AGM MP40 Headache!

[hepcatfox]

Be kind to me, I'm still new to this... ;D

Based on what I've read on these boards about the AGM MP40 I decided to take the gearbox apart, clean out the old grease and replace it with the proper kind (and take out the semi-auto fire function). Although this is the first time I've ever messed with a gearbox, I am fairly well mechanically inclined, I read all the articles, and I went slowly to keep track of everything.

At first, everything went well, I got it all back together, and test fired a few rounds. All was fine, but after about a magazine worth, it stopped firing.

Taking it back apart the piston was stuck in the rearward position. I've now had it apart around six more times and can't figure it out. Everything seems to be back in place correctly, but every time I reassemble and test, it cycles once and then the piston sticks again. It seems to be sticking right at the point before the sector gear disengages from the piston allowing to fly forward. The gears get progressively harder to turn as it nears this point. If I move the bevel gear manually with a small screwdriver I can get it to move and release the piston. But the motor can't do it.

The two things I have suspected are (1) could the piston be jumping off the guide rails? or (2) could the tappet plate be bending somehow and causing the problem? While I'm not 100 percent for sure, I have tried to make sure these two items were not happening and they don't seem to be.

I also checked the charge on my battery with a volt meter, and it seems to be fully charged. (But I confess I don't know much about checking the battery other than seeing how many volts it is putting out.)

Also, one other thing I'm not sure of. On the sector gear, the shim that goes on right hand side of the gearbox, should it be the last piece that goes on the shaft before the bushing? And speaking of shims, since I have the thing apart anyway, should I just go on and order some replacement bushing and shims, and which ones are the correct ones to use?

Any help will be greatly appreciated!

[Obergefreiter Raimund]

Do you have the stock piston in the gun? If so I would replace it with a better one while you have the gun apart anyway!

[summvideo]

Wow. I have the EXACT same problem with my AGM MP40, and I've NEVER taken my gearbox apart. Every so often, the piston gets stuck in the rearward position, and I too have had to release it by advancing the bevel gear with a screw driver. It seems to be a problem with the gears (but I'm far from an expert).

Does anyone else have this problem ? How did you correct it ?

[biged]

A piston twisting off the side rails is not possible. (Unless the gearbox side rail snaps off.)
Tip: Rotate the cylinder and line the hole up so you can see one side of the piston/rails.

The tappet plate is a black & white operator. It's either working, broken, or the spring is not on it.

Here's a list:

1. Gun's will not fully cycle on a uncharged battery.

2. The stock batteries are horrible. Buy a high amp output battery.
www.onlybatterypacks.com/items.asp?db=26
(Battery # 11218)

3. If you upgraded the gun you'll need #2.

4. Spring guide may not be seated properly. Make sure it is in the slots inside the gearbox. Hold it in place with a screwdriver, then slide out the screwdriver when closing up the shell.

5. Shimming... Gears MUST SPIN FREELY.

How to check the shimming:

1. Remove: piston, spring, spring guide, cylinder, cylinder head, tappet plate & spring, nozzle, and anti-reversal latch & spring. The only things you want in the gearbox are the bushings, gears, and the trigger switch assembly.

2. Close up the gearbox and put all the screws in. Do not attach the motor.

3. Reach inside the cylinder hole and spin the gears with your finger or a pick.

4. The gears should spin freely at least 3-4 cycles with bushings and 7-8 times with bearings.

If they do not budge or barely spin a cycle you have to re-shim the gearbox.
If they spin freely check the side-to-side slop on each gear. It should be no greater than .5 mm.

A properly shimmed gearbox will not eat up bushings, and will last 3-4 years with stock gears and a light spring (Stock spring, M100, a PDI120% or PDI150%.).

It can take anywhere from 15 minutes to 2 hours to get a gearbox perfect.

///ed///

[hepcatfox]

Thanks for the list biged. This helps a lot!

Right now, everything in my MP40 is stock parts. I tried your suggestion about testing the "spin" of the gears. With nothing in the mechbox but the three gears, shims, and bushings they are easy to turn, but will not spin freely (meaning they turn as long as I move one of them, but stop when I stop turning them). So it looks like I need new bushings and shims.

I'm also suspect of the battery (especially in light summvideo's problem) but the link you included is to 9.6v batteries. Should I move up to 9.6v or should I stick with an 8.4? If I do move up to a 9.6v are there other parts I need to upgrade?

Thanks!

[Ripper]

I've run a 9.6V 1700mah battery in my MP-40 since the day I got it. I fire it in 5-10 round 'squirts' and the gun is still flawless after 4 months of continous use.
I have yet to have to change a battery during an all day game.
The gun is still completly stock internally and I won't be opening the gearbox for any reason until something breaks. I've never quite understood some guys drive to immediately open up a brand new gun just to look inside. Nothing good ever seems to come from it.

[odinfish]

9.4v really? I've always heard that it puts a strain on the internals. I don't know much of course and my AGM MP40 is my first gun so I'm slowly picking up info from you guys as well as other sites/forums. I won't open up my gearbox either until something stops working or this thing hits a dirt pile to where I gotta give it an overall cleaning.

BTW - and I know this is kinda off topic within this thread, but what is everyone's take on skirmishing in the rain with an MP40 or other AEGs?

Mike

[Obergefreiter Raimund]

odinfish said:

BTW - and I know this is kinda off topic within this thread, but what is everyone's take on skirmishing in the rain with an MP40 or other AEGs?

Mike

After playing as many years as I have… I have never had a problem scrimmaging in the rain with an AEG.

The only things I would recommend for you to do when and after scrimmaging in the rain is: Do not change your batteries in the rain.
Also, after the game it is wise to dry off your gun, wet a cloth with some Remington gun oil and wash your gun with it. Also spray it down the front end of your barrel and swab your barrel. Be careful not to swab too deep where you might mess up your hop up unit. That is all.

