ninja master of coffee Posted July 31, 2016 Report Share Posted July 31, 2016 I have been having issues with variable and low velocity with my CYMA AK-74m (CM040C) for some time now and it has me stumped. At first there were some issues caused by any obvious air leak (original piston head had a terrible seal) but that has now been swapped out. Currently I have replaced the piston, piston head, cylinder head, air nozzle and tappet plate with SHS parts, as well as changing the hop up bucking and nub to Tokyo Marui. Last week I chronoed it at home using .25g BBs (blaster) using an Xcoretech X3200 mk3 and it was hitting around 280 fps. Yesterday I took it to a game and it first chronoed at 245 fps, then a few minutes later it dropped to 220 fps, then 205 fps (still using .25 g BBs). At the time I still had the original CYMA air nozzle fitted. The site tech suggested the issue could be using silicone lubricant rather than silicone grease, as lubricant dries out. I have taken the gun to bits and checked the air seal, for the cylinder, cylinder head and piston head the compression is perfect, the original air nozzle leaked, so that has been swapped to an SHS one with O-ring, compression was greatly improved. I have also added a small amount of silicone grease to to the piston head O-ring and air nozzle O-ring. The velocity has now gone up to 245 fps with hop applied or 265 fps with hop off which is till low with an SP 100 spring. I have also checked for air leaks around the hop chamber using tissue paper place above and below the hop chamber, no leak is apparent. I have also checked the air nozzle and hop chamber for travel and closeness of fit and both seem fine (the air nozzle completes a cycle, the hop chamber fits to the gearbox). The other point to note is that the volicty issues remain regardless of magazine used. Any help or suggestions would be greatly appreciated. Link to post Share on other sites
Guges Mk3 Posted July 31, 2016 Report Share Posted July 31, 2016 Your Piston rails are to fat. SHS parts are not HQ...nor are they consistent. Since you seemed to have checked everything...I suspect your piston is not sliding easily when the mechbox is buttoned up. Take it all apart again. Drop the piston in the mechbox with cylinder and cylinder head. Button it up tight and if you tip the mechbox forwards and back the piston should freely flop back and forwards...if not...remove the piston and sand down the rails on the PISTON. You may have to file the channel deeper too on the piston. Remember...nothing is drop in when it comes to airsoft parts. Some for of fitment it always required. Link to post Share on other sites
ninja master of coffee Posted July 31, 2016 Author Report Share Posted July 31, 2016 Ah, that sounds quite plausible, I've checked for ease of travel with the gearbox open but that won't tell me if the piston rails are too wide. I shall give it a look later. Thanks for the suggestion. I did have a bad experience with a previous SHS piston (it fit too tight on the rails and disintegrated in the gearbox). I shall update once I've re-checked. Link to post Share on other sites
ninja master of coffee Posted July 31, 2016 Author Report Share Posted July 31, 2016 You were correct! SHS piston was quite sticky, swapped the piston head onto the original piston which runs much more smoothly, velocity is now up to 290 FPS with a .25 which clocks in at just about 1 joule. Many thanks for a speedy and accurate diagnosis! Link to post Share on other sites
Guges Mk3 Posted August 7, 2016 Report Share Posted August 7, 2016 Glad I could Assist Link to post Share on other sites
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