Working on a KWA M4A1.
 

It needed a new cylinder head. Replaced that. Piston oring wasn't sealing, so I replaced that. Put everything back together, test fired, and the tappet plate broke. Well crap.

Took it apart and replaced the tappet. Double checked everything, good seal on the piston and all that. Put it all back together and test fired... shooting 150fps.

The thing is... if I turn the gun upside down... FPS will jump to 390. I'm using Green Devil BBs and a midcap.

I've never seen this before. Any suggestions?

have you check the hop up?

Tried with hopup all the way on, all the way off, and in between. No difference :(

Try checking the lower hop-up feeding tube. Often that will get mis-aligned and cause the FPS to drop like that.

Oooh good tip, will check tomorrow and report back. Thanks fellas.

It appears to be okay, and is nice and tight. Wtf :(

Any other suggestions?

I would suggest trying a different mag. but whatever the issue is, it has to be something that is damaged/loose. maybe something with the nozzle, it could be something to do with the hopup assembly (right side up something getting in the way, upside down it moves out of the way kinda thing...) check the hopup chamber for obstructions.

Did you use the proper V2 cylinder head?

Yeah, OEM replacement.

Try swapping out the feed tube with a different one, I had a gun where that fixed it not to long ago, no idea why it went screwy, but its worth a shot.

Also, check the nozzle to make sure its moving its full travel.