m4 acting like battery dies
 

I'm working on a custom m4 for my brother in law which has been running for years. I did a few upgrades to it and when I put it back together it runs fine on semi but on full auto it'll do a short burst but when I try a long burst it dies off like the battery dies. Only the battery is fine and if I pull the trigger again in 2 seconds it shoots again and does the same thing.
Internally the gun has a modify cylinder, piston, nozzle, mask cylinder head and piston. I can't remember if I changed the gears or not but either way I know the shimming is great it was checked and double checked. The piston is moving forward and back freely and isn't jamming (i had to file it down so that it wouldn't). I tried changing the motor to see if that was the problem and it still did the same thing. I would consider the wiring cause it has joiners in it needed to take the stock off, but would it die off like that because of the wiring? Wouldn't it always just lack in power then?

Any suggestions please?!?!? Thanks

I'd say take a look at the trigger contacts. If that's not it, could be you're burning out the motor

Motor or battery.

Try an other battery (even if you don't think it's that), because your issue sure does sound like a dead battery.

If your motor smells when you burst, it's dead.

sorry should have stated I did try 3 different batteries all which I know are working great ranging from 9.9v life to 11.1v lipo and they all did the same thing. Also all three batteries were off of a fresh charge.

It makes no sense for it to be the motor cause I replaced the motor out of one of my guns that has a fairly new motor in it and it did the same thing.

I think next I will have to try rewiring and or changing the trigger contacts.

just tried replacing the trigger contacts and wiring and same results, so thats not the problem!

Could be a crimmped wire

Are you exceeding the C ratings on those batteries?

had the same problem, long ago, pretty sure it was a short. replaced the wiring and it was good to go. maybe a multimeter will help...

You said you checked the shimming. Did you make sure to check it with the cylinder out (so that you can spin the gears) and the gearbox screwed down?

sushicak and horto- I'll check again but I replaced all the wiring when I tried another trigger contact so I doubt this.

ThunderCactus- how do you tell if you exceed the c rating?

Sturak- yeah that's how I test my shimming

How strong a spring did you put in and what gear ratio are the gears. If you tried to push the stock setup too far the batteries are likely to small (low c) to maintain sustained burst fire.

I run 1500 mah 12.8v mad bull lifepo's that do 40c, but to get that power in a safer configuration (not lipo) the batteries need to be larger than the small lipo batteries everyone tries to fit in their stock tubes.

maybe ur run an extremely strong spring? :P

Going to have to assume your shimming is good...cause it's impossible to know via online/forum. Once you think it's set...then it's set and won't change except for one of the tests below.

Long and short of it is that one of two things are happening
1. When it's hot your motor is not providing enough force to crank your mechbox. Either because there's something less than optimal in your setup, your motor is dying/not scaled to your setup, etc...
2. Your battery cannot sustain the power required to force it's way through a cycle. Most any topped off battery can crank a couple of shots...but when after it's dumped a bit of power then you see it's behaviour.

When things are acting weird...it's always worth going back to basics:
1. In another known good gun/mechbox with a known good battery...try that motor.
2. With known good gun/motor...try that battery
- shoot both semi and short bursts
- if you're shooting at a rate of 2 shots/sec on semi there shouldn't be any drop off in trigger response for a long, long time. If that's good on semi...but when you put on a burst of FA and it dies off...then it's your battery.
* more often than not that will pin it down really quickly
* note...you risk breaking your mechbox shell if the front is unsupported...you can help that by putting the front of the shell against a solid table...or putting your finger over the nozzle to create a bit of an air cushion for the piston.
* see #3...you might want to do all this without putting the mech in the receiver since it's just simpler/less work if there are issues.
- if both work then the problem is likely with your switch/wiring/setup

