M4 sometimes fires full auto when set to Semi-Auto

[laladijaan]

hi all,

i have a classic army M4 CQB and an iCS M4A1 that are both presenting similar problems.

when set to semi-auto, upon pulling the trigger both will frequently fire a burst of full-auto instead.

If i snatch the trigger when they are set to semi-auto very quickly i can get a single shot from them most times. Full auto setting works as normal on both guns.

Any help will be appreciated as it makes it very hard to paly inside buildings right now as they are mainly single shot.

cheers.

[johnnyj]

Re: M4 sometimes fires full auto when set to Semi-Auto

cut off or switch block is worn would be top of my list to look at

[guard-sniper87]

Re: M4 sometimes fires full auto when set to Semi-Auto

it maybe the trigger contacts are dirty causing the trigger block to stick in the fired posistion until works lose again

[dope_on_a_rope]

Re: M4 sometimes fires full auto when set to Semi-Auto

The trigger block gets quite a positive push from the cut off lever if everything is working as it should. I don't think it being dirty would really cause it to stick, but some wearing on the plastic fins down the side of the trigger blade could cause the cut off lever to experience problems disengaging the trigger.

Also, there's something in the back of my mind that wants to blame this on the tappet plate, or the little spring by the cut off lever. If it wasn't going to work because of the first option, then you'd think when it missed it's first chance, the second or third attempt might work. If it's because the cut off lever is in the wrong place, or worn down, or the little spring's lost, I'd be happier to think it could sometimes work first time, and other times not work at all.

Whatever it is, really shouldn't be a problem to diagnose if you're happy going this deep into a gearbox. If you're not, it wont be a problem for your local techie to diagnose and fix.

Edit: Having said that about the dirty trigger contacts, I think this single shot to auto thing was a common symptom amongst VFC gearbox's, because they packed the trigger mech full of grease. So it might be solvable by a good clean.
My memory isn't what it once was.

[laladijaan]

Re: M4 sometimes fires full auto when set to Semi-Auto

thanks guys, will check for a worn cut-off lever on the Classic Army CQB.

However, here's the strange thing. i had cause very recently to replace the lower gearbox on the iCs M4A1, so i bought a new one from Zero One and slotted it in, but still have the same problem of somethimes getting fullpauto fire when selecting sem-auto. is there any possibility of the actual selector switch on the body being faulty at all and somehow causing the problem?

[richardy]

Re: M4 sometimes fires full auto when set to Semi-Auto

unless the selector plates half in full auto half in single it wont be that it will deafult to sginle only if you remove it.

you guys have to understand one principle, the posistion of the trigger blade has to be inside the contacts in a forward position in order for the cut off lever to knock the blade back out of the contacts...

If its not there it cant be knocked out or even get a chance to meet a cut of which is worn or not worn.

sometimes trigger contacs get moved closer together there for they will now activate with the blade sooner...

This is why he found if he jamed the trigger hard and fast it managed to get the balde into position fast enough for the cut off lever to knock it out.

id clean / open up the contacts slightly or leave them closed but bend them out wards at the start of the contacts in a v shape. this way the trigger will have to travel further before activating the gun.

[laladijaan]

Re: M4 sometimes fires full auto when set to Semi-Auto

hi guys, update on this:

i changed the trigger, trigger switch, wiring, cut-off lever and selector plate on this gearbox, now it works on semi-auto all the time, and only sometimes on full auto, grrrrr!!

what i observerd is that it will do a few rounds on full auto and then stop with the piston in whatever position was in at the time of stoppage. pressing the trigger again whilel still on full-auto will cause the piston to move to the rear a little further and then stop again (if i switch to sem-auto at this point it works fine and completes a normal cycle). several presses whilst still on full-auto wil allow the piston to eventually completes its cycle a little bit at a time.

semi-auto works fine all the time.


i have also changed the body, and i have to admit (now using a king arms vltor troy body) that the fire selector from my classic army M4 body is not a great fit it feels loose, so switching between semi and full auto modes does not seem nice and crisp. is this the problem as i am not sure it would explain teh weird behaivour on full-auto.

any further ideas would be helpful.


cheers