Umarex ump help

[tomola]

So I fitted a new hop unit (the old one was snapped, 2nd hand gun) to my umarex ump as well a 6.03 barrel and an m100 spring and it was only chronoing at around 200fps which is pretty pathetic although the range was surprisingly good which confused me. I know it's a v3 gearbox and the internals are the same as a g36, any help would be greatly appreciated, I'm thinking it's got bad compression or something?

 

[Lozart]

If the range is good with that low of an FPS then at least your hop is working well!. Sounds like an air seal issue to me. Is the piston o-ring ok? Did you check that he barrel is seating properly onto the gearbox?

 

[tomola]

Yea it's a metal hop that I've sort of adapted to fit cus the plastic ones were pretty weak, I thought it might be an air seal issue just wanted a second opinion before I started stripping the gearbox apart, it's got a brand new m100 spring in so it should have pretty decent power. I didn't think the barrel touched the gearbox it goes in the hop unit doesn't it?

 

[Lozart]

I didn't think the barrel touched the gearbox it goes in the hop unit doesn't it?

While the barrel itself doesn't touch the gearbox, the tapered lip at the end of the hop rubber should be making a seal against the tip of the nozzle. To do this the body of the hop unit usually needs to make contact with the front of the gearbox.

Certainly sounds like an air seal problem to me, you should have plenty of poke from that spring!

 

[dex]

You need to open up the gearbox and check the compression on the cylinder and piston. Put your thumb over the nozzle of the cylinder head and push the piston in. If the piston slides forward too easily you have an air leak. Check the fit of the cylinder and piston heads.

 

[tomola]

I'll check the air seal but the bit about the nozzle not making a seal sounds like the culprit I've just looked up the feed tube with a torch and I can't even see the nozzle so I'm assuming it's not up the gearbox