Fps Drop on Auto . Cant Find the Cause, Any Help Welcom

[Caber77]

HI folks, Im looking for a bit of help/advice if possible with a build that ive been working on. I posted recently regards my batteries getting hot and draining quickly and i got some sound advice that helped sort that out.. so thanks to those who gave their input.

 

Im still encountering a different problem where i cant get a consistent FPS between Semi and Auto and drop by about 70-80 fps in auto. And sometimes a bit of miss feeding too.

 

So the gun is on an M4 platform and consists of the following:

SHS 12:1 gear set - now short stroked by 3 teeth (pick up side)

LONEX 8mm SS bushings

SHS 14 tooth full metal rack piston - 3 teeth removed to match sector gear (release side)

SHS ball bearing Piston and Cylinder head

SHS Air Nozzle

Gate Titan Advanced ETU/Mosfet

Guarder SP120 spring was a 110 in it previously

SHS HT Motor

Madbull SS 300mm 6.03 inner barrel

MAXX Tracer Hop

Maple leaf bucking

 

Mags used... I only have High Cap, 3 x Lonex Flash, 2 x ASG Flash and 1 x G&G standard high cap.

 

So ive tried all sorts of things to try and overcome this

Gradually short stroked 1 tooth at time to slow down the pick up, giving the tappet plate time to reset, and now at 3 teeth less. Made no difference at any stage and have now fitted a Gaurder SP120 to bring the FPS back up (320 on semi)

Sector chip removed, allowed consistent FPS at 320 on both semi and auto but miss fed like a bitch

Sector chip in without trimming gave 320 on semi and around 250 on auto and random miss feeding

Sector chip trimmed off completely on the pick up side, exactly the same as the above

Tappet plate fin trimmed to about 9mm to see if it would let the plate and air nozzle seat quicker, still exactly the same as above giving 320(ish) on semi and 250 on auto. Its now about the same as a DSG fin

Took out the SHS HT motor and tried JBU HT and an 18K G&G motor, other than the drop in rps to be expected it had no impact on FPS. I had hoped a slower motor would decrease the gear speed allowing more time for air nozzle to seat.

 

Things ive not tried..

Sector chip trimmed on release side

Could try a full fin tapet plate with the above combos again but i think im right back where i started and almost at the point of dropping in a short stroked 18:1 gear set and saying sod it but i hate to give up.

 

Theory would have told me that adding a sector chip would allow the tappet plate to hold back long enough to resolve the miss feeding, then i get a loss in FPS on semi so it suggests it being held back too long and the piston is cycling either before the tappet plate/air nozzle is closed or its cycling and opening it again before BB has left the barrel.

Short stroking suggests that im delaying the opening of the piston so should help the seating of the air nozzle

Trimming the pick up of the sector chip suggests that it wont pull back too soon after piston released, keeping it seated for air seal

Trimming the fin suggests that the tappet plate should be released early enough to seat before the piston is released and should have decent enough air seal

 

Am i completely wrong or am i missing something real silly here??

 

Anyone care to comment?

 

 

 

 

[Adolf Hamster]

whats your rate of fire on auto?

 

might be pme although short stroke with a ht motor shouldn't be that bad unless it's a truly monstrous battery.

 

other possibility is tappet plate binding in a manner that's making it a bit slow to close- weak spring or tight fit.

[concretesnail]

Is the cylinder ported?

Thinking that if you set your feed and timing to clear the misdeeds but get too low fps only in full auto, does the cylinder have enough time to completely refill before the piston releases again.

[Caber77]

I've tried 2 tappet plates and the spring is cut by 4 coils now. ROF should be around 40 give or take depending how accurate my chrono is. 

Batteries are 11.1v lipo, 2600 mah 25/50 c so kicking 65 amp semi and 130 amp continuous but need that to cope with the amp draw the titan was reporting and the batteries getting extremely hot.

5 minutes ago, concretesnail said:

Is the cylinder ported?

Thinking that if you set your feed and timing to clear the misdeeds but get too low fps only in full auto, does the cylinder have enough time to completely refill before the piston releases again.

Yeah cylinder is 3/4 ported, I'm half wondering if the shs piston head or o ring is too tight in the cylinder (got signs of scraping/rubbing) and if even though the head is ported to expand the o ring on the release stroke if the pressure against the inside of the cylinder is too tight on the back stroke and pulling air back on itself vacuuming inside the barrel.

