M4 GB FPS problem

[brewer17]

Hi all,

My mate destroyed his G&P gearbox and decided on buying a chinasoft gearbox (against my advice)

The FPS of this GB was 98FPS so he gave it to me to sort for him and I cant get it over 191FPS. Iv re-shimmed, changed the cylinder & head, piston & head, put in an m100 spring and changed the air nozzle. I know the airseal is spot on because i cant even push the piston in.

So this thing has baffled me. Any advise or ideas??

[Sitting Duck]

Re: M4 GB FPS problem

Correct nozzle length ??

Shave front of tappet plate to get nozzle to fully seal on bucking

Bad alignment of box - hop resulting in nozzle not sealing correctly ???

A bad air leak it sounds like if m100 is correct

Was gun firing ok with old box ???

[brewer17]

Re: M4 GB FPS problem

Yeah all fine with the old GB, nozzle is correct. Shouldnt be an air leak if i cant push the piston down the cylinder with my finger over the cylinder head

[Sitting Duck]

Re: M4 GB FPS problem

From piston no

But doesn't mean nozzle is sealing correctly against hop/bucking....

Some boxes at front of shell can be 2.6 to 3.1mm thick, then add in tappet plates....

Found a SRC was very thin - just over 2mm where as most others like SHS are 2.5mm and often people will shave front of tappet a little to ensure nozzle moves forward more helping to seal on bucking lips

Just these deviations alone will throw stuff out, plus some hops / nozzle combos may not work 100% without a little mod/adjustment
TM compatible often doesn't mean 101% compatible with everything...

Get gearbox in lower receiver and cycle gun to where nozzle is fully forward
Just lower receiver and gearbox running...
Place hop/barrel up against it and you should feel about 1mm resistance so in effect it is sealing 1mm no less and no more than 1.5mm as too much push will damage bucking lips

If that works then wrap tape around hop incase of hop leak

Try and fire a bb with 3 pairs of hands holding hop to gearbox see what readings you get you may have to attempt this a few times....

If you get 300+ by this method then the problem is hop not pushed back enough, nozzle not sealing or bad alignment in gun

Also this diy test may show up stuff like hop unit nozzle opening might not be fitting into gearbox correctly so it sits slightly forward if the holes are bigger/smaller
I have had tight fitting hop to gearbox and needed a little sanding to get hop to fully insert into box

On thin front gearbox 2.6mm the larger hop nozzle opening can protrude a bit too much but this is quite rare that hop entry goes a whisker too far into the box and needs filing not to hit tappet plate inside

Also ensure a good hop spring or some o-rings keep the hop pushed fully back onto box or try other mags
What could be happening is that as mag is clipped in it could just tug the hop unit forward creating a poor seal when firing bb's

Excuse any typo's mobile is correcting my bad typing

Oh yeah fun n games these poxy toy guns

[brewer17]

Re: M4 GB FPS problem

Cheers fella ill give it a go.