Featured Tappet Timing

Discussion in 'Gun Building, Modifications & Repairs' started by Jimps, Feb 15, 2021.

  1. Jimps

    Jimps Active Member

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    So Titan, 14:1DSG. SHS tappet plate cut to 8mm. 4 coils cut from tappet spring. Maxx nozzle 21.25mm. When I was building this DSG I had bumps along the way. Amp draw, heat, sluggish performance. Got it all figured out. Last thing I did was use the stock VFC QRS motor grip. I had the PTS grip, and I guess the angle was off a bit. I gained 4-5RPS and lost 15-20 amps of current draw. I took the gun apart 30 times changing shimming and checking motor height and bevel height. I got it good but still not great, changed the grip and now it is near perfect.
    All that being said, now that my ROF is higher, my FPS in auto is low. Semi-auto is 320ish hopping .25bbs. When I was at 35-38RPS, FPS in full auto would be steady, maybe drop 10-15 FPS. Now I'm at 45ish RPS it drops to 210-220FPS in auto. Not sure if I should cut tappet more? I'm at 8mm now. I don't know If I could cut more coils off my tappet spring or not. It is pretty tight. Should I use a POM nozzle? Is there a better tappet plate to use? I will never use auto on this gun most likely, and binary at 18-20RPS is fine FPS wise. I just cant stand it though. I cant help but think something could be better, even if ill never use it, I want it to be right.
     
    Monkey Tech likes this.
  2. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    Do the tissue test in full auto.
     

  3. Jimps

    Jimps Active Member

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    I put a small piece of tissue in the hop hole, fired on full auto. It did not fly away.
    I find that plastic from a grocery bag works too.
     
    Last edited: Feb 16, 2021
  4. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    And how long is your barrel?
     
  5. Jimps

    Jimps Active Member

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  6. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    Seems the pressure impulse wave is just too short in FA mode at that speed. Try a 2S battery to slow your FA mode down and see if that increases your FPS.
     
  7. Jimps

    Jimps Active Member

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    At 25-35 RPS the FPS is stable at 320ish. Anything above 38 RPS really starts to drop.

    I will never use 40+ RPS, but it just bothers me knowing something is not right. I would also like to learn something as well.
     
    hawkchief likes this.
  8. Jay rollz

    Jay rollz New Member

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    Cottage grove
    This is probably a dumb question but did you grind diwn grip a little bit where it meet the lower to gain a small tollerence for alignment of motor?
     
    Monkey Tech likes this.
  9. Jimps

    Jimps Active Member

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    Yea I did, when I screwed the grip to the gearbox inside the lower, the receiver could still move
     
  10. Ben3721

    Ben3721 Active Member

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    I believe you can hit pretty high rps with a standard 11mm tappet. However! Sometimes a dsg tappet will kick the pellet past the hop up and down the barrel halfway before the piston drops. Cutting the spring can actually make that worse sometimes assuming that is the issue. Resulting in a completely unhopped shots in auto and leaving the last round to potentially roll right out of the barrel.

    Is the tappet movement free with the sector gear in and all? Are the shims snagging the tappet fin? Is the gearbox free of casting edges where the tappet sits?

    Is the nozzle snagging anything in the hop up unit? Some hop up units have sharp ridges that nozzles can snag a bit in auto causing fps loss like this.
     
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  11. Jay rollz

    Jay rollz New Member

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    Nice was just wondering heard they can throw off the alignment a bit.
     
  12. Jimps

    Jimps Active Member

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    Gearbox is Retro Arms, Tried two barrel/hop setups. VFC-250mm and Maxx-290mm, same result with both. 35RPS and under is fine.
     
    Last edited: Feb 16, 2021
  13. Jimps

    Jimps Active Member

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    After further testing, I noticed while doing the tissue test, the tissue got sucked in the hop up on full auto. Guess I have some suction. Not sure if the Prometheus piston head is known for this at high rate of fire or not. Should I port the cylinder? Was thinking of opening the holes on the piston head a bit to allow more air to pass through. I watched a science type of video on this. Guy connected his cylinder to a hose with water in it. When he pulled back the piston, there was a tiny bit of suction. He tried different o rings and piston heads. He widened the holes in the piston to stop the suction effect completely while retaining all forward pressure.

     
    Last edited: Feb 17, 2021
  14. Ben3721

    Ben3721 Active Member

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    A properly designed piston head should allow the o ring to slide forward during the backstroke allowing air under the o ring and through those ports from the back. Yes some air will still be brought in through the open nozzle, and if applicable a ported cylinder. Those same air holes and air around them (majority) expands the o ring during the forward stroke when the o ring is forced back. Its important to have a O ring wide enough that it drags a little against the cylinder so that it quickly seals back and front during a cycle, I've found that alone helps with fps consistency a bunch, causing a +-15 to sometimes jump down to +-5fps with a slight fps boost... it makes a difference when the o ring seals immediately rather than from the pressure spike expansion.

