TM mp5 HELP!?!?!

[Junky]

Ok .

I am having feeding issues with my mp5.
I have just rehauled the whole thing.

so here we go, i have just put in a new hop chamber, and bucking. Aswell as completely new gearbox.

When i fire the gun on semi or a single shot on full auto it double feeds. When on full auto "holding trigger down" it feeds fine and fires awsomely!!


Now it seems that everytime the trigger is pulled for a signle shot the piston retracts and releases as the norm and the tappet and nozzle retract and move into the forward position and fire the bb. But then it seems the motor keeps spinning for a split second, enough time to partialy pull the piston and tappet back just enough to alow another bb to enter the chamber. And then you can hear the assembly spin back down until it hits the next notch on the anti reversal latch in which the gun is now at resting state. The next time you pull the trigger there is already a bb in the chamber before it loads the one that "should" be fired. thus being a double shot. And thus really getting on my nerves.
I don't understand the reasoning behind this at all. the motor seems to be switched on for just a fraction of a second too long.

ANY HELP!?!!!!!!???????

[Naglfar]

Your gear timing is probably off, go to mechbox.com and watch the MP5 reassembly video for the proper gear alignment.

The sector gear has to be right up against the tappet when you close up the mechbox.

[Dynamo]

I would say it's either your hop-up or your anti-reverse latch isnt correctly installed. There isn't really anything to time as far as gears.
Maybe your spring is too soft for the rate of fire you got, and it doesn't have enough power to stop the gears from rotating.
I'm really leaning towards a hop-up issue since the gearbox should only draw the nozzle back during the pull back of the piston, and release the nozzle befor the piston is released.

So if there is a problem with the hop-up, it may let two BBs in everytime the gearbox cycles. The BB should not be able to move from the feed tube forward into the hop-up without the nozzle pushing it forward. Check your hop-up sleeve. Remove the barrel and hold it with the hop-up pointing up, then place a BB on the hop-up openning. The BB should not just drop in, but it should be held in place by the hop-up openning. If it drops into the hop-up with out you pushing it (lightly) with a finger, then your hop-up openning is too wide.

If there is nothing stopping the BB from moving forward, then the BB below it will push it with the tension from the mag spring thus the double feed.

[Dracheous]

Hop-up rubber could have bunched up and/or twisted. First before stripping down the entire mechbox again, I'd try reinstalling the hop up rubber. When installing, insert inner barrel into hop-up bucking, and then those two at a 1/3 twist to the hop-up unit, slowly push in while turning slowly till top dead Centre. Once in, pull the inner barrel just a bit, not enough to actually move the inner barrel out, just make sure properly aligned for the C-clip, and then reassemble the gun. Once done this, dry-fire the device to clear some of the excess oil. If this does not solve the issue, then you will have to investigate the timing, but the described issue sounds to me like the hop-up bucking as previously diagnosed.

[Junky]

thanks for the insight guys.



-- i checked the hopup and it seems fine, i just got a new one due to the stock one being worn and letting more than 1 bb in the chamber like you had mentioned from the pressure of the mag spring.

--It fires fine on bursts of full auto because the motion of the tappet is continuous from the motor being held on.

--Semi shots, the assembly is not stopping right after the piston releases, the sector gear keeps turning to the point where it engages the piston again and pulls it back about 3/16" then it stops and travels forward again until it reaches its resting position on the next position on the anti reversal latch.. which is working! you can noticeably hear the motor and gears spin backwards until this point.

Could this be a switch problem. Or motor? That is the only thing left stock from the factory in the mechbox still.

[Jimski]

what is the spring, what is the battery voltage?

[Schwag]

As far as your motor and gears turning back, check your bevel gear.
Some bevel gears have 4 or maybe 5 teeth that connect to the anti reversal latch while some only have 2 or 3. The more teeth, the less "rewinding"

[CDN_Stalker]

Problem lies in the hop up like has been mentioned. I had the same problem with my MP5 last year, the only reason you think the gun isn't double feeding in full auto is you can't tell because of all the BB coming out.

My hop up unit was missing a screw (that holds it together) and the clip at the end (the one that holds the inner barrel solid) was also in backwards, casuing my rubber and the barrel to move forward a slight little bit, enough to allow two BBs into the breech. I tapped for new screws, reset the rubber and fixed it.

[Flatlander]

I'm having the exact same problems as you described on an m4 (also V2). I'm working on a teamates mechbox and some new parts inlcude a modify 100 spring. One thing I've noticed is that there's something wrong with the antireversal latch (also modify)...when I check the shimming w/ the mechbox closed and NO latch in place it spins freely. When I put the latch in and close it up and there's A LOT of resistance and I can't figure out why for the life of me. Still waiting for my buddy to get me his old latch as well as a few other parts so I can see if that will simply fix it.

Someone mentioned above that maybe the spring was too soft and the voltage used too high. I was testing my buddies mechbox on a 9.6V 2200mah and the modify spring is a VERY long spring (regular pitch) so it's not very stiff. It would make sense the the spring is not capable to stop the 'overspin' of the gears. There is also only 2 anti reversal latch 'catches' on the spur gear unfortunately.

Once I get the parts and things figure out I'll update you.

EDIT: I would also recommend checking your piston teeth. It wrecked a brand new modify piston when the piston would return after the overspin. The first tooth (back) was completely broken towards the FRONT of the piston and the first few warn down that way which is OPPOSITE of the normal wear (caused by the slight 'overspin' then returning forward). I'd also recomend staying away from Modify pistons. I had at most a couple hundred rounds through it and when I took it out it also had 'spider web' type cracks running along it. Little bit longer and it would've shattered in the mechbox!

[Colin_S]

I've noticed this issue with a few TM MP5s that I've worked on in the last little while. I talked about my issue here and in the pic you can see the hopup rubber deformed. I found that when I replace rubber with a systema and lube the outside, I can install the rubber properly and don't have feeding issues anymore. Take what you want, hope it helps!

[Styrak]

Originally Posted by Colin_S

I've noticed this issue with a few TM MP5s that I've worked on in the last little while. I talked about my issue here and in the pic you can see the hopup rubber deformed. I found that when I replace rubber with a systema and lube the outside, I can install the rubber properly and don't have feeding issues anymore. Take what you want, hope it helps!

Link fixed

[Colin_S]

WTF? That was an odd one, thanks.

[Junky]

ok i will get a new hopup rubber. I did have an issue putting this one in, i guess i didn't lube it enough cuz it was a bitch!

BUT what the heck is up with the overturning and then back spinning of my gearbox?? i have systema spur gears and the bevel has 5?? i think notches for the reversal latch. i've played around with the timing and can't seem to get it straight. this isn't normal is it??.... oh and the spring / battery is Prometheus m90 and 8.4v 600mah batt.

[ThunderCactus]

Originally Posted by Naglfar

Your gear timing is probably off, go to mechbox.com and watch the MP5 reassembly video for the proper gear alignment.

There's no such thing as gear timing, all functions that would require any sort of timing are all operated by a single gear, the sector gear, this eliminates any need to time gears.
The reason you set the sector gear to that position when putting the mechbox back together is so it's not pulling back the tappet plate and it's not meshing with the piston, there's no actual gear to gear timing involved.

[Junky]

ok. That does make complete sense.

Ugh. i just wish i could pinpoint my problem with the mechbox..
I will get a new hopup rubber and try that out i guess. but the unwinding of my gears/piston is really buggin me!