Internal revamp for my UMP

T

THE FNG

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Hi all!

Right, I've decided it is time to upgrade my UMP's internals when I get paid mid May.

The situation is that has started to loose it's power by around 5 - 10 fps (not necessary the issue) and the range has dropped considerably (more the issue).

I put in a m105 spring made by element, but it now only hits 260ish fps as opposed to stock fps of 325. I have concluded that element springs are shite as I had one for my p90 (an m100) that only reached 240ish fps.

My questions are:

What spring would you recommend?

What hop rubber would you recommend?

Would a tightbore be recommended? If so, what length?

Would any cylinder heads, pistons etc be recommended? (I think this part of the gearbox is fine; gears all still look pristine).

I would assume an m100 spring from madbull, g&g green or blue hop rubber and a madbull 6.03 barrel at a length closest to the stock barrel, but I wanted to see what you guys think before I act.

Also worth noting; 205mm stock barrel is currently installed. Adding a 363mm barrel made range and accuracy worse and increased fps to 380! Compression appears to be fine at the moment.

Thanks in advance,

Andy

 
Madbull M100, gives 330 or thereabout consistently

Madbull Blue rubbers work well in everything I've tried them in and aren't horrifically overpriced, they're a bit chunkier than TM standards (the other good but cheap option) and tend to give a better airseal

yes, always upgrade barrels. Those and the hop are the only thing that effects where the BB goes, doesn't matter about tightness, just get quality (again, I'm going to sound Like I bum of Madbull, but I've always had good results)

It depends on your compression. A Piston head O ring solves the problem 9 times out of 10 without any further work and they cost next to nothing.

 
Fantastic, cheers Dave. Here's a potentially daft question...do you hydrate the piston O-ring at all?

I'll probably get the rubber and spring now and then get the barrel with the wage packet in May.

I'll probably post results when I get them. I can vouch for the green g&g rubber. I put one in my SCAR and it is very good. Added a nice consistency as well as a bit of range that I couldn't hit. Not tried a blue yet. Do the fish bone nubs make as much difference as the websites say?

 
I usually end up with the stock nub in as the ones supplied are very hard and usually over-hop with 0 hop applied

Don't hydrate the O-ring, just get a new one for a quid or so, worth replacing every time you open the gearbox really.

 
I've ran into another problem. I've installed the madbull m100 and chrono'd but only hitting 280 fps at max. Could that be air seal related or a barrel issue? Stock length is 205mm. When I added a 363mm barrel fps went up to 380 fps from 320. The stock spring is a little longer and does feel a bit more tense than the m100.

 
Def sounds like air seal problem, either the piston head O ring or the hop rubber not sealing properly in the hop unit/barrel? Also has the cylinder got an air hole or is it solid?

 
And is the piston head vented (IE a ring of holes in the front face) or non vented?

 
I've found that the problem was in fact the hop rubber! There was a small hole underneath and it wasn't sealing properly. Having replaced it, I am hitting 330 consistently....well...I was.... :/

I've now got an issue with the gearbox. When the trigger is pulled on semi auto, it clicks and nothing happens. Sometimes it will fire, but it isn't a proper cycle and it sounds weak.

Having researched, it could apparently be due to the cut off lever....but I have no idea how to solve this problem or how to troubleshoot it.

Any advice or help would be much appreciated!

 
What make is it? Only ask as I've had that problem on my JG UMP a couple of times after putting the gearbox back together during upgrades. Basically the cut off lever, trigger mech and trigger contact switch all interact with small tolerences, and while I can't explain WHY this happens, I have always put it down to something not going back EXACTLY in the right place during reassembly - basically I now know if the trigger clicks when reassembled it will not work properly just as you describe, and the wiring will get hot when tested with a battery connected. I suggest you check:

1. The battery is fully charged, wiring is not damaged or preventing the fire selector plate moving fully, and the motor connections are secure.

2. If the above doesn't cure it open the gearbox and check the trigger contacts aren't worn excessively and they contact fully when the trigger switch is pushed in. Also check the trigger cam engages correctly at the back of the trigger switch, a little grease on this area may help too. Finally check that with the trigger switch pressed in, the cut off lever will push it up so that it disengages from the trigger cam and is pulled back by the small spring (this last bit is fiddly to check as you really need 3 hands. . . .)

3. Reassemble and try again. This has always worked for me and been fine until dismantled again so hope it helps!

 
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Thanks for the advice!

It's a Classic Army UMC, a clone of the G&G UMG. I've taken it apart and re-assembled it so many times last night and every time it just wouldn't work. The only different outcome was that on one disassembley, the gearbox would be able to cycle after 2 or 3 trigger pulls on semi. Makes a wierd noise as I do it. Can't really describe the noise...just sounds a bit like an underpowered cycle... :S

 
Sounds like the trigger contact switch needs looking at then, either cleaning or replacing if arcing has eroded the contacts? I know the noise you mean, like a normal cycle slowed down about a hundred times lol!

 
I've fixed it!

One of the trigger contact screws had started to loosen up, making the connection very weak. I've tightened it up and now it cycles again perfectly!

So, the revamp has been finished! I installed an m100 spring by madbull and a blue shark rubber. There is some over hop so I am now playing around with different softness nubs today (day off!!).

Thanks everyone for the help and advice, much appreciated again!

 
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