3. Try the motor/battery/mechbox/grip outside of the receiver...weird/poor setups can bind in the receiver/grip and it's good to isolate those factors
- if it doesn't cycle well then...
...try a different grip (some are tight with some bushings and they'll bind the bevel gear bushings. The grip shouldn't be sqeezing the sides of the mechbox/bushings...a good fit is good, but it should slip on easily. The sides of the grip where they ride over the bushings can easily be relieved with a file/dremel.
...loosen up your shimming. Some mechboxes flex quite a bit and once it's under load it doesn't work as well as when there's no spring in it. I've run into some where I can rig it up with motor/batt/grip/no spring and run the gears....then pinch the sides of the shell hard enough to alter the "tune" of it. A good test to see if your stuff is binding under load is to just slightly back off the top mechbox screws (starting from the back). Not falling out loose, but just enough. On a lot of problem setups this will indicate that either the piston/shimming is binding under load...and or your bushings/gear axels are binding.
...consider reseating your bushings...or swapping them out. A cockeyed bushing will add a lot of drag to things...which just gets worse under load.
...check that your piston is completely free in the mechbox (take the sector gear and spring out). Screw up the mechbox. You should be able to freely push the piston all the way back by pushing a long allen key/rod/whatever down the nozzle. If it's binding either on the rails or the spring guide then swap parts (a non-bearing "plain" spring guide is often handy when troubleshooting)
...check that your piston can go fully over your spring/spring guide...place the spring guide on a solid table...put the spring over it...put the piston over that. Line it up and press down evenly. There shouldn't be any catching/binding/stuck-spots...it should go over nice and smooth. If it's hitting the bearings on the spring guide...swap the spring guide.
- set the mechbox up with everything but the spring guide, spring, piston, tappet plate and cut off lever. Setup the grip/motor/batt and run the mechbox...obviously it will not turn off without letting go of the trigger. Run it for a while...listening to changes in pitch or rubbing. Then take it apart and look for wear/scratches/worn spots. Areas to check for are the back side of the large flat gear rubbing the mechbox shell...or any of the gears hitting/rubbing the mechbox shell, reinforced sections, the post that the spring for the switch block hooks onto. It's also a good way to check if any of your gears are cockeyed enough to be rubbing.

4. You don't mention what spring you're using...if it's anything up to a M120 you should be able to simply test your setup with a known good large cell 8.4v/2000+mAh...9.6v/2000+...7.4v/2000+/20C (like an RC race pack). If it works with that but not with the battery you want to use (i.e. stock pipe/mini's that fit into a certain stock, etc...) then you know definitively that it's the battery.
- from there you can alter your setup for the battery you want...i.e. improve gear ratio, lower spring power, "plain" motor, etc...

That's just off the top of my head...would need more details of your setup.

motor height too high/low? how's it sound when it cycles?

Interesting ideas.

I wonder where the heat is causing the power drop though, since the Curie temperature for rare earth magnets is extraordinarily high (even neo magnets for example, which have among the lowest Curie temperature, don't start to lose magnetism until they're between 300-400 Celcius). For airsoft applications, these would be very high temperatures. Do you think it's in the wiring somewhere maybe?

Also, I like theory #2, mainly because I've seen so many weak-sauce batteries out there. One way to be sure is get a nice big R/C battery with a high C value, say, 7.4V 5000mAh and 40C, and hook it up to this gun. The problem though, is that it might mask another issue by powering through whatever real problem lies beneath.

edit: Just noticed you suggested the same thing. Shame on me for not reading your whole reply.

I have seen cheap neodymium magnets losing magnetic field around 200-ish C... especially in cheap Chinese brushed motors (in RC cars). Once the process is started, it just goes exponential (motor has less torque, but the load stays the same, so more current has to flow, resulting in more heat and less torque...).

To feel the grip getting hot, the motor core would be close to 200-ish...

Tsk, tsk...shame...LOL JK.

I've had more than a few motors over the years come across the bench that seemed ok, but just didn't handle as expected under load. (IMO)...once a motor is heated past it's operating thresholds it is weakend and needs to "work harder", requiring more power, than before to accomplish the same tasks. This further tasks and compounds issues with a marginal system. Simplest thing to do is to just pin it down to the motor and chuck it.

I've also had the odd motor that seemed to work sort of...but couldn't sustain long strings of shots (in multiple setups/batteries). I suspect that it had to do with a fault in the motor that worked when it was cooled and then failed as the wiring heated. Those would just stop dead though and not slow down as described.

Re. just putting a "BIG" battery to it...yes, I think that it would really only serve to mask other issues. Unless you're running a very powerful setup you shouldn't need more than a regular large battery to cycle a mechbox.

Re. wiring...yes certainly. Some setups have come across the bench with poor joints/splices that just starved off the motor. I think the record I've seen was one that had 7 splices in one wire...and only 2 were soldered!!! Kinks/nicks/pinches/shitty wiring will starve the system. Poorly attached connectors, poorly done splices and poor soldering jobs at the switch will do the same. Wire that is tired should be replaced...the wire is probably oxidized/corroded in it's sheathing. Connectors need to all be soldered...splices should be avoided...all wiring should be insulated...all solder joints should be able to withstand a tug just shy of hard enough to bend/break what it's attached to. Wiring should be a MINIMUM of 18AWG solid strand (not solid core, but it should not be braided) copper wire...or good quality high efficiency "silver" wire (warning...there's crappy "clone" silver wire out there). If find the silver wire to be a pain sometimes with soldering...and actually prefer copper wire. The thing "standard" wiring used in so many AEGs was ok (not nice, but ok) for old stock guns when you were pushing m90/100 stock springs with a 8.4v mini....that's not the stock builds now but the wiring hasn't changed. ACM type companies just shitty solder too and it's not uncommon to have wiring simply pull off the switch tabs once you're into the mech.

