FPS loss in Cyma RPK

[Ninja_En_Short]

Hi everyone,

I calling my fellow doctors because I have a sick CYMA RPK that is messing with my brain quite hard :

I must put the gun at 400 FPS but as I test the cylinder seal before reassembly I immediately find a leak with the nozzle even though it doesn't look really worn down. I put some grease to restore the seal and put the RPK back together with a M120, something that should give about 410/420 with this gun (I expect).

Test fire 300/310 FPS, first "WTF" of the evening.

I find a 1 mm gap between hop up block and gearbox, I put some cardboard between the gearbox and body to restore the seal as much as I can but still about 320...

I have disassembled the hop up yet could find a leak anywhere and the tiny "pshit" I get in the BB conduit is the same as my G36 which fires at 410 with that spring...

I'm at a loss on this one so did anyone ever encountered anything like it or has an idea where I could have missed anything besides the nozzle ?

PS : it has a Madbull Python inner barrel.

[HKGhost]

If there's a gap or you're not sure the nozzle and bucking is making a seal, here's a tip you can try. File down the face of the tappet plate about 1mm, and that will for sure you have a solid contact and seal. Make sure you have the correct nozzle length if you're off more than 3mm. Check compression parts and correct cylinder for barrel length. Hop up chamber position when the mag is inserted as sometimes it will move and cause a gap between the nozzle and bucking. Make sure you're not missing any sector gear teethes which will cause short stroking and lower the fps.

[Hectic]

Also how is the seal around the hop rubber, barrel, hop unit?
Put the rubber on the barrel put your finger over the end of barrel and blow through the hop end. See if air is escaping between rubber and barrel.
Dont worry if a lil is escaping. Put the hop up on and do the same. Plug barrel and blow through hop assembly with the barrel plugged. If there is still leaking put a wrap of teflon on the barrel install the rubber and put another wrap over the rubber where it ends on the barrel side not the feed side and reassemble. Should be air tight.
Im sure you likely know that but its something thats overlooked at times

[ccyg8774]

Originally Posted by HKGhost

If there's a gap or you're not sure the nozzle and bucking is making a seal, here's a tip you can try. File down the face of the tappet plate about 1mm, and that will for sure you have a solid contact and seal.

Thanks! I am working on a different problem, but this is very helpful for my situation as well.