FPS variance....what is even going on...

Discussion in 'Gun Building, Modifications & Repairs' started by Stab70, Oct 24, 2021.

  1. Stab70

    Stab70 New Member

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    Eureka
    I am using a Perun V2 Hybrid in a CYMA Platnium M4 with the 30k Warhead brushless motor, and an 11.1 1 100c brick battery. This CYMA comes with 13:1 gears.

    I am achieving 38rps on the ssg, but the fps is varied by up to 70fps. Approximately 7 shots @336-337, then two @407, then back to 336. I have very good compression and seals on all air components, gotta trust me here lol, despite what it sounds like. Also, it used to regularly shoot 420s, +- 2fps with .20g.
    I have never had this problem on lower speed builds, so it's definitely a stress situation. I am hoping it's not a problem with the high speed gears, high speed motor, insane battery, and Perun.
    The m4 Platnium has a unique sector gear. It a 12 tooth, but they made it with 2 missing on the front pickup side, and 2 missing on the back end. The timing lobe underneath is adjusted for this of course, but I wanted to include that detail in case there is something wrong here.
    My kid has basically the same build, but his battery only allows him to reach 34rps. No feed issues, very consistent fps @ +- 1.5fps. It rocks.

    I can't figure out the problem, but i think it's from a very slight PME? I don't see any wear on the gears tho. Probably 700+ rounds through it testing various adjustments so far? Perhaps the sector over spins just enough to catch the tappet before the nozzle can fully seal on the bucking. I know there is an AB problem between Warhead and Perun, but has anyone found some way to keep this from happening? Is my tappet spring too weak? I took off 2 wraps so far. That's kind of the last hope as I have tried everything else. Because this isnt a dsg, I haven't gone crazy on modifying the tappet fin or taking more off the tappet spring.
    Anything you can recommend?
     
  2. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    The gears, motor and battery has no direct affect in FPS.

    You should focus on the compression parts.

    Have you done a tissue test to determine you don't have slight variance at the seal and nozzle.

    Packing durometer, shape and nozzle length can form a perfect storm and give you bad compression or good compression if it's "just" right. With excessive HS builds (like yours), many times harder packings don't return to shape fast enough to form a good seal and you lose FPS.
     

  3. Ben3721

    Ben3721 Well-Known Member

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    I'm going to trust your judgment on the compression.

    If compression and timing is good and there are no signs of PME which would show fps issues but with damage. Then the last thing is feeding. If the mag is behind causing the nozzle to close late it will have less fps. If the hop up lip protruding into the feed tube? Are you testing with quality midcaps?

    Also sometimes a tappet can snag up on the upper sector shims. You also should make sure the nozzle is pushing tight against the hop up rubber and sealing tight.

    I wouldn't say this is a lost cause, just something go overlooked.

    Edit: since the messed with timing that much the bottom half of the tappet might need either an angle cut or shortened.
     
  4. Stab70

    Stab70 New Member

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    Eureka
    @Guges Mk3
    Interesting thought on the rubber hardness. I had not seen that explained by any techs. My nozzle is a maxx 21.25 with double o-ring. It's been on my build for 40k rounds Again, this was never a problem when it was at 34rps, so I am hesitant to blame that o-ring seal. The length has been checked to the hop up and it does not protrude to hinder bbs, and at full stroke, makes good contact with the bucking. (The MAXX ME Pro hop up has windows to easily view this)

    The Piston head with o-ring is pretty new. This is a part I got from RealDealAirsoft. Not sure what brand it started as, but it's the best one I have used to date. Here is the o-ring specs on that. Used for about 5k rounds before this last round of upgrades.
    "New design uses super slick Viton O ring. This O ring has superior properties to the standard material o rings that come with most piston heads. You will not have to worry about losing the flair on your O ring. Its made of a stronger material that can really take a beating. Super High RPS builds will appreciate this O ring. Incredibly friction resistant as well as temperature resistant. Can be used in as low as -12 degreed F."

    The bucking is a lonex 70 with MAXX ME Pro hop up. PTFE tape wrapped around barrel to bucking contact for extra seal.

    There's the extended details. Anything I may not be thinking of?
     
  5. Stab70

    Stab70 New Member

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    Eureka
    @Ben3721
    The bucking lips do not protrude into the feed tube, and neither does the nozzle during the feeding phase (tested by manual gear movement of course.) This is also visible through the windows on the MAXX ME Pro hop up.
    The mid caps I am using are the Evike BAMF 190rd. I have heard reviews saying they work for DSGs, and I did not have problems at the 34 rps rof. I don't honestly know if they are considered quality or not. Have never gotten consistent advice on what mid-cap to use.