Wow, crazy. The exponential rise in temperature is really interesting, kind of like a vicious cycle. In a sufficiently-large R/C car I could imagine there being a few options for introducing a cooling system. I can't quite imagine what we would need to cool an airsoft motor in-situ. A grip designed hand-in-hand with a motor housing, with the housing having a dense array of fins. The question becomes.. where to dump that heat?

I have noticed fairly huge differences in heat output from different neo motors which pretty much have the same magnetic stickiness and torque when tested "in the lab". My JG Blue hardly produces any heat at all, while at the same time the SHS High Torque seems to get fairly warm -- but only the long version, where as the short version doesn't produce much heat.

I've gotten the impression, but haven't been able to solidly verify, that some motors seem to produce less heat if they've powered by a larger battery with a higher C value. I'd love to hear someone explain why the heck that would happen.

my EF1300 produces very little heat. Until I start laying down the hate, after half a box it feels like my grip is gonna melt lol
The impressive part is it doesn't lose any power at that temp

Because the battery and motor don't have to work as hard. Think about it like this, if you had to move a huge rock and had a 2 inch long stick as a lever (mini cell battery), you'd have to work REALLY hard to move it. If you had a 4 foot long stick as a lever (large cell battery) it wouldn't take any effort at all.

The power gain in large batteries vs mini batteries even in the same voltage is huge. Not to mention the internal resistance differences and probably a couple other differences.

Physics 101...

Electro-magnet coils produce a magnetic field proportional to the CURRENT flowing through and the number of turns. (Higher number of turns = higher resistance = lower current).

Current is measured instantly. That is why batteries are rated in mAh or milliamps-hours, the the amount of current that can be sustained for 1h before the pack is empty.

The rate (or C) of a battery is the maximum current it can give. For example, a 1400mAh NiMh 2/3A battery (mini packs) can deliver around 10C, so roughly 14A at MAX.

A better battery for example a 4500mAh NiMh Sub-C (large packs) can deliver around 10C also (limited by the chemistry), roughly 45A at MAX.

Given both packs are 8.4v, the same mechbox and motor, the magnetic field of the motor running on the larger pack will be more than 4 times stronger. That directly results in more TORQUE.

The problem when a motor struggle to accelerate is that it will ask for that MAX current for a longer period of time. The curve of required current vs. speed is logarithmic (starts at +infinity, dropping VERY fast to almost nothing), so if you prevent the motor from accelerating by starving it, it will generate more heat for a longer period (we talk milliseconds, but the "cycle" only lasts a few hundread milliseconds anyways).

So the larger the battery, the better. Contrary to what youtube seems to say, running your AEG on a car battery won't break it any faster than running it on a 11.1v LiPo. In fact, the car battery will put a lot less stress on your motor.

Thanks for the awesome explanation. This backs up many observations I've made over the last year but couldn't explain with my weak knowledge of the physics involved.

For what it's worth, I've got a ZIPPY brand battery coming in from Hobby King this week that's 7.4V, 6000mAh and 35C. Should push everything in my stable pretty effortlessly, though it's fairly certain none of my motors will be asking for anything close to 210A :)

Man some very good, detailed answers here...

Short and simple dude, your battery and/or motor cant handle your current "custom" set up. Motor and Gears cant handle the spring. If shimming was too tight, it wouldnt shoot semi very well either, and you said it works fine with that. Not to mention it would sound terrible and be putting even more stress on your motor.

Well said. Fix up that shimming and follow my golden mantra: MORE POWER

:D

Yup, you should always use the biggest damn battery you can stuff into your gun. If your gun only fits minis you may want to think about using a PEQ/battery box or battery bag with a large. Even an 8.4v large is a ton of power that should run almost anything up to 450-500fps.

So I got a large battery and it seems to be working great now. I'd like to see him game it and then say but we got rained out here this weekend. Thanks for the great info everyone.

Yeah! If it doesnt fit, make it fit :D

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Tamiya connector? Heresy!

Old pic, its Deans now of course.

Deans connector? Heresy!