TM MP5 air loss issue

[Ben33]

Hello all,
Hoping somebody can help,
Recently got a used TM MP5. I knew there was a Gearbox issue before I bought it but got it cheap

I have rebuilt the gearbox (Version 2) into a new case and shimmed all the gears.
It has a G36 length barrel fitted so have also fitted the correctly ported cylinder plus a new piston. I have checked the piston head and it has a tight seal plus the hopup bucking seems to still be in good condition.

The FPS seems too low, even with the hop set high it drops after about 20 or so feet. any one have any ideas, the tech at groundzero seems to think its an air pressure issue but not sure what else could be causing this?

Cheers in advance
Ben

[Ells]

Re: TM MP5 air loss issue

-using the new cylinder may have created a leak around the cylinder to cylinder head
- you may have put it back together incorrectly, either at the gearbox or the hop
- could be an airseal issue around the hop
- is the barrel clean
- did it work with the tm barrel, before you changed the barrel?
-how does it fire if the hop is set how it should be, it doesnt really matter how it fires when at full

you should try and take a laser beam style approach to it, try and locate the area of the issue (gearbox, or barrel), then it will be easier to diagnose

just realised how horribly written that was, hope you got what i meant

[Ben33]

Re: TM MP5 air loss issue

Hello mate,
thanks for replying!
It could be the seal between the head and cylinder, although it felt like a good seal its hard to tell by hand. I might try some PTFE tape as a temporary fix just to test the theory. I have pretty much ruled out the gearbox now as I have dismantled and checked a fair few times
The previous owner changed the barrel over so I cant really make a comparison although I do have access to a friends longer barrel i could try.
Any idea how I could tell if its losing air at the hop?
Thanks again
Ben

[Ells]

Re: TM MP5 air loss issue

well, to start with i would take the hop apart and reassemble it, there could be a rip in the rubber that was causing air loss, this would be pretty easy to see. Especially if the other guy didnt know what he was doing when installing the barrel, he could have ripped it without noticing when installing it.

But if you take it apart, clean the barrel, perhaps put some ptfe tape over where the hop meets the barrel (on the side away from the gearbox) and double check to make sure that the hop has a good range of movement before you install it back on the barrel.

given that you don't know exactly whats been changed in it, it there could be an issue with the nozzle distance to the hop chamber, but i would check the barrel first, then the gearbox afterwards if the low fps is still existin, when you say low, how low do you mean btw?

[Ben33]

Re: TM MP5 air loss issue

Great thanks mate, I will give all that a try this evening (wife and kids time permitting)
and report back

[HunterAndy]

Re: TM MP5 air loss issue

In my experience the hop up units in TM Mp5s can wear out if using upgraded springs. I have previously changed the old one for an ICS mp5 hop unit and it fixed the issue.

[Ben33]

Re: TM MP5 air loss issue

Thanks, I'm not sure of the rating on the current spring as it was in it when I got it.
I have it open at the moment, just checked the hop up unit seems to seal but tricky to say by blowing down it and sealing the ends. I may well have to change it out anyway but
Wifey has implemented but spending restrictions for the rest of the month .

[Ben33]

Re: TM MP5 air loss issue

Ok not sure if this will make any sense but:

With the G box open, when I test the air pressure in the cylinder with the nozzle attached and blocked off with my finger I can push it all the way to the head.
When I take the nozzle off and block the end of the head with my finger I cant push it much past the ports.

How tight a fit should the nozzle be on these?

Sorry for all the questions and thanks for all the help!

[colinjallen]

Re: TM MP5 air loss issue

It sounds like your nozzle is not sealing well on the cylinder head. Does the nozzle have an internal o ring? If not, replace it with one that does.

[Ben33]

Re: TM MP5 air loss issue

I think you are correct.
I've just ordered an o ring nozzle and also a new head just to make sure from zero one (collection from warehouse tomorrow as work close)
Will update tomorrow evening after reassembly.

Thanks to you all for your help

[sparrowhawk]

Re: TM MP5 air loss issue

Before installing the air nozzle, you should check it's seal in the hop unit, push it in the unit and seal up two ends and blow for leaks, then you can check again with the complete hop unit and installed inner barrel pressed against the rebuilt gb,just to see how they all mate up, the unit should be nice and snug against the gb

[Ben33]

Re: TM MP5 air loss issue

Just a quick note of feedback

I fitted a new nozzle and checked the seal both to the gearbox and into the hop which was better on both counts.
This seems to have solved the airloss issue which is great. But still need to test over distance ( not to sure I fancy trying it in the car park at home)

I also put a new spring in as the old one looked a little ropey and was also an unknown. I've fitted an M110 but now I'm a little worried this has raised it above 350fps. I guess the only way to know for sure is to get it Chron'd.
Does anyone know the original spring rating on an older TM MP5?

[sparrowhawk]

Re: TM MP5 air loss issue

If you have improved your air seals then you put in a m110,the likelihood is that you have gone over to around 360 or higher, some springs do look ropey by design but as long as it isn't snapped and does its job then there's no need to change it

- - - Updated - - -

Tm guns should be around 270 in their original state, you should of used an m90 or 100

[Ells]

Re: TM MP5 air loss issue

cool that you managed to fix it- personally instead of just bunging in a bigger spring. I would put in like an m90 and a bearing spring guide- as it will most likely have just a plastic non bearing type, and linear spring. Then you should be around 330, and be able to use it at both woodland and sites with lower limits - without any noticeable difference in performance.

[Ben33]

Re: TM MP5 air loss issue

Thanks guys
Yep a bit of a newb mistake!
Have ordered an M100 for now will see what its like with that
Still doesn't hurt to have to many spares!