New gnu double-feeding

[RostokMcSpoons]

My Classic Army Scarab has really impressed me so far, but I have found a couple of issues now, after firing a few hundred BB's through it.  
They may be related!

1) My Lonex flash mags were a sloppy fit in the mag well, able to rock backwards and forwards (and also a little up-and-down too).   The supplied mag (a Nuprol high-cap, not the Classic Army one it should've been) is a better fit

 

2) Firing on semi-auto, and also occasionally when burst firing full-auto, the gun will shoot two BBs at 50% power, then go back to feeding properly.  It doesn't seem to skip firing a BB at any point.

 

3) The motor is getting pretty warm in the pistol grip.   I've wound the height adjuster screw off half a turn, it seems a little better but maybe that's psychology and I'm just not running it as hard!  I'll experiment further with that.   The gun sounds nice (to my untutored ears) when fired.

 

I found that pushing the mag forwards in the well seemed to improve the feeding greatly, so I've now shimmed the rear of the mag well using one of those little adhesive cupboard "stop 'em slamming" nubs that Ikea always provide.   I filed that down to give a good snug fit for the Lonex mags (snug enough that the mag doesn't drop free, and so snug the Nuprol barely fits in).   

 

I've tried both 11.1 and 7.4v batteries, same results.   Both are rated 25C or below, as per a warning I read not to go to 30C or above.   
I've not chrono'd the gun but I'm guessing its running at 16-18rps on 7.4 and 20-25 on 11.1.   According to the listing it's at 335fps on 0.20's.

I'm using decent BBs which don't give me problems in anything else.   The Nuprol and Lonex mags are both in good nick, relatively new, and giving plenty of feeding pressure.

 

The gun itself is in as-new condition.  The metal hop unit looks great, the barrel is clean, and I can see the rubber impinging nicely as the hop is wound on.   I've not dissembled it to examine the rubber closely, but there's no sign of Tatty Lip Syndrome (tm).

Any ideas on possible fixes?

Edited April 4, 2023 by RostokMcSpoons

[mightyjebus]

before you end up going down a rabbit hole try different BB's and then a different mag if it's still the same. 

[RostokMcSpoons]

I think I need to go down the rabbit hole ... somewhat.    Because they're the BBs I can get at my local site, and the Lonex flash mags are the only ones I want to run - so I need to get the gnu running optimally with that combination, or I'm not in as good a place as I could / should be.

 

To be honest, even with the gun as it is now, I would run it like that - the full auto is great.   The double shots will be an annoyance where full auto isn't allowed though -  a good chance I'll miss a kill or give my position away

 

I suspect the main thing I need to investigate more is the location of the magazine within the mag well.   I may have shimmed it too much... I don't know.
The next stop will be looking at the hop rubber... but then we're onto stuff like swapping the nozzle, or adjusting the tappet plate.   Opening the gearbox is something I'd rather not do, if it can be helped.

 

Edited April 4, 2023 by RostokMcSpoons

[ak2m4]

do you only get the double-shot with 11.1?  overspin.  check the cut-off of course but it's most probably too weak a spring for the motor/battery combination.  Short-stroke with higher spring and/or 22tpa motor can often fix, or just run with 7.4v

[RostokMcSpoons]

Nope, that was my first thought because I had to swapped from the supplied Big Foot 7.4v 1300mAH 15C across to a Turnigy 11.1v 1200mAH 15-25C.
So I swapped back - the same thing.  No obvious difference in how often it happens either.    Maybe I'll do a more 'scientific' test of that at lunchtime.

[mightyjebus]

down the rabbit hole we go....

 

ok so the problem is simply 2 BB's are getting into the top of the hop unit when only 1 BB should get there.

 

things to check:

1. is the nozzle binding on the cylinder head? rubber O ring nozzles without any lub can do this. It doesn't need to get stuck only slowed down for this to happen.

2. the the tappet plate spring weak? it might be borderline and occassionally not have enough energy to bring the tappet plate forward fast enough allowing 2 BB's to enter the top of the hop.

3. is the tappet plate lub'd inside the gearbox shell or is it lub'd but binding (turn all the gearbox screws by 1 flat to see if the issue clears)

4. not sure if the AEG has a mosfet so there is a chance the nozzle gets into a position after firing that there is enough time for 2 BB's to move into the top of the hop before the gearbox cycles.

[RostokMcSpoons]

Thanks that's a good checklist for me to work through...

 

It does have a MOSFET.  I dry fired the gun with the upper removed, and because the CA guns use to have an EBB system there's a big slot in the top of the gearbox so it's easy to see the position of the piston... It did seem to do a good job of completing the cycle when in semi auto, but I could occasionally catch it out and it'd be pre-cocked in full auto.  So there's no active brake or pre-cock features working, as far as I can tell.