ASG M1 Carbine problem ,help needed please

[dadio]

Right this is an ASG M1 Carbine Co2 powered blowback semi auto rifle , out of the box these are hot and i was given this one to reduce the power to sub 350 fps , so I simply fitted a choke to the nozzle to reduce gas flow , this worked but the knock on effect seems to be that the blowback engine no longer works efficiently , I'm thinking that the reduced gas flow is affecting the rocket valve , it fires perfectly when no BB's are in the mag but load up and 4-5 shot's and it vent's , I'm thinking a softer rocket valve spring might help but it might also reduce the power again ..... any ideas ?