Same M90 spring in 2 guns with huge FPS variation

[ao_tenshi]

Hey all,

Just wanted to get some experienced opinions from everyone here in the community on this. I've used the same SHS M90 spring in the two guns i have and the chrono'd FPS is vastly different. I wanted to know if this is normal/expected, or is something wrong (or operating inefficiently) in one of the guns. Both guns running on 7.4v Lipo.

M90 Spring in:
1) KWA M4 = 360FPS
2) G&P TR = 290FPS

Thanks!

[▼Stripes▲]

sounds to me like you've got an air leak in the g&p

[ao_tenshi]

Originally Posted by ▼Stripes▲

sounds to me like you've got an air leak in the g&p

I'm quite new to airsoft... any suggestion on how to check and fix the air leak?

[lurkingknight]

check compression from rear to forward... piston to cylinderhead by blocking cylinderhead. It can either leak from the pistonhead or cylinderhead o rings.

Piston/cylinder assembly to nozzle... block the nozzle. If compression elsewhere is good the next point of failure is the nozzle o-ring... if it has one.

After that it's nozzle to hopup, which is the hardest to deal with depending on the severity of the leak. It could be as simple as a bad hopup rubber, the seating of the rubber on the barrel, the assembled barrel in the hopup unit or the hopup unit itself not being tight enough to the gearbox.

[ao_tenshi]

Thanks guys, I'll check the gun for leak based on your instructions.

So the ~70FPS variation on the same spring is definitely "abnormal" correct?

[Danke]

The one with the low FPS is the same one that's full of plastic shavings from the piston?

[ao_tenshi]

Originally Posted by Danke

The one with the low FPS is the same one that's full of plastic shavings from the piston?

Yes. I've noticed very tiny plastic shavings around the spring and piston in the G&P when i changed the spring.

[Danke]

It sounds like you have a few things going on.

You can go at this a few ways. You can treat the AEG as a trial and error project, post questions here, buy parts, try the new parts and then carry on if the situation is not remedied. That takes time, patience and cash. It can wind up causing you more grief if other parts fall victim to the process and also must be replaced, or just get lost in the multiple tear downs.

Or you can hand he AEG to a gun doc. That gearbox is the most common one out there and it's possible you have a simple and well know issue with an easy fix. That costs you more cash than fixing on your own and it can take time too.

There is no right or wrong way to do this. It depends on your skill and patience levels.

[suzenonest]

Are both barrel lengths the same?
Are the cylinders ported?

That's where I'd start

[ao_tenshi]

Originally Posted by suzenonest

Are both barrel lengths the same?
Are the cylinders ported?

That's where I'd start

KWA barrel is about ~3" longer than the G&P
all cylinders are stock (as well as everything in both guns).

[suzenonest]

Are the cylinders ported tho?

If not, then you must be leaking air somewhere as earlier stated. Checking this will require taking the gearbox apart and testing by hand.

For the kwa, does it still cycle and shoot normally, just with low fps?

[ao_tenshi]

Originally Posted by suzenonest

Are the cylinders ported tho?

If not, then you must be leaking air somewhere as earlier stated. Checking this will require taking the gearbox apart and testing by hand.

For the kwa, does it still cycle and shoot normally, just with low fps?

Both guns are not ported. I guess i'll have to test for leaks. Both guns cycle and shoot normally. Just the FPS on the G&P is significantly lower than the KWA.

[HKGhost]

Start checking with the hop up bucking on the G&P. Then move into the compression system once the hop up internals are ruled out. I have a feeling that you may have incorrectly installed the nozzle, but you can check that without opening up the gearbox.