Cm030 Double Feeding

[pLaGuEd]

Hi guys and gals, new to the forum and mostly new to airsoft (after a 20 year hiatus) but I'm about to get back in with my son and I've been given a "broken" cyma cm030 to see if it's any use to me.

Now I don't know the background to this gun but it looks almost new, so there's a possibility that this issue has been from new.

The gun seems to work well all except it always double feeds, not misfeeds, even the first shot with a fresh clip will fire two bbs.

I've stripped it down and can't see any obvious issues. The hop up seems to be functioning correctly, and setting it to max does not resolve the double feed.

The Bucking looks normal and is providing a small amount of resistance (I did try a small layer of ptfe around the outside edge of the bucking put some more pressure on the entrance but that caused the bb to practically flop out the end rather than fire.. so too much pressure).

I've tried 3 brands of BB all with the same issue.

Voltages seem good but I don't have another battery to try ..... I could rig up a bench power supply to test with?

I've read that gearbox timing is reset each round so shouldn't be an issue, as my initial thought was it almost feels like the feeding mechanism is withdrawn for too long or something, but I would expect the bucking to stop the first round irrespective of how long the feeding mechanism is open.

The last avenue I was looking at is I've seen people blame the hop up housing allowing the bbs to flow in too easily and drilling a slight indent to make it harder for the following bb to push it round the corner.

So, before I get the drill out, does anyone have any suggestions of things I may have missed.

[Dusix]

Hi pLaGuEd.

I had the same problem with pistol asvel.

For my toy was broken anti reversal latch spring. I just unwinded one coil, made similar like hook that end and problem for me solved so far 😉. Maybe the same with your pistol gearbox.

[pLaGuEd]

Hmmm interesting. I'll have a listen and see if I can hear anything after firing. However did that cause the first shot to double feed? I could understand that impacting multiple shots, but my pistol is 100% of the time firing 2 bbs

[Dusix]

Story of my cm030.

Bought second hand, changed battery to lipo type. Played a lot. Pistol started double feed. One of marshals advised to check ARL spring. I checked, he was right. 

When ARL spring snapped it started double shot, first bb goes in normal fps second at low fps. By sound it looks like gearbox fine but ARL not doing its job and let gearbox double feed. 

I know that its PITA to pull out gearbox from pistol but its your choice  of what to do with it.

[pLaGuEd]

gearbox looks fine, while I can't see the actual ALR spring as it's on the back of the arm it's definitely sprung and engaging, and I don't hear any wurr as it winds backwards a bit after a shot

I'm at a bit of a loss, without actually knowing how much pressure the bucking should resist it's hard to tell if it's really faulty, but presumably it should be stronger than the spring in the clip meaning it shouldn't be possible for the clip to force two bbs into the barrel.

so my current thinking is that this is either -

The bucking is too loose and needs replacing, the clip is able to force one bb through the bucking up to the hop up and another behind it

The air nozzle is retracting back too far allowing 2 bbs to enter between the bucking and the air nozzle

 

[Dusix]

AEP gearbox has cut off leaver, if its worn or gearbox badly shimed can not engage and gearbox shooting couple  or more of bbs in one pull of trigger. Take off slide, black cover from top of the gearbox and check is it lifts up trigger leaver. It's at top of gearbox white plastic small piece.

https://eagle6.co.uk/shop/catalog/product/view/id/6172/s/tokyo-marui-g18c-cut-off-lever-spring-for-aep-series/

This part is next to trigger contacts, in the centre of picture.

Hope you will find what's causing double feeding.

[pLaGuEd]

Thanks, but there doesn't seem to be an issue with the cutoff lever. Semi and full auto work as expected and it looks fine. 

I have made some progress though. I manually moved the air nozzle back and marked where there was good clearance to load a bb. I then did a slowmo video of it firing where I could see it moving a good few mm beyond my mark. At risk of breaking an already broken gun I tried adding in 4mm of rubber behind the air nozzel to prevent it going as far back as it does. I almost expected something to snap as presumably there no give in the gearbox??? Anyway this "fixes" the issue and the gun fired perfectly in both semi and full auto. I've seen people shim the lever to make the air nozzel slide back more, but I've never heard of someone having to grind off a bit?

While blocking the nozzle from sliding all the way works, I'm fairly sure that it's not good for the gun, but I'm baffled as to how this can even be an issue, but other than opening the gearbox and grinding a tiny bit off the arm, I can't think of an alternative. I can't see any moulding/manufacturing issues on the slide the pivot or the arm. 

[pLaGuEd]

OK, I went for it. No idea if there was another simple explanation, but I split the gearbox, pulled out the lever and filed it down. I do have to admit I filed it too far and using the slowmo video again I can see it about 1mm short of the mark I was aiming for, but it now loads a single round and fires 100% in semi auto and misses about 1 round in full auto.

I'm going to shim it back to where I want it, but irrespective of what the real cause of the issue was, this has fixed it. 

[Dusix]

Good to hear that you found what cause problem. You can try JB weld to shim "tappet plate" it should hold well. 

About problems with tappet plate I read that they can crack but never read that it can be too long.