F2 Low fps issue

[bigtom]

I recently built a full custom HPA M4 but ive been having an issue where my fps is really low? Im shooting around 325fps with 0.2s at 120psi with a 290mm 6.02 barrel. Ive replaced just about every damn part in this gun trying to fix this issue including the inner barrel, bucking, hop unit, nozzle sleeve (I have a gate ion hop unit that comes with a nozzle sleeve for HPA. Ive checked the alignment about 8 times and shimmed the gearbox along with taking it apart and cleaning and lubricating it fully 3 times. Ive also messed around with the FCU a lot and even with the DP as high as it goes im shooting low fps. Ive triple checked the air seal and there isnt any air leaking at all between the inner barrel and the engine. I had a small air leak from the IGL but I removed it and tightened it more along with adding teflon tape on the threads and all around the outside of the igl to stop the leak. I really dont know what it could be now ive tried replacing literally every part except the engine itself and the IGL ive tried multiple tanks and lines, different fcu settings and at this point i dont know what to look at next. Any help would be greatly appreciated.

Edited Wednesday at 12:30 by bigtom
adding some info

[Adolf Hamster]

Some things i can think that you haven't specified.

 

First is the hop setting, being too far off can give weird shenanigans with fps dropping off to the point the bb can almost be just rolling out the barrel. Likewise too far on you can hit a second "sweet spot" where the bb is being massively restricted but spun so much it flies anyway.

 

I typically aim for visually ~1-1.5mm of protrusion into the barrel (flatter buckings like ml macaronss you want less than standard buckings) as a vague ballpark when actually setting it on a range isn't viable.

 

Second is midcap syndrome, if you're trying on a fully stuffed mag or one with a larger capacity (ie bigger than ~140 rounds for a standard m4 midcap) then the first few shots can fail to feed well due to the pressure on the bb stack, this will typically cure itself as the mag clears. A good test is to try with a hicap.

 

The third is that there might be a restriction somewhere, from memory on a ~250mm barrel i was running barely 20psi to get 1j, had to swap out to the low flow poppet so i could run it at ~80psi at 1j with the higher pressure helping it cycle. A kinked line or crap somewhere blocking a port.  I'd be double checking to be sure the ptfe added to the igl isn't covering the end or had a peice blow into the system.

 

Final thing is it'd be worth checking if you're running in closed or open bolt mode. i think it's dr is the one that controls how long it waits for the nozzle to push the round into the chamber before sending the main air blast. The manual tells you its something like 14ms without any restriction so you might wanna raise it above that (or just max it out) see if it does anything. Thats unlikely to be the culprit specifically for semi auto when in closed bolt mode as the round is starting already in the chamber.