    I had considered that the shims on the sector gear might be causing an issue with the tappet, but I'm not sure how I would alleviate that. Currently there is a tiny bit of vertical movement on the sector gear, a .2mm shim would make it too tight. Perhaps tightening the sector gear shimming would help. I had figured a little play would help since the sector has so much room to contact the spur and rack.

    So...I was trying to eliminate the need to modify the tappet. From the time it starts the return cycle to the end the cycle is roughly 2/3 of the sector. I understand that this is moving at light speed, but DSGs only have about 1/6th of the sector to return to start of the next pickup tooth. The difference here is that DSGs usually are 16 or 18:1, and not 13. I have definitely seen videos of 10:1s, but the majority are going to be rotating slower, so the tappet mod may be the solution.

    Based off this info and the post above, any further recommendations? Could it be an issue with my main spring? M130 systema with ~60k rounds and 15k test fires lol
     
  6. Ben3721

    Ben3721 Well-Known Member

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    Sounds like everything is on point. Some do not like the Maxx hopups due to their loose internal tolerances which can sometimes cause feeding issues.

    As for the tappet snagging the shims a slight beveled edge helps prevent that.

    When it comes to 35 rps and up fps issues from the tappet being pulled back before the piston is done can happen. But not at all in semi unless the timing is horribly wrong. I'm not sure how that manufacturer timed their tappet cam.

    The o ring on the piston needs to be large enough to drag open and seal. If it struggles to do so it will cause fps issues. A piston head is actually a one way valve, many manufacturers struggle to understand this. That one way valve helps with fps consistency in full auto and rps due to less backdrag.

    Is a sector chip being used? If so what type?

    Also 10:1, 12:1 and 13:1 can hit like 40 rps with a little short stroking. The difference is how much overspin they have, not really anything else if they are paired with the appropriate motor for the same desired rps.
     
  7. Stab70

    Stab70 New Member

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    Eureka
    @Ben3721
    Semi auto has the same fps issues.

    The piston head is a 6 port aluminum head. With the nozzle plugged, it compresses immediately in the cylinder. I tried to upload a video but it won't let me do .mp4.

    No sector chip, but the post on the sector gear is over sized. I would guess it's the same diameter as the gear shaft.

    It's overspin that I think is causing the problem. I really feel like sector tappet post is re-catching the tappet just before it makes 100% seal, but I am trying to eliminate that possibility. The Perun and Gate mosfets (mine is Perun Hybrid) both have interface issues with the Warhead ESC when it comes to AB, and I think this might be causing a situation with inconsistent locations where the sector gear stops, giving it that very slight overspin that contacts the tappet before it makes a full seal. I *think* this is why it jumps to 407 every 5th-8th shot...its just operating correctly for those couple shots. I think I will cut the tappet to see if I can gain .001 seconds on that seal.
    I won't be able to get to it for a few hours, so please reply (if you have time ) if you see an error in my approach.
    Thank you for your time
     
  8. Ben3721

    Ben3721 Well-Known Member

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  9. Stab70

    Stab70 New Member

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    @Ben3721
    So...nothing worked.
    For no particular reason I loaded up my .28s which are much higher quality than the cheap .20s I was testing with. Not 100% sure why, but they shot 50 in a row +- 4 fps. The mags can't push the heavier bbs at that rof, but I don't care. I'll get new mag springs.
    The good news...at this point it wouldn't take much to upgrade to dsg lol.
    Thanks for the help
     
  10. Stab70

    Stab70 New Member

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    Eureka
    @Ben3721
    So....I did some interviews...
    My 13 yr old kid's friend mixed our .28 game bbs in with my .20 test bbs that I had given him to use a couple weeks ago, not fully understanding the importance of different weights. I have spent over a week doing diagnostics and combing over every little part to check for damages. Not to mention the countless hours I spent on research for interface problems with high discharge batteries and brushless/perun interface....not much info out there on this.
    Ugh...lesson learned. If it's something super out of whack, start at the very basic solution first...
     
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  11. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    Mixed...bbs...yeah that will do it.
     
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  12. Ben3721

    Ben3721 Well-Known Member

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    Well that's infuriating lol. I have a bag of mixed ammo from leftover mags but it's only for grenades or basic test fires
     
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