After my abortive attempt yesterday to upgrade the MP44 spring (which I'd previously downgraded to a 1J spring which gave a measly 270fps) in which I managed to lose the anti-reverse latch spring
Try the motor, mine is so damn magnetic half my tool box gets stuck to it. 
You'll be pleased to know mine is still doing something similar despite trying everything I can think of- I've never spent so long checking and reshimming a gearbox. Going to get someone who know what they're doing to have a look on sunday as I'm sure it can't just be the spring. 
After my abortive attempt yesterday to upgrade the MP44 spring (which I'd previously downgraded to a 1J spring which gave a measly 270fps) in which I managed to lose the anti-reverse latch spring
Try the motor, mine is so damn magnetic half my tool box gets stuck to it.
I'd suspect an air-seal issue Rich. Could be piston O-ring, cylinder head, nozzle or hop rubber or nozzle to hop rubber seal. hop rubber could be damaged too maybe? I have found in the past that some ventilated piston heads dont give a very good air-seal and changing the O-ring for a stock Marui one has fixed it!
“I wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So don’t think I marched there for this reason – it could carry another name – but because there is a very important goal... this goal I wanted to take – and you know – we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
If there is an air leak so far I've been unable to find it. The piston and cylinder head seem to have a good seal- the piston will push the cylinder head out of the cylinder if I cover the end and push hard enough. The nozzle is a snug fit on the cylinder head and I have tried 2 different hop units with the same results so unless both the hop rubbers are damaged I don't see how it can be that.
The air seal nozzles in them are poor, the point where the BB is fired the nozzle is in the forward position which means all that looseness looses air, the systema M4A1 / M16A2/A3 is the one to use, although 290fps is quite low,
Steve.D
I've split this discussion off from the sten thread...
I have changed the nozzle to a guarder M4 type- also changed the piston head, spring guide and tappet plate. An last time I chronoed it it actually did 230 but that was some dodgy shimming on my part which I've now sorted. I'd guess its doing 250-260 now.
Could be air escaping between the front of the hop unit and inner barrel - seems a common problem with a number of weapons at the moment. Easy fix is some insulation tape around the join (be careful how much you use, it might not fit in the outer barrel). A more elegent fix is heat shrink around the join. I'm going to do this with all my AEGs the next time I open them up and I've found the right size heat shrink!
Jon Steele
1st Sgt, Fox Company, 506th, 101st
OC
I have changed the nozzle to a guarder M4 type- also changed the piston head, spring guide and tappet plate. An last time I chronoed it it actually did 230 but that was some dodgy shimming on my part which I've now sorted.
In theory bad shimming should only really affect ROF and not FPS. Hmmm.... sound like you have already checked most of the obvious things Rich, tis a bit of a mystery 
The hop is spring loaded against the nozzle and gearbox right? so is it possible that the magazine is pulling the hop unit forward slightly and thus causing a lack of seal between the nozzle head and the hop rubber seal? I wonder how it would chrono with no mag in it? just turn it upside down and drop bb's into the hop unit 1 at a time? or am I talking Bo***cks
“I wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So don’t think I marched there for this reason – it could carry another name – but because there is a very important goal... this goal I wanted to take – and you know – we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
asy fix is some insulation tape around the join (be careful how much you use, it might not fit in the outer barrel)
We tried that at Die Glocke (yes that was me) and it made no difference. The hop is glued into the barrle anyway so as far as i can see there is no way for the air to escape that way. As I said I've tried a different hop unit and barrel with no decernable difference; other than the other hop being slightly better.
In theory bad shimming should only really affect ROF and not FPS.
You didn't see how badly shimmed it was. 
Lesson 1- don't do things in a hurry.
It doesn't look like the mag is pushing the hop forward at all, still look like it satys firmly agains the front of the gearbox.
It doesn't look like the mag is pushing the hop forward at all, still look like it satys firmly agains the front of the gearbox.
It was a long shot! Well I'm out of ideas then, other than chuck a bigger spring in it! 
“I wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So don’t think I marched there for this reason – it could carry another name – but because there is a very important goal... this goal I wanted to take – and you know – we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
It seems to have come down to that as I can't think of anything else either. As I said I'll try to get someone else to have a look at it this Sunday to see if they can sus it out. Thanks for the help though. 
did you put the cylinder in the right way? meaning the hole in the side of the cylinder is to the back.
The cylinder isn't ported. 
Don't know if this will help but when i made my sten gun i used a MP40 gearbox and hop up..When i cut of the barrel for it to fit the gun it lost fps like crazy but everything was fine before i did it..Could it be as simple as barrel length?
I have some lose.half baked idea in my head that high Rof and a short barrel makes for lousy fps..
