Random FPS Drops

[madmogga]

 

Hoping someone can help me on this one, I purchased a G&G P90 (PDW99L long barrel version ), the gun had lots of issues when I got it, most have all now been fixed but one problem I still can not work out.

 

Basically I get a random drop in FPS on semi auto. the gun is currently running 380 fps but once or twice in about every 10 shots the fps drops down to 200. I have in the course of the rebuild put on a new cylinder head/ new piston and new nozzle. I have tested for air leaks and cannot find any, also this does not happen on full auto and when the gun shoots correctly the FPS is within 3-4 fps so quite consistent also when it drops the low fps is very consistent at 200.  Its not double feeding as far as I can tell, and I have replaced the hop/bucking and nub still the same. I have low/mid and hi cap mags all of which cause the same problem however if I drop just one bb in the hop at a time the issue does not happen. The mags are feeding ok as I have no miss feeds and I get a bb shot every time. now if I put 2/3 bb’s in the hop and put a small amount of pressure on the bb’s (like the spring from the mags)  then I get the FPS drop, so I am thinking that the next bb waiting to go into the hop might be pushing the nozzle out of alignment somehow? but I cannot see how as the pressure from the mags are not that great. Either that or it’s a tappet plate issues not returning every now and again but if it was then why when I drop one bb into the hop dose it still not randomly happen?

 

Oh and I am using G&G .20 BB’s

 

Any thoughts or things to try ?