By Rich on Wednesday, July 15, 1998 - 07:27 am:

Errr.. Did you try adjusting the grub screw on the pistol grip? Tightening that can cure the problem. They can vibrate loose, so once you've adjusted it, seal it down with a drop of locktight - not too much or it will never turn again!

I had a similar problem with my Sig 550. All it was was vibration moving the motor out of the correct position. Turning the grub-screw fixed it no problemo.

Rich,

I tried that this morning.... I tightened the grub screw about 2-3 turns... and funny... the trigger does nothing now!!!

Perhaps my bttery is so low, that it won't turn the motor under load....

More later... (I'm disassembling the gun this evening, so... we'll see what's up)

Ryotsu

Ryotsu, you've probably overtightened the grub screw so much that it's seized the motor shaft against the AEG mechbox preventing the motor from turning. I would *strongly* suggest that you back the grub screw out 3-4 turns (farther than you screwed it in), remove the grip bottom, unhook the motor and remove it, looking very closely for any damage or 'burned smell'.

It's very possible that if you've attempted to fire the gun repeatedly with it in the 'trigger-not-working' condition, you may damage the trigger contacts at the very least, and if on full-auto, could have fatally burned the motor.

What happens is that the motor is trying to turn, but the current isn't exerting any force of spin on the shaft. Since the shaft is stuck, the current continues to flow across the brushes which carry current to the commutator (these are what you see through the inspection/cooling holes on the side of the motor). If it smells 'burnt' or is brown/black in color, then you've likely burned out the motor. Unfortunately, if this is the case, the insulation on the windings wrapped around the rotor will have melted down, and it will be useless.

The 'grub screw' on the bottom of the grip is for very, very minor adjustment of the pinion gear to sector gear mesh, only, and should be carefully adjusted while the gun is firing full auto (with a plugged barrel) on a fresh battery. By adjusting this mesh, you'll be sure to get the longest life out of your gears, as any out-of-tolerance gap will quickly chew them to pieces.

After you check your gun, re-assemble the motor (if it survived) and reinstall the bottom plate. Tighten the grub screw with the gun firing full-auto with a covered muzzle (to provide back-pressure to the piston and prevent 'slamming' which will damage everything) and listen for excessive gear 'whine'. With the motor still running, tighten the grub screw until the whine becomes more smooth.

Hopefully, your gears will be in good enough repair to allow you to smooth this out. If the whining just gets worse, back the grub screw out. BY ALL MEANS, if the motor starts to bind or screech, LET OFF THE TRIGGER and back the grub screw out again. Screeching is a sure sign that you're overtightening this gap and will lead to immediate damage (motor or gears)...

I hope that this helps you with the repair. Normally, this doesn't have to be adjusted for quite some time on a new gun, and is only used to extend the life of already disintegrating sector gears. A gear-change is imminent if the whining continues. Good luck, -LD

OK,

Ryotsu here. I heald my breathe and jumped into the gun this evening.

No problem!!

A friend was with me, who has taken HIS gun appart, and we got to the gearbox in about 10 minutes.

After experimentation and close inspection we found something pretty obvious; the fuse had burned out!! I didn't even think of that. Tightening the grub screw put enough stress on the motor to burn it out. Fine with me. Turns out the motor is fine!!

My friend and I looked at each other and smiled, as we started taking the gearbox appart. I was pretty worried about it, but we had Almighty's pictures and papers in hand...

The large section of the spur gear had lost 5 teeth. (I found them laying in the lube) And something else caught my attention. The straight section of gear-teeth on the piston had been ground up too! The very back tooth was 60% gone, the second was 30%, and the third had started to wear.

Befor the gun jammed, it was firing lame, 50% power shots. I figure that the pull-back gear would slip a few teeth befor it would grab the piston, and therefor only get the cylinder back so far befor it let go, resulting in lame power.

Also, when the back teeth on the piston got worn past a certain point, the big "pull-back" gear may have jammed itself against the worn teeth and while the motor tried to power on through.... BA-Bzzzzzrrrrgggg the spur gear gave way..

Cool.... now I know what happened. And I'm no longer afraid of that silver and brass gearbox!

I have a new gearbox on the way.... but how do I keep this from happening again?

Silicone, every 5-10 clips?

No more .13 -.165 gram glow in the dark BBs? (that's what I was using when the gun jammed)

What?.... I don't want to have to do this again...

Thanx for any more advice, and thanx again to Almighty for reposting the info!!

Ryotsu
Hope my story was fun to read...
Cool, now I know what

Excellent work, Ryotsu,

I couldn't have done better. You got the part about the piston's rack quite correctly. Don't think that if you replace the piston with metal the problem will go away. It might even be worse. That is why I only trust using the individually sold Systema piston (matched to the helical cut gears).

The only way to lessen the wearing out of the rack is - lessen the number of shots you fire. Wearing out is inherent in the piston and gear(s) service life - one simply tries to make as many shots count during the period.

But how come you're getting a new gearbox? Was your old one damaged? Oh well, you can keep one of them in reserve anyway...

I thought I'd just add...

The motor adjustment grub screw should be treated with respect, but I reckon a lot of gear problems are caused by this screw vibrating loose and causing the gears to fail to mesh properly. I noticed that this happened on my Sig because the grip assembly was loosening.

You've really got to keep an eye on it. The best method is to get the adjustment right and then seal the screw with a drop of locktight (but not too much in case you want to adjust it again later). Just put enough on to stop the vibration loosening it.

I only found this out after reading a translated manual for my MP5!