High RPS VFC M4

[Governor]

Hi,

 

I opened up my VFC box after two years of use, and sadly the spur gear's shaft snapped along with all of the bushings grinding nearly all the way through the gearbox shells - I decided it was about time to get a new gearbox in there and had a few extra (new) parts lying around. So... I've put together a nice little gearbox with:

 

- King Arms 7mm shell w/ bearings

- ASCU v2 Gen3+

- SHS full steel rack piston (polymer body)

- SHS Piston Head

- SHS cylinder head

- SHS tappet plate

- SHS air nozzle

- SHS 13:1 high speed gears

- VFC bearing spring guide

- Standard VFC spring (M100/M110?)

 

I put it together in the M4 with:

- SHS High Torque motor

- 6.02 TBB

- ProWin Hop Up

- Prometheus Purple Bucking

 

When using an 8.4v battery, the gun fires fine (slow than I would like, but fine). However, when using a fully charged 9.6, and a fully charged 11.1 the gun double feeds every couple of shots on semi, and a hell of a lot on full auto. Any ideas?

 

So far I've tried it with the stock VFC barrel/chamber/bucking along with a G&G Unit/bucking with an King Arms barrel but they both still gave the same results as the ProWin/Promy setup. I have also tried the gearbox in a King Arms upper and lower, with the same results...

 

Could it be something like needing a sector delayer chip? New tappet plate? Longer air nozzle?

 

Cheers for the help.

Albert

 

[cyrexx]

Sounds like your getting over spin, usually happens with weak springs on high speed setups. Whats the FPS?

[Sitting Duck]

my guess is you have used a regular spring - m100-ish

and are approaching aprox 25rps so she is overunning - double firing

(or say 20rps on a weak/used spring of about m90 strength)

 

This is quite normal - gun is running faster and failing to slow/stop very quickly

 

your options are

 

1- stay on lower volts - easy one that

 

2 - if using a mosfet then use a mosfet with active braking as this reverses the polarity for a split second as trigger is released

thus slowing the motor very very quickly - but can make motor run warmer due to the slight increase in stress

 

3 - on higher speed guns "Short Stroking" is performed

This means fitting a heavier spring - say m120 which means the fps would be too high

but to compensate for this some of the sector gear's teeth (at beginnng) ar removed

So the higher spring is only pulled back say 85% of the way to obtain a more moderate fps

The tension of the m120 slows the faster running gun very quickly and over spin is greatly reduced

 

This is the way many mofo's setup higher speed guns...

you barrel length needs to be a moderate length to still achieve the correct air volume ratio

but unless you are running a 400+ barrel you should be able to short stroke or SS 2 or 3 teeth on a m120 spring

(piston must be say 3 or 7 metal teeth as the last tooth used must be metal on release)

 

get gun chrono'd at tell us the fps and rps

delayer is only for feeding issues at higher speeds - don't stop over spin

[Rock-climby-Dave]

Shouldn't overspin with an ASCU in it unless you've turned the active brake off

Would probably say short stroking is the way forward. You don't need to delay the tappet, you're already getting too many BBs as is

 

What is power like? is the right nozzle fitted?

Is power level stable (within 10fps shot to shot), and the same as on semi, when in full auto?

[Governor]

I don't think it's over-spinning due to the active break of the ASCU (I've also put the ASCU into test mode to ensure the cut-off switch is working correctly).

 

I'll pick up a friend's Chrono tonight and get the FPS and RPS on it. Using the cheap RPS detector (audacity) I got the figure of 27 RPS with an 11.1 - that was, however, with a cheapo DBoys motor.

[Sitting Duck]

FFS - test the gun on 7.4v first !!!!!

 

13:1 gearset + shs torque all fetted/deans/16awg etc....

you should be hitting about 25rps aprox on 7.4v 25c with a shs torque

 

NOW if you shove in 11.1v with shs motor you should hit another 50% nearly

35 to 37rps - before you get all excited and stuff a warning.......

 

m100 spring guns:

as you get to 25rps guns start to over run a bit more and close to double fire on non ab fets

even if AB kicks in to stop this over run - DO NOT THINK you can continue to push it more safely

as gun starts to fire at 30rps the risk of Pre Engagement greatly increases

More dangerous is when you have a full metal piston in there and not ultra light either

(lighter pistons release/return a little quicker and help to avoid PE but don't mean you can keep going nutz)

 

So if you connect up a beefy or even moderate 11.1v lipo on a m100 UK gun and pull trigger.....

