Going to "tackle" a unfamiliar gun (A&K Mk43) with unsure issue - Looking for help and advice

[ccyg8774]

The gun in question is a A&K Mk43 with the "brick" gearbox. I have some experiences on V2, V3, V7 (G&G, not TM) and RS T1, T3, but I don't have any experience on A&K Mk43 or M60VN or any similar LMGs.
The major issue with that gun is it fires at about 260 FPS.
The minor issues include the existance of the microswitch and the wiring to the box magazine being wrong. But I will worry about those later.
I did tried to take down the front end once to look at the hop-up chamber, which is known to have issues in Mk43, but can't really tell if there is a problem in the nozzle-rubber sealing by just looking. The only thing I got from that attempt is finding out that the hop-up rubber looks like an aftermarket one, probably a flat-hop.

My plan for the next few steps includes:

1. Swap to a spring with known FPS and chrono again, so I know how much is the FPS drop. (It has quick spring changing so this do not involves opening the gearbox... yet. )

2. Check if the piston is moving all the way forward. (if the travel of the piston is too long and the spring being too short).

3. Open the gearbox, shim if necessary.

4. Take out the piston, cylinder, and nozzle. Check if there is a leak between the piston head to the cylinder, the cylinder and the cylinder head, the cylinder head and the nozzle. Change piston head O-ring, the nozzle, or teflon wrap the cylinder head if necessary. If the problem is with the nozzle, I heard the nozzle for A&K MASADA will work as the length is close (is that true?), if not then I guess I will have to make one.

4.1. Check the strength of the tappet plate return spring. (Thanks to Windows)

5. Dissamble and reinstall the hop-up chamber, check if the rubber and nub being installed correctly, teflon wrap the rubber-barrel seal if necessary.

6. If all of that don't work... Guess I will do one of the following and hopefully improve the nozzle to rubber seal. (Seems to be the only place left.)
a) If I am using a custom made nozzle: Making a slightly longer nozzle.
b) If I am using a MASADA nozzle: I will file the tappet plate to make the nozzle extent more forward (I assume I can do that to the M60 gearbox like to a V2 or a V3? ), only a little bit each time.

7. (Only if it is working properly) I will deal with the electronic part.

What do you think? Any thoughts? Suggestions? Things I should pay attention or look closely when working on a Mk43? Do you think this is a good sequence to check (I'm trying to start from things that are easy to check and fix)? Any additional things you think I should check?

Thanks.

[Windows]

Everything seems like you know what your doing. You may also consider the strength of the tappet plate return spring. If it is unusually weak, the loading nozzle may seal under normal testing but leak when air pressure spikes during a shot.

Just an idea, good luck.

[ccyg8774]

Originally Posted by Windows

You may also consider the strength of the tappet plate return spring. If it is unusually weak, the loading nozzle may seal under normal testing but leak when air pressure spikes during a shot.

Good call, thanks.

[ccyg8774]

Update:
Checked the air seal between the piston head to the cylinder, it is horrible.
Later find out that the part of the piston head near the piston guide are being break off. No idea how that could happen, as it seems there is no way that can hit the piston guide. Had to replace that. So glad I have some spare parts.
Tried different combination of piston head and O-ring to get best air seal. Also telfon wraped the cylinder head. The airseal between the piston head and the air nozzle, which I was worried the most (as the previous owner told me that has a problem) is actually not bad, for a nozzle without O-ring. I won't bother to change that, but I will add some air-seal grease.
Will have to remove some wires to proceed further, but I left my soldering iron at work. Will get that tomorrow.

[Stealth]

Your methods are sound but you're not really solving the problems you're coming across (ie: using air seal grease? there's such a thing?...)
As a general rule, if anything is qualified by the phrase "actually not bad", then you should probably revisit it until it is "excellent" or "bulletproof" - you are after all diagnosing an airseal problem.

