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My gearbox is giving me all sorts of problems ever since I upgraded it. I have no idea why as the only upgrades I last did were a new gear set and mosfet. My problem deals with fps though.



First the cylinder to piston and cylinder to cylinder head air seal was terrible but that is mostly fixed and I changed the spring too but it is only shooting 360 fps and the spring is an m120 so it should be shooting hotter than 360 fps.


When I test the gearbox without the barrel, the air is powerful. When I pressed the hopup to the gearbox and turn it upside down to do the air nozzle to bucking seal test, the air does blow out into the mag well but I have to hold the barrel just right, other wise it seals fine.


After the test I sanded the tappet plate to fix the problem, but instead I got the same exact fps AND now the bbs feed horribly as if the air nozzle is too far forward, blocking the bbs.


I only have $30 to spend on amazon (because it is a gift card) on upgrade parts.


All help is appreciated. Thanks.

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What type of cylinder do you have? It's plausable that there is an under voluming issue. You could have put the cylinder back wrong when you were fixing the seals so the port was much closer than it should be. Also Maybe you could get a sector gear chip so that the gun feeds properly again. They're normally around $5.

Edited by MrGearbox

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Madbull 3-in-1 hopup

Madbull m120


Unknown bucking

ZCI 6.02mm steel tightbore

ZCI aluminum cylinder head (teflon taped and sorbo)

ZCI aluminum piston head

ZCI air nozzle with o-ring

Modify type 1 cylinder (installed correctly)

Unknown tappet plate
APS Hybrid Gearbox shell
If the problem is the bucking could you guide me towards one without the mound so I don't have to remove it for the IR-Hop.
Edited by Hunter74

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And when the details comes out...the answer is easily attainable.


Sadly, I think you assumed that you can just thrown any parts together and have it all work in harmony like Video Game gun upgrades. Well this is reality and with that...things do not always work well together. Many things are not going to be pretty without a lot of fine tuning.


IR Hop will knock your FPS down a bit due to the larger contact patch on the bb. Losing 20fps with an IR hop does happen.


3in1 Hop-up by MB is a bit odd in dimensions...I have heard from others it does not IR hop easily. With known characteristics of the 3i1 it is a bit tight and ideal position is off on most AEG's. Position shimming is required. To far forward and you get mag well blow by.


I would tinker with hop-up positioning first and then hop-up tweeking to get your AEG to about 380fps.

Edited by Guges Mk3

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The MB Hop-up is actually just hard to m-nub, which I haven't done yet. The thing is it was shooting around 390 fps before I upgraded the cylinder and spring. The barrel assembly was exactly how it is now before I the previous 2 upgrades. I got even better compression with the upgrades and the spring felt much stiffer.


But, I did switch to the APS gearbox shell after these upgrades because I liked the QC spring guide feature and it had more space for shimming my bevel as it was pressing on my pinion more than it should, which made a horrible squelch. Also the magic box 5.9 mm bushings were too lose in the shell and gluing them could have made the gears lock up.


I am looking to get a new shell anyways. The only one that looked decent enough within my price range on amazon was this one:



I can't be positive on the quality as I have found no reviews. As long as it isn't a tight fit around the bevel I think I might get it.


Also I will try the barrel shimming method. Thanks.

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