TM M4 gearbox issues

[gkagan]

Hey everyone.

I've got a TM M4 that I got second hand. I was wondering if anyone knew what to put in the mechbox to make it run at the equivalent of a new classic army M15A4 gearbox, that is to say, ~350 to 400 fps at ~15 rounds per second. My boss just got the M15A4 and his does a nice 401 at 15.54 rps on an 8.4v.

It has had some work done on it, and my understanding is that it's got an old systema M120S with polycarb piston, steel piston head, standard speed steel gears, yada yada, but it does 395 fps at FIVE ROUNDS PER SECOND. WITH A 9.6 VOLT. I do not think this is normal, and I would like to be able to empty my 30 round mags in 2 seconds.

Basically, I'm asking what's in a newer stock CA gearbox. Is the motor different?

Thanks all.

[Styrak]

There's something very wrong with your gun if you're getting 5 rounds per second. What did you measure it with?

[dpvu]

A STOCK CA would not be able to do 401 FPS at 15rps. It obviously has some upgrades as well (although some CAs are coming upgraded from the factory).

Make sure your battery has a good enough discharge rate and is fully charged. Even 9.6V mini batteries have issues pulling heavier springs and a M120S is decently heavy.

Part of the problem as you suggested may be the motor as well but I suspect your friend is running an 8.4V LARGE and you are trying to use a 9.6V mini or your battery is just on it's last legs.

[Crunchmeister]

A stock CA is usually in the 310-320 fps range, so it's quite likely that gun was upgraded.

As for your gun, I'd suspect either one of a few things. The first thing I'd check is the battery. An M120 spring is pretty hard to pull. You have to have a good battery that can supply enough current. If you're using a cheap battery pack or a nunchuck style AA pack, you may have trouble turning that spring over. What type of battery is it? That info would be of great help.

Motor height could also be an issue. If the motor is too high then it'll slow down your mechbox significantly. If you can confirm the battery is good, then try lower the motor height a bit. A motor that's too high will impede the motor from moving at full speed.

If those things don't fix it, then I'd suspect it was shimmed too tightly and would need to be opened up and reshimmed.

[ReSpEcT]

From my understanding you can upgrade your gun to 400FPS without changing the motor, so you can keep your current TM motor. I believe stock TM's shoot at around 280FPS.

You would need a new battery, if your current battery is a MINI type and probably stored in the handguard then you will need to purchase a battery pack with C type cells (larger) in order to supply the amount of current being requested from your motor. The motor will draw more current in order to pull back the stronger spring, that is why a larger battery pack is required. The C cell type battery pack will be too large to house in the handguard so you can either house it externally or re-wire the mechbox to the rear of the gun and place it in the stock.

Once you have purchased the new battery then you can try upgrading the mechbox internals (gears, spring, bushing, piston, etc.).

There are other ways to improve your FPS without modifying your mechbox. You can try upgrading your inner barrel to a tighter one (6.04mm) which may increase FPS by about 0-10. Also try adjusting your hopup, to make your BB's travel farther. These upgrades do not guarantee an FPS boost as every gun will have different results. Good luck and read some stickies. Just be cautious of the increased rate of wear on the gun, like the shell of mechbox, motor, etc.

[kalnaren]

Originally Posted by ReSpEcT

From my understanding you can upgrade your gun to 400FPS without changing the motor, so you can keep your current TM motor. I believe stock TM's shoot at around 280FPS.

You would need a new battery, if your current battery is a MINI type and probably stored in the handguard then you will need to purchase a battery pack with C type cells (larger) in order to supply the amount of current being requested from your motor. The motor will draw more current in order to pull back the stronger spring, that is why a larger battery pack is required. The C cell type battery pack will be too large to house in the handguard so you can either house it externally or re-wire the mechbox to the rear of the gun and place it in the stock.

Once you have purchased the new battery then you can try upgrading the mechbox internals (gears, spring, bushing, piston, etc.).

There are other ways to improve your FPS without modifying your mechbox. You can try upgrading your inner barrel to a tighter one (6.04mm) which may increase FPS by about 0-10. Also try adjusting your hopup, to make your BB's travel farther. These upgrades do not guarantee an FPS boost as every gun will have different results. Good luck and read some stickies. Just be cautious of the increased rate of wear on the gun, like the shell of mechbox, motor, etc.

I don't know anyone who uses "C" cells in thier gun. Large batteries are usually Sub-C cells, and a smaller battery using 2/3A cells will crank and upgraded mechbox fine, assuming the wiring and shim job are good. I run a 370 FPS setup using 1500mah mini bats.

Hop-up also won't do a thing to increase FPS.. it will decrease it if anything.

Regardless, the OP's issue was ROF, not FPS.

I vote along the lines of what Crunch said.

[coach]

Originally Posted by kalnaren

I don't know anyone who uses "C" cells in thier gun. Large batteries are usually Sub-C cells, and a smaller battery using 2/3A cells will crank and upgraded mechbox fine, assuming the wiring and shim job are good. I run a 370 FPS setup using 1500mah mini bats.

agreed.

I have my M4 with a PDI 150% installed and a 9.6v mini (2/3A) 1500mah pulls it just fine. Roughly 380fps at last chrony test. My drum mag can't keep up sometimes. I also have no issues pulling the same spring with 8.4v sub-c's all day long with either a 3300mah or 1400mah

[gkagan]

Thanks everybody.

I measured the fps and rof with an xcortech chrony. As for my boss' CA M15A4, I believe you're right when you say it came up graded from the factory. What I know is that he certainly didn't upgrade it, and just got it in the mail from airsoft atlanta a couple of days ago.

My understanding of batteries is that the voltage mainly affects ROF, and amperage the battery life. For the record, it's a 9.6V nunchuck at 1950 mAh. Of course I'm open to suggestions on either a) a new battery, or b) a completely different type of battery.

I have tried playing with the motor height, and I see no improvement. I will open the thing to see if someone shimmed it like a doof.

It is fitted with a systema tightbore and a metal hopup, which I have adjusted.

Does anyone have a feel for what gearset belongs in this thing?

[Crunchmeister]

I'd suspect the battery isn't strong enough. Those AA size nunchuck packs don't deliver a lot of current. But it could also be the shimming, as one of those nunchucks (2000 mAh Intellect pack) could turn over my 395 fps SAW for a few hundred rounds before slowing down. The easiest would be to try another battery to see if that makes a difference. Then you'd know if it's the battery or shimming without having to tear into your mechbox.

[kalnaren]

I'd check the cell type too. Some cells, despite higher Ma, aren't designed for high current drain.

Higher Mah will also effect ROF simply because it can handle a higher current drain than lower Mah cells (assuming the cell is designed for higher drain)

[gkagan]

Today I tried a few different batteries with more or less the same results. I decided enough was enough and tore the thing down. The culprit: gears and cylinder swimming in old dry white lithium grease.

Thanks for all your help, guys.

[NoGear]

clean that stuff off/ re lube ur gears and shims. check and relube the piston etc and ur rof should be normal if not higher don't forget to lube some of the inserts so it runs smoother. www.mechbox.com do it there way and u should be fine

[dpvu]

Although voltage is the main factor in determining ROF between 2 similar batteries of the same size, mini batteries even of higher voltage cannot match the discharge rate of large batteries. For example, 9.6V mini-nunchuck batteries use the same type of AA cells you see everywhere and have extremely poor discharge rates. They have a hard time pulling tougher springs at a good ROF.