High FPS variation

[SealTeamNorm]

Rebuilt a gun with the following now getting huge FPS variation(80 FPS between shots) 250 to 330 and possibly a little low.

G&G warthog

Tight bore barrel

13 to 1 zci gears - short stroked 2

22tpa zci high torque

M115 asg spring

Perun ab++ MOSFET

11.1 v lipo

16awg wire

X ring piston seal o ring

Double o ring zci cylinder head

I've tried turning active brake up and it tightens the spread a little but still present

Any thoughts would be appreciated!

Thank you all

 

[SeanM]

Sounds like an air seal issue somewhere. Have you tested the compression in the cylinder and piston? If you have, and you've got a good compression. Next check that the nozzle sits in the hop correctly. It should make contact with bucking so make sure it's not too short (or long) you should also shine a light into the hop unit and then look down the barrel and make sure it's centered nicely. If all these come up as good and there's good compression. Check the bucking and make sure there's a nice seal there and it's sitting on the barrel correctly and isn't loose or lopsided.

 

[Sewdhull]

Check the short stroking hasn't messed with tappet plate timing and all the above from SeanM

 

[MrTea]

Did you change the bucking or barrel? Try going back to the stock bucking first. I've had bucking cause this exact issue when building my DMR. If the stock bucking is producing the same variation, change back to the stock barrel and try again.

 

[SealTeamNorm]

Cheers for the responses 

So I short stroked 2 from the pick up. That shouldn't cause a timing issue but I know it's possible.

Should the nozzle make contact with the bucking at rest? If that's the case mine is a good 10mm away. See the picture though it's dark!

I'll take the bucking off and see.

I think compression should be ok with all new seals etc but never know l.

I've tried the tissue test and I'm definitely losing air down the feed tube when firing just not sure how to fix it .

@MrTea same bucking and barrel that was working pre tinker!

Thanks AllView attachment 119927

View attachment 119925

 

[SealTeamNorm]

Also interesting maybe nothing but when I took the motor out the air nozzle jumped forwards a few mm. 

 

[ak2m4]

You've got a double o-ring piston head (don't use them if you can), and a X ring piston seal o ring - check that both can slide easily.  Check piston head vents don't have too much grease on them.   AB shouldn't affect it, don't run an AB, will only heat up your motor and reduce life.  How long is your nozzle?  

 

[SealTeamNorm]

Stripped it all down again both sliding ok 

Nozzle is the original gng one, not sure how long 

Gun cycles full auto and on semi fine when not attached the barrel/ feeding pellets.

Get loads of air leaking from the nozzle to bucking when shooting with out a mag attached

Got a feeling the tappet timing has been thrown off by the short stroke hmm

Thanks everyone btw 

 

[SealTeamNorm]

Another update, changed the spring to a m120, removed the lip on the bucking , corrected ape and same issue high FPS variation. 150 FPS to 350 with a m120

Time to trim the tappet plate?

 

[Sewdhull]

I would carefully observe the movement of the tappet plate and see what the timing looks like. If it is fully forward in good time before the spring is released and stays there a bit before being pulled back by the sector gear, then you should be looking at the nozzle interaction with the bucking, some buckings are shorter or longer than others at the nozzle end.

Since the nozzle and tappet plate are original then I would not expect the tappet plate to need any adjustment, unless the new cylinder head has pushed the nozzle and tappet plate forward causing fouling on the case. The tappet plate and so the nozzle should be able to go as far forward as before the changes were made in any case. Except perhaps if the cylinder head was off center for some reason.

You do have a new sector gear so this may affect the timing and movement of the tappet plate, potentially preventing full forward motion against the spring tension.

Where did you remove the teeth from on the sector gear?

Another update, changed the spring to a m120, removed the lip on the bucking , corrected ape and same issue high FPS variation. 150 FPS to 350 with a m120

Time to trim the tappet plate?

Removed what lip on the bucking?

Oh and how well does the nozzle fit on the new cylinder head?

 

[SealTeamNorm]

Hi sewdhull,

Thanks for the response.

I removed teeth only from the pick up side of the sector gear. Interestingly the center axle is thicker on this gear Vs the stock 18 to 1. 

Timing wise when cycling by hand looks ok? Bbs fall into the chamber and get pushed into the lips before the piston comes forward.

I removed material from the bucking lips.

When dropping a BB down into the BB barrel hop emulating the mag. The lips stopped the pellet falling into the barrel.

 

[ak2m4]

At this point I recommend taking the build back to stock gears and see what happens.  

 

[The_Lord_Poncho]

View attachment 119925

I think your problem is that you are clearly using the wrong wooden cylinder. Tropical hardwood is what you want, just not that Beech!

 

[SealTeamNorm]

Haha yes perhaps it needs more varnish

@ak2m4 was definitely the next port of call however trimmed the tappet down...

First 2 shots come out weird then it quickly settles to a consistent 340 344fps( shot 80 odd times). Which tbh is bang on.

I need to strip it again as full auto won't go on now but it definitely seems to be the tallest timing , going to take one more but off then leave it.

 

[Colin Allen]

I think your problem is that you are clearly using the wrong wooden cylinder. Tropical hardwood is what you want, just not that Beech!

Lignum Vitae is the best; it is self lubricating so is ideal for a cylinder.

 

[SealTeamNorm]

So it's shooting fine now semi.

Full auto it's misfeeding then after a few bursts jamming up.

Sector delay switch might be needed ? 

Thought I'd be ok on a 22tpa motor!

 

[Rogerborg]

misfeeding

Could be midcap syndrome, just too much pressure on the nozzle.  Do you get the same behaviour with a hi-cap, or a nearly empty midcap?

I've not tried it, but I'm intrigued by the idea of using a tapered punch to (very!) gently flare the cylinder head so that the nozzle sits more securely on it without weeble-wobbling.

I mean, you might need a delayer, but I prefer to start at the front and work backwards.

 

[SealTeamNorm]

Unfortunately only own the one mag but I think it was pretty consistent throughout the mag load. 

I tried spamming the trigger with no mag attached and all cycled fine no jammin etc so definitely a feeding issue.

I'll get hold of a Hi cap and see what happens!

 

[SealTeamNorm]

Another installment. 

Swapped out the bucking and selector plate for a shs one. Shoots fine now!

20rps around 348fps.

The new bucking lips extend slightly over the barrel so I think the issue was the zci gear shaft was a bit bigger than the stock gng one so impacted sealing.

Rps feels a little low but maybe missing the odd shot?

Plus the hop doesn't impact FPS. Frustrating as every now n then a shot comes out at 350.8 or something. 

Any ways to reduce this by a few FPS and thoughts on the FPS?

Reassembled it anyway ready for games.View attachment 120742

 

[Rogerborg]

Plus the hop doesn't impact FPS

Not at all?  That's... very unusual.  Is it (observably) applying hop?

FPS

Do your local sites test using 0.2g?  If they're operating in the 21st century, I'd highly encourage thinking in terms of Joules, using the weight of BB that you intent to use, and ideally with the hop dialled in to lift it.