You may - well quite likely hit PE at 35rps

plus as this takes place - a plastic toothed piston will usually just strip to bits and limit further failures

However - on a full metal rack piston there is no weak point - solid metal motor pinion to gearset to metal tooth piston

This can result in a bigger crunch and numerous metal teeth smashing f*ck out of each other inside

 

The Short Stroke setup is required for normal 350fps guns trying to shoot at 30rps+

shorter piston stroke and stronger spring returning a light piston means the piston avoids getting smashed at high speeds

though to be fair at 30rps you will probably need a delay clip if feeding becomes a problem (likely depending on mags used)

 

Many of us have learnt the hard way when pushing guns too fast

please check stuff first on 7.4v like I said and maybe stay at a steady 25rps aprox unless you do your homework

 

going too nutz may result in.......

 

[Governor]

Ok, so with .25 BBs it's shooting with a mean FPS of 289.8 (min was 275 and max was 315).

 

WIth a 7.4v it fired at 23.4rps and with an 11.1v it fired at 30.7rps.

 

When testing, it still double-fed on the 7.4v, but not as often as with the 11.1v.

[Rock-climby-Dave]

that's a huge variation in fps. Should be (with a mean of 290) 285-295fps

make sure AOE is right and the second tooth is removed

 

Make sure the tappet is moving freely and the return spring is good enough.

 

and by double feeding, you mean that for one cycle of the gearbox (one audible pop sound) two BBs exit the barrel or one pull on semi cycles the gearbox twice, becuase people still seem to be convinced it's an overspin issue (although I doubt it personally.)

[Governor]

that's a huge variation in fps. Should be (with a mean of 290) 285-295fps

make sure AOE is right and the second tooth is removed

 

Make sure the tappet is moving freely and the return spring is good enough.

 

and by double feeding, you mean that for one cycle of the gearbox (one audible pop sound) two BBs exit the barrel or one pull on semi cycles the gearbox twice, becuase people still seem to be convinced it's an overspin issue (although I doubt it personally.)

 

It is indeed double feeding. It was reading about 120fps on the chrono when double feeding. I'll get some sorbo pads and sort out the AOE, the tappet is moving freely and it is the correct air nozzle.

 

What could be causing this FPS variation, though? The Piston, Piston Head, Cylinder Head and Air Nozzle are all SHS, the cylinder is VFC but I have a spare SHS cylinder in the packet if that could help?

[Rock-climby-Dave]

Check compression through the piston and cylinder to air nozzle. in a proper setup slowly but firmly pushing the piston forward should seal the piston against the cylinder.

Is the Pro-win mounted against the gearbox nice n tight?

 

FPS variation could be an air leak or signs or pre-engagement.

 

What BB are you using? Barrel? Hop rubber a good one and not got any wear?

[Governor]

Air seal is great, will have to check the ProWin after work tomorrow. It's using a new Promy Purple bucking. I've also tried it with a nearly new (100 shots) guarder bucking, but somehow that has grown and won't fit into any hop up chamber...

 

Just found the invoice for the barrel and it's an Angry Gun 6.03 300mm stainless steel TBB. Using .25g Nuprol, .25g Blaster, .25g ASG Cursed and .25g ZeroOne BBs. All of which still give a double feeding issue.

 

I'm going to be SSing the piston and sector gear tomorrow, along with ordering an M120 (only spring I have spare are M140 and M90 ).

[Rock-climby-Dave]

The gearbox case lines up with the hop unit and isn't at an awkward angle or anything?

[Sitting Duck]

Hi,

 

I opened up my VFC box after two years of use, and sadly the spur gear's shaft snapped along with all of the bushings grinding nearly all the way through the gearbox shells - I decided it was about time to get a new gearbox in there and had a few extra (new) parts lying around. So... I've put together a nice little gearbox with:

 

- King Arms 7mm shell w/ bearings

- ASCU v2 Gen3+

- SHS full steel rack piston (polymer body)

- SHS Piston Head

- SHS cylinder head

- SHS tappet plate

- SHS air nozzle

- SHS 13:1 high speed gears

- VFC bearing spring guide

- Standard VFC spring (M100/M110?)

