m4 acting like battery dies

[Visser]

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

[suzenonest]

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

[Kos-Mos]

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.

[Visser]

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.

[Visser]

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

[sushicake]

Could be a crimmped wire

[ThunderCactus]

Are you exceeding the C ratings on those batteries?

[horto]

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...

[Styrak]

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?

[Visser]

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

[MADDOG]

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.

[phloudernow]

maybe ur run an extremely strong spring? :P

[m102404]

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.

[lurkingknight]

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

[MaciekA]

Originally Posted by m102404

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.

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.