AEG Issues
 

So I have a VFC KAC PDW AEG and I'm trying to modify it. It is shooting perfectly fine and everything, however I can't get it to stay at the desired FPS. I originally got it to shoot at the FPS I wanted (375 AVG) and used it for a couple weeks. After these couple weeks, I chromo'd it again and it was shooting 340 FPS AVG. I have no idea what to do for keeping the FPS at 375 AVG consistent, any opinions? (It was shooting 325FPS with an M110, then upgraded the piston head o ring to a silicon 70D o-ring, tappet plate sanding and notch in fin, and M120 and then that is how I got it to the 375 AVG, sealing before putting together was great)

Parts List:
Gate Titan
Tienly 30K motor
SHS Aluminum O Ring nozzle
SHS Double O Ring Cylinder Head (Teflon Taped & Sorbo + Neo)
SHS Ported Cylinder (Port roughly 3/4 or so)
SHS Ported Aluminum Piston Head (New 70D Silicon O Ring)
SHS Steel Tooth Piston
SHS 13:1 Gears (SS'd 2 teeth, Sector Delayer)
Guarder Tappet Plate (Front Sanded & Find Notched)
Guarder M120 Spring
VFC Bearing Spring Guide

ProWin Hop Up
Prommy Purple Flat Hop'd
Prommy Flat Nub
Prommy 6.03 275mm Barrel

Only thing I could assume would be the O ring, but I am not having any consistency issues, but then again I'm not a fancy tech. Also, maybe grease, but also could be a wrong assumption. Hoping someone could help me out.

Silicone makes a poor piston head oring. It’s too soft. It wears prematurely.

I'm thinking of getting a PTFE O Ring to try, but also buy a couple X-Rings, Viton & Buna-N also. However, wanted to make sure that it was the o ring before I bought them. I just find it odd how there isn't much inconsistency, but FPS went down by 30 in two weeks in two games.

get viton and use teflon grease. Double check the compression.
temp can also affect fps greatly depending on the grease you're using

Parts will also settle down a bit after use. That’s quite a bit after just a few games though.

Yea, I know it would settle down, but figured that be only like 5-10 FPS, not 30 lol

I’ll get a bunch of o rings to test, including the ones you stated, and get some superlube silicone grease (with PTFE). Hopefully that may solve it. Or would you recommend different grease?

How is the grip with the prowin hopup and the bucking? Can the hopup unit move freely?

I also had issues with my prowin before, I switched to a Lonex one after.

I purchased a VFC Rotary Hop as I heard they are good... Prowin doesn't move freely, up right against the gearbox.

You put all this tech work in but did you do a compression test by many chance? There's a lot of talk about the main piston o-ring but did you check the compression of the cylinder head and the nozzle?

Afaik every tech including myself has considered the Teflon tape mod as a temporary fix. The tape will eventually degrade or flatten with exposure to oil and that might be the cause of the air leak where you installed it on the cylinder.

Yea, I always do a compression test right before I put my guns back together. Nozzle was at fully extended position and test was done and was flawless.

As for the Teflon mod, I just did it as a precaution, but I always do it anyways. The cylinder head by itself with no Teflon is a really tight fit so don’t think it would be there.

Nozzle?

Rip in the bucking?

How’s your selector switch? Is it tight? Weird as it sounds? I’ve gotten a gun that was like that. Maybe a little more sporadic but noticed the selector was tight. After upon examination I noticed whoever shimmed the gears had it tight and the selector plate was pushing on the bushing for the spur gear. Removed a shim and sanded down the selector plate for clearance and it solved the problem. If you’re getting decent contact though and it’s all good there it’s probbaly the bucking or maybe the Hop Up is on too much :P is check the nozzle.

did you use the Teflon tape and dental floss on the bucking as well? with a flathopped bucking, there will likely be some issues without these mods as there will most likely be air escaping between the bucking and barrel when hop is applied.

Don't think there is a rip in the bucking, and nozzle should be in perfect contact with bucking... Shined a light in the mag tube of hop up when nozzle was all the way forward and no light was seen at and of barrel. Selector switch is not tight, switches perfectly and easily

I did not dental floss the bucking as I didn't know how to do it correctly until recently, however the bucking is teflon tape'd. Maybe this is it... But sounds strange for it to drop FPS so quickly then not continue to drop. Why would it be perfect then lower after a couple games? Did air just start to find its way out?