Low FPS

[Ells]

An easy one to fix - apparently not this time!

FPS is coming out at ~240 with an m100

In my Cyma MP5 I have the following;
Guarder M100
SHS Red Piston + piston head
Full cylinder
Cylinder with sorbo
new tappet plate
SHS air nozzle

All the other bits are stock. FPS is reasonably consisten (+/- 5fps), but seems to be super low. I've already switched out the;

- Main spring with another - slightly lower FPS
- Cylinder for a ported type - slightly lower FPS
- switched out the the piston - no difference
- Switched to a different barrel - higher fps - 50 fps higher, it was longer and tighter - im guessing it was due to this, and the acceleration of the bb
- Switched hop rubbers - no change

The alignment between the nozzle and hop chamber seem fine, as does the internal compression, but I'm sure I must've missed something - as I should be getting about 350,

Any ideas?

[Sitting Duck]

Re: Low FPS

Guarder springs are usually a little more than they are marked up.....
so you should EASILY be hitting 350fps imho as they can be another m10 more than other springs.

Now this figure or any final figure will depend on other factors:
bearing piston head - nah use a plastic spacer to save weight
bearing spring guide - adds fps though I've found if correcting AoE you lose a bit but bearing spring guide gets you back aprox to original figure
barrel length or joule creep that does exist on longer barrels than baby short mp5k barrels
a few other possible factors like a very heavy or very light piston may affect it a smidge.....

Oooh that is a thought could be a tight sticky binding piston perhaps but first thing I'd test is:

box running + hop unit, load up bb into hop unit, hold hop right against box - yeah you will need numerous pair of hands
and you probably have to repeat this 3 or 4 times and yes it is a pain....
now chrono the bugger - perhaps with ptfe wrapped all round the hop unit to reduce airloss
you are trying to see what she may shoot at if it all went together, sealing well as best and as steady as you can get

Hopefully this will show a decent figure with hop dialed off, and you should feel as you push the hop up against the box about 1mm of resistance as nozzle presses against bucking lips though in truth you would have to get the gun to cycle in auto and get ya finger off trigger quick to leave nozzle fully forward
(many people panic that guns overcycle if they see nozzle starting to retract but in fact it is quite normal unless you are running a very very slow gun)

You gotta carry out a few basic tests or a simple process of elimination first to hopefully pinpoint where the main loss of air is
You can start changing out loads of things - which in fact will usually just make it worse

Also you need to ensure the tappet plate is correct, allows max possible travel - there are few mods like shaving front of tappet plate
plus it is Soooooooooo important to ensure the nozzle itself is the correct length or you will get fps or feed problems
There is an aprox 8mm "window" that allows the bb to enter hop, nozzle shove forward and seal against bucking lips
This is fine for most instances but if wrong nozzle or it is catching then bb's will not load & seal perfectly especially at higher speeds

But back to your problem - that is quite an airloss on possible 350+ spring and I think the hop/bucking area is going to be first thing I'd check
if you still got poor results after that test then check hop unit/bucking is sealing - ptfe and stuff to improve upon it
if you can't feel a bit of resistance if nozzle is fully forward when you place hop against gearbox then that nozzle or position/operation of tappet plate needs looking at. Whilst you are about it with box open, remove cylinder/spring and ensure piston slides back n forth freely on its own in case piston is binding.

Everything needs to be double/triple checked and often sand/file the odd component to operate 101% smoothly
Many people think you just chuck bits n bobs into boxes and they just work - only if you are very lucky is the hard truth I have found

best of luck

[Ells]

Re: Low FPS

Thanks for the long reply!- turns out it was a problem with the sorbo on the cylinder head - which I really wasn't expecting - its back up to a tad under 350 now