I suspect you have overshimmed the sector gear and it's grabbing the tappet plate.



From your original post, I suspect that the tappet arrangement is not moving fully. This is either allowing multiple BB's to feed for one cycle, or the gun is cycling but not shooting. Either way, it's the tappet that is causing it, It may, however, be caused indirectly through the tappet, If the Airseal nozzle isn't fully compatible it could be binding on the cylinder head and stopping movement, The gears been badly shimmed could cause it, Or a spring failure.

The lack of FPS is caused by the tappet plate in this instance. Because the tappet isn't fully forwards it's not making any attempt at an air seal.


Since you spooned the piston as well I'll need to see pictures of the piston arrangement and the o-ring. You may have done some damage you haven't noticed, Like crushing the rear, or chipping the sealing face.
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I watched multiple videos of the shimming process and took great care when shimming but I will do another strip down and start again from scratch , the only thing that I was a little unsure of was the bevel gear position and how it interacts with the pinion I replaced the bushes with graphite bushes which a faction thinner which I accounted for with extra shims but your definitely right with a group of errors , this is the first gun Iv worked on since they were kits so I am a bit rusty lol

the piston problem I’m hoping is just a case of using oil instead of grease and the feeding problem must be the Tappet plate then,.. I did notice the Tappet plate wanted to raise up over the sector gear Tappet pin not massively but I’m thinking this might be a good place to get right ,

the other thing I was unsure of was which was the setter gear , ie the sector gear being dead in the centre of the piston teeth or the bevel gear being perfectly meshed with the pinion as I write this I’m thinking it has to be the pinion and bevel gear as that doesn’t really have much of a tolerance in terms of side to side adjustment as the teeth on the piston do have a bit of extra width to play with , thanks Iceni really good points and I’m sure with these pointers I’ll get it right ??the air nozzle I’m a little uncertain aswell but I’ll get these issues sorted test it then move on to the air nozzle can I carefully wrap PTFE tape around the cylinder head to create a better air seal ? 

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Willyg1981 said:
I was a little unsure of was the bevel gear position and how it interacts with the pinion
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Split the box, attach the grip and motor to the half you're working on. You can then set the height of the bevel gear and the height of the motor to match. Once that's done you can remove the grip and motor and the rest generally goes from there.

This vid for me was a great help to follow as he explains a lot in detail. Far more helpful than my words will ever be:




?

 
20 minutes ago, Immortal said:



Split the box, attach the grip and motor to the half you're working on. You can then set the height of the bevel gear and the height of the motor to match. Once that's done you can remove the grip and motor and the rest generally goes from there.

This vid for me was a great help to follow as he explains a lot in detail. Far more helpful than my words will ever be:

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Awesome ? thank you buddy 

 
That piston head is fucked.

The big crack is only going to get worse and with it the piston head has a good chance to lose a chunk. When and if it does that you will damage the gears and piston.

39983425423_54bd90eb84_o.jpg


Willyg1981 said:
the sector gear being dead in the centre of the piston teeth
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Central sector to piston engagement is not critical. Provided the piston teeth and sector teeth have full mesh it doesn't matter if it's out of line (most gearboxes are off a little).

Do yourself a favour. Stop buying marked up parts.

https://www.ak2m4.co.uk/index.php?route=common/home

Use this shop.

Your cylinder head that you paid £20 for...£6.50 on there.
https://www.ak2m4.co.uk/internal-parts/cylinder-heads/ra-m4-cylinder-head-pad


Piston head £6.50
https://www.ak2m4.co.uk/internal-parts/pistons-heads/ra-piston-head-double-oring-aluminium

If you have a soft tappet plate I'd swap that as well. Soft been the flag end is easy to bend upwards, A little spring in the tappet plate is normal, but it should be stiff enough that it doesn't deform and bend when the chip engages the plate. Since you have had a minor smash I would change it just to be 100%.

£3.50
https://www.ak2m4.co.uk/internal-parts/tappet-plates/ra-tappet-plate-v2

And since you are putting in an order you may as well have an air nozzle at £4.50
https://www.ak2m4.co.uk/internal-parts/air-nozzles/ra-m4-aluminium-nozzle

 
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Yep I saw that when I took it apart after the initial failure to fire so ordered the Dytac piston which was £10 ,

to be honest I was struggling to find any decent next day online parts supplier , and was convinced with the branding and obviously the material , that place looks good though ?

 
The Rocket gear advertised on his website is made in the exact same factory as the SHS marked up products.

 
Airsoft is a bit odd with branding. It's more of a culture of brands than a culture of performance.

If it looks the same a good portion of the time it is the same.

The rocket airsoft brand that @ak2m4 sells is very good quality. And that isn't based on its price, it's based on the performance.

I've got RA parts in a lot of my guns, and will continue to use them. I'm also quite fond of makes like ZCI and Element. The more expensive gear has to offer a significant advantage that is proven for the upgrade to be worth the cost. Most of the time this is not the case for AEG's. For spring snipers the opposite is true, the branded stuff is normally significantly better.



 

 
Thank you so much for the help Iceni , I alrecieved the bits from ak2m4 this morning and got it all put together , I actually realised what I’d missed the air nozzle clips into the Tappet plate something I’d missed completely but thankfully I’d discovered it this morning Iv just tested it and now it’s firing 365fps ??? so I’m gonna cut a coil off the spring and test it again ,sooo pleased is and understatement lol ?.

 
Very nice!

Watch a video on cutting coils, There are a few tricks to it. A single coil is about 10fps. but you can cut half a coil as well if needed.. So cut less and work up to the value you want.

 
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