    Unfortunately many manufacturers including some decent ones fail to understand this important function of this and they make no room for the o ring to move inside the piston head, literally preventing this function, which then means all the air must be dragged through the nozzle, if the nozzle isnt slotted on the tip, it can seriously cause some rps drag with horrible fps loss in auto as the air pressure will be less. Because the next pellet may be sucked right to the tip of the nozzle.

    Make sure the piston head o ring has room to move freely, make sure it drags a bit against the cylinder, widen the port holds if necessary, and if possible use a nozzle with slots in the tip, lonex is starting to update their design with slots.
     
    aotsukisho likes this.
  15. Jimps

    Jimps Active Member

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    Also what would be the most likely cause of scratching the cylinder? The very end of the cylinder has wear on it. The piston head does not interact with it. I changed cylinders and it is fine now, but what usually is the cause of that? Sector shimming?
     
  16. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    Scratching? Were you using an internally tapered cylinder?
     
  17. Jimps

    Jimps Active Member

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    No, They are Maxx Model Full cylinders. They are not deep scratches. It looks bad but when you touch, it feels minor. Cant tell anything with gearbox open, and when it was closed I could not feel or hear it moving the piston by hand. Weird. Was just curious. Want to prevent from wasting $18 again. I could have prob still used it because the piston head did not come near it. just bothered me.
     
  18. Jimps

    Jimps Active Member

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    Augusta
    So I opened the holes on my Prometheus piston head. Gained 40-50FPS in auto. I'm now at 280ish FPS with .25s before I was 200ish FPS. When I chrono in full auto, while holding the trigger it shows 250ish and when I let go, it goes to 330. So the last shot of auto fire goes back to normal FPS? Before it was 200FPS when I let go. I have shot 8000 bbs through the chrono to come to this conclusion.

    I have two of the exact same pistons and piston heads. I set up the hose test, to test for suction. The before and after was a big difference in suction. I was too lazy to film or take any pictures. I took a tiny drill bit and just made the holes in the piston head 25% bigger. I could probably make them bigger, did not want to ruin a $30 piston head though. After seeing the difference in the suction test, I wanted to chrono and see if there was a difference in FPS in auto. Amazingly there was. Tech at a shop told me to use/make a ported cylinder. Mabey the combo will completely solve the problem. Might make the holes just a tad bit bigger as well. A thinner o-ring could also make a difference as well.
     
    Last edited: Feb 18, 2021
    kazuchan likes this.
  19. Jimps

    Jimps Active Member

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    After even further testing, I have some more results. In the vacuum test on the modified Prometheus piston head, I reduced the suction by 60% by increasing the the holes in the piston head by 25ish%. I gained about 61% of my FPS back in full auto. I fired 2000 rounds in full auto before modifying, average FPS - 206FPS with .25 gram bbs. After the test, another 2000 rounds fired, average FPS - 279FPS with .25 gram bbs. FPS on semi average 325FPS, 1000 rounds fired.

    The numbers in the vacuum test are not super accurate. I just took a hose with a ruler next to it, then measured how far the water was drawn back while pulling the piston out of the cylinder by hand. I tried to be as consistent as I could pulling the piston back. I also eyeballed where the water line was. So a close estimate is about 60% of reduced suction. Its just crazy for me to see close to the same % of FPS gain in full auto. I have 2 of the same piston heads, both had the same results, so not a defect.

    I have another untouched Prometheus piston head. When I get at least 90% of the FPS back, I will make a video of the results. I will show the vacuum test with the pistons and I will chrono with the stock piston and the modified piston as well. This all started with the tissue test. Never heard of that before. When I looked it up and tried it, the tissue literally got sucked into the nozzle. Interesting. Hell, there could be a better piston head option and this is all a waste of time. The lonex piston has less suction than the Prometheus, but the lonex piston weighs a tad bit more. The lonex piston has about 25% less vacuum over the Prometheus.

    I have 12 tappet plates cut to every length possible(shs, guarder), different length nozzles, tried different hop units.... All when there is no problem in those areas. Hopefully this can help someone in the same position. I'm pretty sure I'm not the only person that has had this problem. Maybe it is the particular combination of parts I have. I have not had anyone mention this problem, or read anything about this. I just happened to watch the Gel Blaster tech on YouTube talk about it a while back. I might buy a couple more piston heads and try them as well, just to see if there is a better option to avoid vacuum issues. Ill post a video later this month or next month.
     
    kazuchan likes this.
  20. Guges Mk3

    Guges Mk3 Administrator Staff Member Administrator Supporting Member Lifetime Supporter

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    If you want tappets I have a bag of generic V2 units I could send you for 5.00

    Like a handful of them.