Did you check to see if your piston is binding against the piston rails?
SCs Top Tips - Piston Check - YouTube

[Danke]

This is what I"d suggest. Check the quick detach barrel to see if it's moving too far forward.

If it's construction is like the M249 then it can walk out of place and take the hopup chamber and lower barrel with it. Causing a massive gap between the mechbox and those parts.

[ccyg8774]

Originally Posted by Stealth

Your methods are sound but you're not really solving the problems you're coming across (ie: using air seal grease? there's such a thing?...)
As a general rule, if anything is qualified by the phrase "actually not bad", then you should probably revisit it until it is "excellent" or "bulletproof" - you are after all diagnosing an airseal problem.

Did you check to see if your piston is binding against the piston rails?
SCs Top Tips - Piston Check - YouTube

Sorry, I shouldn't use vague words like "not bad". I actually meant "no visible leaking after I replaced the piston head, but a O-ring nozzle probably could make it better". I do want to improve the seal further more, but this is a very weird nozzle, and there is hardly any aftermarket parts for that. (A&K masada nozzle may work on this, which I am still trying to find its exact length to make sure)

Making one from raw material is not easy for me either, as I don't have direct access to the machines. I can probably beg for an hour on a lathe (no CNC), but I am not even sure if I will be good enough to keep the tolerance down and make anything better than the stock one.

The grease I used was a vacuum grease, which is just a thick silicone grease. It is meant to lubricate and seal the glass joints in a vacuum setup. I found it can improve the air seal a little bit when used in tightly fitted moving parts (like the nozzle), and not too thick that prevents parts from moving. Probably a Mickey Mouse solution to you, but hey, "If it's stupid but works, it ain't stupid". And I don't have much choices when it comes to improve the nozzle seal on this gun anyway.

And thanks for the guide, very helpful.

[ccyg8774]

Originally Posted by Andres

As well, the springs will be weaker than their rating due to a longer gearbox if it's the same as the M60VN. I'm using a spring rated for 440~ and getting about 390 out of mine.

The Masada nozzle should work, I have one in mine as well.

Which brand of Masada nozzle are you using? I found the Apple Airsoft Masada nozzle is 30 mm which is too short. I am still trying to find out the length of the Wii Tech Masada nozzle.

[badrelko]

The Wii tech is also 30mm

I was taking mine apart and noticed this your post do I measured mine :P

Good luck

Sent from my Galaxy Nexus using Tapatalk 2

[ccyg8774]

Update:
Problem solved after replacing the piston. Fires at 380 FPS with a Element M95 spring. (which is meant to shoot at 370-390 FPS despite it saying M95)
Find out the gear set is actually SHS 13:1. Installed an SHS delayer to prevent feeding problem under high ROF.
Then added the MOSFET and switched to no bouncing microswitch, also rewired the main wire with 16AWG wire. The "AUTO" set of the box magazine do not like the MOSFET. The "Continum" works fine. Built a tail switch for the box magazine and wired the magazine so the switch will work with "AUTO".
All problem solved. Thank you guys for the help.

[ThunderCactus]

should have looked at this earlier lol
its a PGC mechbox
well known for having airseal issues, although it's usually hop chamber issues
And the BEST thing you can do for a box mag, is run it off a pressure switch directly to the battery. None of that audio switch or pressure feed bullshit.

[ccyg8774]

Originally Posted by ThunderCactus

should have looked at this earlier lol
its a PGC mechbox
well known for having airseal issues, although it's usually hop chamber issues
And the BEST thing you can do for a box mag, is run it off a pressure switch directly to the battery. None of that audio switch or pressure feed bullshit.

I heard the MK43 hop-up is troublesome, the good thing is the hop chamber of this gun was reworked by the gun doc of the previous owner before I get it. It works nicely so I didn't touch it.
The pressure switch, that is what I did for the box magazine, with a homemade pressure switch. It was wired in parallel with the main motor before, so the magazine motor will turn when the main motor turns, but that didn't work well.

Second last gun on the "to fix" list... Now fixed! Yeah!!!