Low FPS causes?

[Rubberducky]

Right, recently got my hands on a TM mp5k that's apparently firing at 180fps although I've yet to chrono and confirm this. I took it apart tonight to check the gearbox casing for cracks and check the spring too. The gun looks to have only been fired a handful of times as internally it's mint. Gears are all perfect, motor is perfect and the casing is fine. All the seals look good on the piston and the head so don't think the gearbox is an issue. Spring doesnt really feel weak. I do have a TM one I can put in there to try but it doesn't seem to come out of the piston (a knackered one from my other mp5 I've just rebuilt) so any ideas on how to get that out?

Other than the 2 things I've spoken about above is there anything I should be looking for?

Thanks, James

[Montesa77]

Re: Low FPS causes?

Done the airseal test? Mabye it lies with that hop up(knacked rubber).

[jagillham]

Re: Low FPS causes?

TM springs are attached to the piston by a screw.

Should be around 290fps. I'd get it crono'd before going mad trying to find a problem that does not exist!

[Rubberducky]

Re: Low FPS causes?

Agree but my local site aren't running till next year. If I can get down my local shop I'll take it there. I must admit I'm starting to think it's probably ok.

What is the air seal test. My hop rubber knackered if my other mp5 and that still punches out 320.

[Montesa77]

Re: Low FPS causes?

Put your finger over the air nozzle then push the piston down through the cylinder. If it goes down all the way = bad airseal, if it stops and bounces back = good airseal.

[Rubberducky]

Re: Low FPS causes?

I'll give that a whirl. Thank you. Also check the hop rubber while it's apart.

[richardy]

Re: Low FPS causes?

chrono before, change spring and chrono after and go from there.

[Rubberducky]

Re: Low FPS causes?

Right had the Mp5k chrono'd today and it was just shy of 200fps. I've got another TM spring I will try but if that doesn't solve it could it be anything else?

[Montesa77]

Re: Low FPS causes?

Air seals.....

[Kev132]

Re: Low FPS causes?

did you do the cylinder test the guys have already stated ? an fps that low imo wont be caused by the spring.... id say the airseal or the hop....

[Rubberducky]

Re: Low FPS causes?

No I haven't. Do you do this with the gearbox apart?

[Montesa77]

Re: Low FPS causes?

Yeah.

[Rubberducky]

Re: Low FPS causes?

Ah in that case yes. I covered the air nozzle (the part that would stick ou the front of the gearbox when built up) and pushed the piston down the bore so to speak and it went right to the end. So I'm guessing I could do with a new seal on the piston and the head.

[jagillham]

Re: Low FPS causes?

Well..... the leak could be from the piston head, cylinder head or air nozzle.

First step, make sure all the rubber seals are lubricated well with silicone oil - then re-rest!

[Rubberducky]

Re: Low FPS causes?

Ok. Is there a seal on the air nozzle too then? I thought there wasn't.