 

I put it together in the M4 with:

- SHS High Torque motor

- 6.02 TBB

- ProWin Hop Up

- Prometheus Purple Bucking

 

When using an 8.4v battery, the gun fires fine (slow than I would like, but fine). However, when using a fully charged 9.6, and a fully charged 11.1 the gun double feeds every couple of shots on semi, and a hell of a lot on full auto. Any ideas?

 

So far I've tried it with the stock VFC barrel/chamber/bucking along with a G&G Unit/bucking with an King Arms barrel but they both still gave the same results as the ProWin/Promy setup. I have also tried the gearbox in a King Arms upper and lower, with the same results...

 

Could it be something like needing a sector delayer chip? New tappet plate? Longer air nozzle?

 

Cheers for the help.

Albert

 

 

Building a great perfect box is not as easy as we all first think

The gearbox shell's can vary so much for a start and the front of the gearbox can vary in thickness

(I don't have a King Arms box)

I'm sure I have an old VFC v2 which is 3.1mm thick at front

and a D-Boys spare shell measures 2.65mm - does this matter - YES

coz that 0.45mm means nozzle is projecting more or less into the hop

resulting in a good seal but possible poor feeding

or reliable feeding but loss of fps

(though must state there are numerous other factors as well for poor feeding/seals etc... but nozzle lengths do matter)

On top of that the red shs nozzle is one of the more longer M4 nozzles and people have found feed issues in some builds

 

7mm bearings - hmm ok if they are good quality ones and shimming is dogz nutz

but may crap out over time - bushings are best or at least bushings under gears

 

if piston is a shs blue metal full rack either complete or already lightened to f*ck - nice piston either way

piston head - hopefully not metal one with bearing as it adds weight but suppose not too bad as shs blue full rack's are quite light

(if you have a bearing on piston head - please ffs make sure you used threadlock on it - they WILL undo no matter how tight you tighten)

 

After correcting AoE with sorbo/neoprene - which you should/must correct

the rest of box seems very nice - but think a delay clip is wise

 

The issue with feeding/double feed/cycling....

7.4v should be ok - double cycling can take place on 11.1v and risk of PE

The double firing issue is usually not quite double firing but more a case of it misfiring/part-jam then another bb is loaded

which emits 2 bb's at once giving the impression of double firing eg:

 

1 - 1 - 0 - 2 - 1 - 1 - 0 - 2 bb's come out the end

 

This is likely to be either the nozzle not clearing enough to allow a bb to feed into hop 101% everytime

or bucking/bucking lips catching on bb or being a bit tight or tappet spring being way too weak etc.......

test bb will pass the bucking lips with light moderate assistance (thin screwdriver pushing into hop nub)

new tappet spring or lop a couple of coils off to increase the return force/speed

 

Shs tappet plates usually are very good at pulling back but mat still need a delay or some other modding

check operation - but think the nozzle itself "might" be a smidge too long perhaps for this build.....

 

Before you go too nutz on this that n the other it seems you have more of a feeding problem than double cycling as first thought

 

I suggest forget SS for the moment, stay on 7.4v and get you feeding issue sorted first at say 24rps

once the gun is firing ok reliable and little variation - then and only then consider faster options

The saying - Trying to run before you can walk springs to mind...

 

It is very tricky - bloody very tricky getting a gun to fire faster and still feed great and last

 

but in stages you may be able to make steady progress and achieve a nice performing improved gun

which eventually after a lot of stress cursing and god knows how many times you opened her up...

 

if/when you do get there you kinda feel like.....

 

 

soz but meaning to add a couple of funny's

 

I'm chiming out as Rock-climby-Dave is bound to be far more experienced than me who is still fumbling my way though

[Governor]

Cheers, the box is apart and I've already done the SSing earlier today. I've used blue Loctite on the piston head thread (which is indeed metal). Sorbo has been ordered, M120 spring on the way. Will have a play around getting the FPS consistent, ensuring the hop up is sitting flush etc. Cheers for the help so far, should post back here by the end of next week when the stuff has arrived and I've had a little play.

[Sitting Duck]

ok but go easy on SS

 

do 1 or 2 teeth max at first - can't put them back on

and make sure they came off the side that first engages piston pick up tooth first

I followed a bad guide and removed the teeth at the end of sector which messes up tappet timing

 

best of luck sir

[Governor]

SS'd by 3 teeth, delayer chip, new tappet plate, M120 spring.

 

It's firing very well now, getting 23 RPS with 7.4 and no double feeding. Cheers for the help.