[concretesnail]

If the cylinder is 3/4 ported you shpuld be doing okay air wise. Have you tried taking the maxx hop out and swapping for a stock unit to eliminate that. I know a few folk have had issues with mazx units of late.

Edited April 15, 2021 by concretesnail
typo

[Caber77]

2 minutes ago, concretesnail said:

If the cylinder is 3/4 ported you shpuld be doing okay air wise. Have you tried taking the maxx hop out and swapping for a stock unit to eliminate that. I know a few folk have had issues with mazx units of late.

No but tempted to do that, along with trying a different hop there is also the possibility of trying a different  (slightly shorter) air nozzle just in case that has a negative effect for feeding on auto.

It's like I need to go back to the drawing board, return everything back to stock and then replace internals 1 piece at a time until I find the root cause. I kow that sounds sensible but I don't want to start again haha

[Adolf Hamster]

1 hour ago, Caber77 said:

ROF should be around 40 give or take depending how accurate my chrono is. 

 

that would be your problem then.

 

that's way too fast for a 1j gun.....

[Caber77]

So is there a possible solution you have in mind other than saying that would be the problem.

I appreciate that it's an SSG build with high ROF rather than a DSG but I'm by no means the first to use a short stroked 12:1 gear set with an HT motor. 

I can't actually categorically say it's on 40 rps as its miss feeding, could be mid to high 30s... 

[Adolf Hamster]

38 minutes ago, Caber77 said:

So is there a possible solution you have in mind other than saying that would be the problem.

I appreciate that it's an SSG build with high ROF rather than a DSG but I'm by no means the first to use a short stroked 12:1 gear set with an HT motor. 

I can't actually categorically say it's on 40 rps as its miss feeding, could be mid to high 30s... 

 

the short answer is you're pushing it too fast, you need to drop the speed either switching to 7.4, a much slower motor, or lower ratio gears.

 

it's also worth noting that even if the gearbox isn't screaming in agony 40rps is when you start to get magazines acting up (ammo/mag dependant ofc)

 

as a more general note, you don't need that level of rof, i get the allure hell i've been there myself (i went full airline....), but if your target aint gonna call 20 rounds then they won't call 40. only real solution to that problem is a better target.

[Caber77]

1 minute ago, Adolf Hamster said:

 

the short answer is you're pushing it too fast, you need to drop the speed either switching to 7.4, a much slower motor, or lower ratio gears.

 

it's also worth noting that even if the gearbox isn't screaming in agony 40rps is when you start to get magazines acting up (ammo/mag dependant ofc)

 

as a more general note, you don't need that level of rof, i get the allure hell i've been there myself (i went full airline....), but if your target aint gonna call 20 rounds then they won't call 40. only real solution to that problem is a better target.

Thanks yeah I totally get that about someone calling hits....

So possibly what I'm looking at is dropping in a 16:1 or 18:1 short stroked in your opinion? I wouldn't be opposed to that as high rof isn't the ultimate goal with the build, + 25 - 30 is sufficient to satisfy me so long as I get reliability. I think if anything, I want to know the cause of the timing issues and how to resolve them but if I'm gonna have to admit defeat then so be it. Don't want to drop to 7.4's cos just forked out over £100 on a few batteries with higher amp output to cope with the draw. New gears again it is then and try again...

[Adolf Hamster]

2 minutes ago, Caber77 said:

Thanks yeah I totally get that about someone calling hits....

So possibly what I'm looking at is dropping in a 16:1 or 18:1 short stroked in your opinion? I wouldn't be opposed to that as high rof isn't the ultimate goal with the build, + 25 - 30 is sufficient to satisfy me so long as I get reliability. I think if anything, I want to know the cause of the timing issues and how to resolve them but if I'm gonna have to admit defeat then so be it. Don't want to drop to 7.4's cos just forked out over £100 on a few batteries with higher amp output to cope with the draw. New gears again it is then and try again...

 

question is are you trying with the 12:1 gears to reduce gearbox noise or increase speed?

 

assuming your setup is actually running at 40rps on that battery then the motor speed is ~29k rpm which working back to 18:1 is ~27rps (assuming the motor isn't speeding up)

 

awaits duck coming along to correct my math on that  

[Caber77]

21 minutes ago, Adolf Hamster said:

 

question is are you trying with the 12:1 gears to reduce gearbox noise or increase speed?

