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Unidentified noise

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This is a new one.

So. It started as an ICS M4. New m120 spring made it better, but the motor struggled. Added a G&P 120 motor. That fixed that but the fire rate was not the best. Switched to an 11.1v lipo. Worked well but worried about the contacts. changed out all the wireing, installed a firestorm mosfet. That blew the fuse. Unjammed the gearbox. Worked fine for about 5 seconds then it ripped the back off the piston. Installed a new piston....

And now this. It shoots wonderfully. Nice trigger response, nice ROF, great power. And yet.

It twangs! When you take your finger off the trigger it sounds like a tuning fork!

You can hear it rather clearly toward the end of the video. Any ideas? Did I leave something loose in there? Anyone else had this?

I love this gun, but it has been a problem child!

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you only hear it when you dry fire

and yes 1 of my guns has a slight "twang" on dry fire

(and I did install a bearing spring guide to reduce spring twist)


your piston snapped off at back due to not doing AoE on piston

you need a little homework before your faster gun stops breaking quickly

that I can confirm for 101% deffo fact as I broke 2 pistons before I realised


"Oh is that why they go on about Angle of Engagement"


m120 spring can put you over - well it will be well over the 350 limit - 400fps unless seals are really pi$$ poor - dire in fact


on a full stroke piston when going past say 25rps she can start to double cycle on m100 spring

going past 30rps on m100 you get Pre Engagement until you short stroke


depending on what your gun shoots at and with a m120 you have a bit extra before double cycle & PE starts to happen

next time you have her in bits - once you know what is what fps wise


then look into shortstroking a tooth or two

each tooth will reduce fps by about 6.6% or multiply the current fps by 0.94 = 1 tooth

then if too high at 400 do the maths again on 376 x 0.94 = 353


also if you have done AoE this will bring down fps by about 15fps aprox

but you claw this back by fitting a bearing spring guide that compresses the spring a smidge more


Trust me - I've broke LOADS trying to go too nutz too quickly without a bit of extra work inside

not expensive or difficult but leanrt the hard way that you can't just drop in beefy motors and 11.1v


in fact I don't use 11.1v - just 7.4v's and still get 30+ on a 12 or 13:1 gearset & m120 SS 2 tooth

that is more than plenty - going beyond 30 is silly, wears quicker and other crap like feed issues on some mags

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It does indeed have a bearing spring guide. Now also a piston with all metal teeth.


Interesting on the AOE Stuff. That might be tricky to look into with the split gearbox....

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If your AOE is out and you're running an 11.1v lipo on full metal teeth, your gearbox is in for a bad time. Previously the piston stripped because it was a weak point (which in a piston can be a good thing), but now that it's been strengthened you will quite likely be looking at something else breaking.

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not tweaked any of my ics's yet


but gotta be do-able, the sorbo/neoprene just goes on the cylinder head to push the piston back a smidge

the piston removal of 2nd tooth and reduced 3rd tooth - all do-able without open main lower gearbox

just the upper gearbox you need to work on, check the postion of pick up tooth and just clearing reduced 3rd tooth ok


pretty straight forward - ensure you got great seal on piston to cylinder - or replace o-ring

polish cylinder with siicone cloth, leave good fitting o-ring in a some silicone overnight - cheers Ian & others for tips

bit of silicone grease assembling top box back together - with a few bits of lithium/teflon grease on piston's teeth

chuck it all back together - job done without removing whole gearbox


Reason why AoE is important is coz without it

the sector picks up the initial pick tooth so very early to miss 2nd gear it actually forces piston upwards as it pulls it back

it can't go upwards coz the guides/runners so though this resistance is ok-ish on stock speed guns

when you start to push it faster it WILL snap off very quickly as it fires faster n faster


metal rack has not solved the problem like was said so you should deffo look into this and consider doing it

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This is really helpful stuff guys, thanks.

So here's the AOE as is.


I take your point.

So If i've understood correctly, I want it to be like this.

Which I measure to be 5.5mm further out.

So I need to add a 5mm spacer to the other end of the piston, And probably grind off just a fraction from that second tooth to ensure it clears. Is that right?

Other thing, obviously this will shorten the operating range for the spring (by 5mm). Will this cause any problems?

Thanks again.

By the by, I've been looking at this http://www.airsoftforum.com/board/topic/221337-correcting-angle-of-engagement-necro-exempt/ . I'm assuming thats what you are talking about?

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