why low fps on full auto?
 

modified the gun to a high ROF setting at approx. 40 rnd/s. it shoots good at semi at a very steady fps around 400fps. however, when shooting full auto, the fps drops to below 300 fps.

plz advice what could cause this issue? and what problem should i look into. thanks very much in advance!

And you confirmed this with a chrony that can measure FPS of full auto fire?

yes sir...

Airleak might cause the problem

Just a guess, but is your high-RoF setup possibly short-stroking the piston?

What gun? On some platforms, the high ROF will not allow sufficient time for the tappet plate to push forwards to create a good airseal before beginning the next cycle.

^ that
Some aftermarket tappet plates also fit like crap and are too tight.
Also, your tappet plate my be too flexible
A more rigid tappet would be best for high speed application

Are you having any feeding issues?

if it's airleak, the gun should have low fps on semi either.

nope, no modification to the piston.

it's a AK74U, V3 gearbox. the gun was at high ROF and had no issue before i replaced the bushings lately.

Modify your tappet return spring to make it stronger. Also, loom where your tappet rides to make sure there are no obstructions causing it to slow its return time.

i have no feeding issues. the thing is the gun was modified to high ROF at 45rnd/s previously and had no issues at all. somehow, the bushing cracked during the last game, after i replaced the bushing and upgraded the spring to m110, with other parts remained the same set up, the mentioned problem came up... eh...this is really frustrated!

Tappet plate is a good place to start, but since it hasn't been mentioned.

Barrel length vs Cylinder type is an issue where unported cylinder heads are concerned, the suction action of the compression assembly can rapidly reduce the pressure in the barrel leading to a moment of suction and extreme turbulence while the BB is traveling down the barrel.

Ensure your using a ported cylinder head and that the ports are unobstructed by grease, check the oring on your cylinder head and ensure that it has room to move forward and back allowing clearance for air flow... if its swollen and cant move it cant function properly.

Spring strength has nothing to do with this. By the time the piston is released, the nozzle has long since returned into the hopup unit/rubber.

then what would you suggest the problem could be?