 

assuming your setup is actually running at 40rps on that battery then the motor speed is ~29k rpm which working back to 18:1 is ~27rps (assuming the motor isn't speeding up)

 

awaits duck coming along to correct my math on that  

In all honesty the the purpose of the 12:1s were to increase rof and see what I could get from it running efficiently but as time as goes and trial and error proves this ain't working I'm leaning more towards 16:1 short srtoked and taking one of my other gear boxes and trying a dsg for a higher rof. 

It's all very much a learning curve for me and I see/read so much knowledge on the forums that I try to replicate.

 I've done a few home builds before for my son and never had issues but then they have only been low stress boxes so simple to do. This has been my first attempt at a high stress/speed box.

 

[Adolf Hamster]

as much as i get the call of the tinkerer, speed chasing on an aeg isn't really worth it, once you start pushing beyond 25rps with a precocking mosfet and a motor with decent pickup it really becomes a very rapid dimishing returns curve that ends with an air-line.

 

of course that said, i didn't listen either when duck told me that years ago when i was blowing racks out of pistons every half game day......

 

*looks at 12:1 ak build waiting for it's brushless motor to arrive*

 

come to think of it i still haven't learned......

[Caber77]

8 minutes ago, Adolf Hamster said:

as much as i get the call of the tinkerer, speed chasing on an aeg isn't really worth it, once you start pushing beyond 25rps with a precocking mosfet and a motor with decent pickup it really becomes a very rapid dimishing returns curve that ends with an air-line.

 

of course that said, i didn't listen either when duck told me that years ago when i was blowing racks out of pistons every half game day......

 

*looks at 12:1 ak build waiting for it's brushless motor to arrive*

 

come to think of it i still haven't learned......

Probably many of us in the same boat. I guess I'll take this one on the chin and just go with a lower gear ratio to get it running.... and start again on another one.

I'll never learn, doesn't matter what it is, that the whole point of having a work bench in my garage and a dam comfy stool... can't help but tinker with stuff👍 

[Iceni]

The only thing I can think that might help this is to expand the holes on the piston head. Doing this should allow the piston head to suck air faster and with less barrel resistance on the draw. It might also be worth using a marginally oversized piston o-ring that rides the walls of the cylinder a bit harder, You'll wear that ring out faster, but it should provide a quicker airseal as well. Typically an AEG piston o-ring is 19x2.5mm (24mm) but moving up to a 19.5x2.5 (24.5mmm) should give better high speed compression.

There might be an issue with that oversized o-ring however. If it slows the piston too much it's going to cause PME. I've never built a 40rps gun and don't know anyone that uses one, so take the o-ring advice as an idea to perhaps try, and not a this is going to work statement. I use oversized rings in all my own guns but 25rps is about as fast as they go.

Edited April 15, 2021 by Iceni

[Caber77]

10 hours ago, Iceni said:

The only thing I can think that might help this is to expand the holes on the piston head. Doing this should allow the piston head to suck air faster and with less barrel resistance on the draw. It might also be worth using a marginally oversized piston o-ring that rides the walls of the cylinder a bit harder, You'll wear that ring out faster, but it should provide a quicker airseal as well. Typically an AEG piston o-ring is 19x2.5mm (24mm) but moving up to a 19.5x2.5 (24.5mmm) should give better high speed compression.

There might be an issue with that oversized o-ring however. If it slows the piston too much it's going to cause PME. I've never built a 40rps gun and don't know anyone that uses one, so take the o-ring advice as an idea to perhaps try, and not a this is going to work statement. I use oversized rings in all my own guns but 25rps is about as fast as they go.

Thanks, no harm in trying that. Ive seen posts/comments of people drilling out the ports a little and suggesting it helped so may give this a go too. Ive got spare piston heads that although have fewer ports in them, they are larger so i guess i could try swapping out aswell before changing gear sets

[Caber77]

So after messing about for a bit with this again I think I've gotten to the bottom of the missfeeding issue. Turns out the feedlips on the mapleleaf bucking were protruding into the hop chamber just a lite too much. The bottom of the lip had curved/squashed upward ever so slightly causing the bb's to stick. 

Dropped in another bucking that I had (think it's a Gaurder 70) and the lip is fractionally shorter allowing the bb's to feed much better.

Now just to sort out the timing issues for the drop in fps when on auto. Kinda going back to scratch and have a new cylinder, cylinder head, piston head and 16:1 gear set on order.

Better bloody work this time!!