[cairo1]

I am having very simular problems except for one not mentioned abnormalaty, here is the whole story.
i pulled the trigger expecting to fire one shot (set on semi auto) the piston came less then half way back and then stoped. so i took it apart and did what i did last time this happened i advanced one of the gears near the trigger rod. eventualy i got it all the way back and i compleatly cycled. i atemped to test fire it but the same thing happened so this time i decided to for go the one gear and went to the one the motar is attached to eventualy i got it to cycle and the whole thing happend again but now here is the abnormal part for some unknown reason the battery cables became hot when i pulled the trigger and became hotter with every second i held it down.

so I have just dis charged my battery and am now charging it as the low power may be the problem I then as a procautionary measure came here hopeing for some other support.

Could something be wrong mit my gears? i can manualy cycle them but the motar cannot shood i get a high torque motar or a new diffrent battery (i tried mu brothers mini for his MC-51 but their was no success), please respond

EDIT: the trigger requires extra force to pull after about a half pull i will inspect the trigger and fireing assembly

[biged]

If you have tried TWO fully charged batteries and have not been able to get the gun cycle you have either:

A. Have accidentally moved a shim and now the gears are being "pinched" inside the gearbox.
(Sometimes the shims fall off the gears when you open up the gearbox you may have put one on the wrong gear. You will have to re-shim the gears.)

B. Motor height needs to adjusted.
(This happens quiet often (about 1 in 5 guns I work on) and you may need to apply locktite blue on the threads of the screw to keep it from moving again.)


Before doing anything see if the wiring and trigger switch assembly is functioning.

Test the motor to ensure that it is functioning properly.

1. Disconnect the motor from the gearbox but leave the wires connected.
2. Connect a battery to the gearbox.
3. Hold onto the motor and pull the trigger.

If the motor does not spin you now have a wiring issue.
If it functions disconnect the battery and reconnect it to the gearbox.


Adjusting the motor height:

There is a motor adjusting screw under the bottom of the motor. You use an allen wrench to adjust how the motor pinion head meshes with the bevel gear in the gearbox.
If the height is TOO high. The motor will apply too much pressure on the side of the bevel gear and the gearbox will not function properly.

1. Unscrew (counter-clockwise) the motor adjusting screw a 1/2 turn.
2. Quickly pull the trigger. (Do not hold the trigger or you can fry the switch assembly.)

If the gearbox does not cycle, repeat step 1.
If the gearbox screeches after unscrewing the screw 4-7 complete turns you have problem A.
If it does cycle LISTEN to the pitch of the motor. It should sound "dull" and not whiney. You have to listen for the sweet-spot which is not too high and not too low.

///ed///

[speirs]

i have a different headache lol, i recently removed the fire selector switch and the part from inside the gear box which only allows the gearbox to fire in semi, after putting the gearbox back together, ok it fires on full auto fine but as soon as you let go of the trigger it does a half cycle in reverse making a noticable sound, i have had the g-box apart countless times now and have made sure the anti reversal latch is in place, its probably somthing really easy im forgettin to do when putting the g-box back together but atm im clueless, anyone got the similar problem or a salution?

[cairo1]

thx biged the motar was the problem she works fine now ;D

your instructions were quite helpful!

[biged]

Anytime, Cairo1... I'm glad I could help.

Speirs, Anti-reversal latches are picky & tricky.

Next you have the gearbox apart. Remove the Piston, spring, cylinder, cylinder head, and tappet plate.
Leave the gears in the box along with the anti-reversal latch. Spin the gears with your finger through the cylinder hole. Or reach inside with a pick to spin the bevel gear.

If you can spin the gears in both directions with the anti-reversal latch in place you may just need a new latch. Or the latch may not be installed correctly.

If you can only spin the gears one way so the latch is working properly. Then you have assembly problem. When you are re-assembling the gearbox the anti-reversal latch spring may be slipping out of the "tension" position. That's when you may need a second pair of hands during re-assembly.

Anti-reversal latch install:

1. Put the anti-reversal latch spring on the latch.
2. Put the latch & spring inside the gearbox.
3. Load the latch by moving the latch and tensions the spring - keep holding onto it.
4. Put in the bevel gear (won't be easy because your holding the latch) and let go of the latch which should now making contact with the ratchet teeth on the on the gear.
5. Rotate the bevel gear until the latch engages one of the ratchet teeth. It will usually click once.

TIP: Remove any and all grease from the anti-reversal latch, and the holes it is seated in
inside the gearbox. This will help during gearbox re-assembly.

///ed///

[speirs]

i done all you said bighed but still no luck :S all i can think is happening is that due to the fact there is only 4 ratchet teeth on the bevel gear i think that it must be falling inbetween the teeth and reversing untill it comes in contact with a tooth but i dunno this for sure, any more ideas because sooner or later my tapet plate is guna go for sure i know it

[biged]

How bad is the "backlash"? Is a complete reverse cycle? (Watch the nozzle with the gearbox out of the gun.)
(Another tell-tale sign is hearing the gun make the reverse stroke and having two bb's being fired the very next shot - double-feed.)

The anti-reversal latch might be "pinched" between both halves of the gearbox and not operating properly.
I would try another anti-reversal latch first, also order another tappet plate as a backup. If the problem still persists - you have to take a few thousandths off the face of both latch holes in the gearbox with a dremel.

Prometheus gear sets ($$$) have 5 teeth and have very little backlash. I recommend them for 500+ fps guns because of the price, but they are strong as hell.

///ed///

[speirs]

just put the anti reversal latch from my 74m in the gearbox and sure enough it worked fine, looks like i need a new latch, thanks for the help biged ;D

[biged]

Anytime Speirs.

///ed///