My gun has low fps! What do I do??

[ThunderCactus]

*Sticky please!

Making a FAQ since we seem to do this really often and it's always the exact same diagnosing process. If you've got other methods to diagnose generic air leak issues, post them up and I'll throw them in here!

Firstly, if you know how to open a mechbox, doing a compression test and testing the tappet plate functionality will quickly determine if it's an internal mechbox issue. Be sure to do the compression test with the air nozzle in the FORWARD position. Having the air nozzle in the rear position will bypass a possible leak.

Secondly, USE GOOD AMMO. You wouldn't buy a new car and put the lowest quality/cheapest gas available in it, so don't use crap ammo in your gun! Don't use .12g BBs. I've seen immediate 10fps gains between polished and unpolished ammo. I've seen BB's that work in everyone's gun jam someone else's. I've seen BB's that have traditionally been very good all of a sudden have a really shitty batch. And Bio BB's deteriorate in wet or humid conditions.

Things that generally cause air leaks or low fps

Dried or undersize piston head O-ring
Common in guns that have been stored for a long time, also possible on new guns. Also happens when someone uses a grease with petroleum in it. Replace the O-ring.

Damaged piston head
Obviously more likely to happen in well used guns, but can happen to any gun. More common on G&P's with their "explosive" (that's the actual name of them) piston heads.

Cylinder damage
VERY common with PTW's. dents, scratches, abrasion, oblong construction. Anything that would cause a smooth surface to lose seal. Can happen in heavily used guns when grime, gravel dust, etc gets into the cylinder.

Over/under greasing
Common with new guns. Too much grease will cause compression failure, not enough will create a poor seal. A thin film of grease is enough, I generally use a small pea worth of grease and wipe the inside with my finger. The piston head will pickup it's grease from the cylinder after a few strokes. Grease in the piston head air holes will also cause compression failure.

Broken cylinder head
Most common issue in stock KWA guns as they use a poorly made 2 piece cylinder head, and with guns that have long air nozzles or require moving the air nozzle in and out of something during take down. If they don't break under normal operation, someone has bumped it against something and broken it.

Loose cylinder head
Uncommon but possible. Typically when using mismatching parts. The cylinder head is too loose in the cylinder causing a leak. Corrected by wrapping it with teflon tape or using a new O-ring. If you require more than 3 layers of teflon tape, you need a new cylinder or cylinder head.

Sticky/broken tappet plate
Either it's over sized, wrong dimensions, the springs broken or stretched, the sector gear has been shimmed too high and it clamping it in place, or it's just plain broken. This will prevent your air nozzle from moving. Best diagnosis for sticky plates is to just see if it moves back and forth freely when you close the mechbox. For broken plates, the air nozzle will stay forward and not react to the position of the sector gear.

Air nozzle leaks
Also possible in any gun. Air nozzle with no O-ring generally cause air leaks, except on the KWA where they're very closely fitting to the cylinder head. It's not uncommon to see very consistent 30-90fps air leaks from a leak between cyl head and air nozzle.
Make sure you have the CORRECT air nozzle. Not all M4's take M4 air nozzles, some are proprietary. The easiest way is to visually compare against your stock nozzle, however even a .01"/.25mm difference in length can cause issues.
Even if your air nozzle has an O-ring, it can still leak. They are a tricky thing, I usually go through 3 nozzles of the same company before I find a perfect one, and this is usually where the small air bleed exists that allows the piston to creep forward in the compression test.

Piston is the wrong size
You may have bought a new piston not knowing that it's either inferior quality or your mechbox requires a proprietary piston. If it's jamming up in the piston rails it will move forward much slower than normal and cause low fps. When you close the mechbox, run a long allen key or thin screwdriver through the air nozzle to push the piston back. Move back and forth and there should be no delay in the piston moving around. You'll definitely notice if it's sticking.


Outside the mechbox

Air nozzle alignment
Typically an issue after someone has worked on a gun. With a flashlight, either shine down the barrel and look for light coming through the hop chamber, or look right down the barrel (it's tricky but possible) and see if the air nozzle is aligned.

Dirty barrel
Believe it or not, I've charged people to clean their barrel before. Try to clean it after every game. Fouling occurs in every type of airsoft gun, from dirty BB's, grease being gradually sprayed out of the cylinder, and dust or dirt in the air or in your mags.
Not only can it cause jams if it gets bad enough, but it also causes accuracy loses early on.

Hop rubber
Make sure it's not on all the way. And actually make sure it's not OFF.
Surprising to most people, but the hop rubber is what keeps the BB in place once it's loaded, so if the hop is off and the air nozzle pops a round in, it could potentially have rolled down the barrel just a bit and that would cause very low fps since there's now a large volume of air between the air nozzle and BB.
Hop rubber could be torn and jamming the BB's a bit.
The sealing lips at the back of the rubber could be torn, warped, or otherwise damaged.
Also surprising you could be losing some air between the hop rubber and barrel, although typically not very much air.

Hop chamber
The hop chamber could be damaged, warped, mounted incorrectly. It could be the wrong hop chamber type if it's a replacement.
With M4's make sure you have the spring above the barrel to help it push back into the mechbox.

Mechbox alignment
Uncommon but can happen in cheaper, heavily used, damaged, or especially plastic guns (STAR M249). Sometimes you have to shim the mechbox into the proper position if the body warps.

Defective construction
Specific to certain guns. For example the CA249 is well known to have a crap chamber. The gun will shoot great then one day develop a 60fps air leak that no gun doc can figure out. It's usually a hop chamber issue.

Gun damage
Body cracks, breaks, having someone step on your gun. Anything that would cause the body to change dimensions. Sometimes the cracks aren't very visible.

[cannonballer1985]

air is leaking out around the hop up unit try the tape used to seal threads for pluming. it's only good for a temperary, as that is what i am doing with my p90 till i can get a new hop up unit