SpudFiles

so this week i put my sprinkler valve cannon on a diet and cut down the chamber to half its size and i also gave it a shorter barrel. it worked perfectly before now it honks and is less than stellar. i took it apart and didn't find any debris in the valve and i have modified the blowgun about as much as possible and there is no hos connecting it. i am confused as to why it worked perfectly and now it sucks.

Apparently chamber size is an issue. I don't personally know, but I think that is your problem.

is there a minimum chamber size to be used with sprinkler valves? this cant be, because i know i have seen smaller chambers on sprinkler valve cannons.

I'd like to see a picture of your cannon, maybe that it will help discovering what's wrong.
What pressure are you using? Did you try different pressures? And does the ammo have any effect on the honking (how is it without or with ammo?)

Many sprinkler valves require a minimum pressure to seal, technically with a smaller chamber and same filling method you should be reaching this pressure quicker

so i have used different pressures and seen that the honking starts to quiet down the higher the pressure i use. it goes completely away at about 90 psi with ammo. i would like to be able to use it at lower pressures though. i have modded the blowgun even further, to the point where it cant be modded any further. it seals fine. im thinking i need to make the pilot hole smaller, but i don't know what method to use. i don't think epoxy will stick very well to the rubber diaphragm. here are some pictures of the gun also.

There are several ways to reduce the size of the pilot. Using epoxy to fill it in is an excellent idea. You have several options, 1 is to plug it completly and then re drill it with a printed circuit drill bit (smaller than normal bits) Either numbered bits for the US or metric bits work fine. I use either a 0.8mm or a #58 bit. I have both.

Another option is to wax a thread of wire by dipping it in melted candle wax and placing it through the pilot before adding the epoxy. After the epoxy is fully cured, heat the valve in a pan of hot water to soften or melt the wax and remove the wire.

The pilot size is often not the cause of the honk. The cause of the honk is the chamber pressure falls much faster than the pilot so the pilot area pressure then closes the valve several times to the chamber pressure drops in steps. This problem is compounded by the length of plumbing and elbows in a small size between the chamber and the valve. I recommend the chamber and elbows all be the same size and then reduce at the valve to fix the plumbing problem between the chamber and valve. (this increases the pressure to the valve for more power) This mod will increase the strength of the launcher and reduce the chance it will break.

The fix is either a larger chamber and plumbing restriction removal so the pressure drops slower, or a larger pilot valve so the pilot pressure drops faster. Try a 1/4 inch ball valve instead of the blow gun or try one of the blowgun modifications.

Check the brass fitting to make sure it is not coming into contact with the diaphragm. Most sprinkler valves taper and when one taps into the side instead of the center of the lid, this problem is often overlooked if the fitting is turned in too much.

I used the plastic stick from a lollypop once..if it fits in it's perfect..
Heat up the ends and they swell up, sealing it into the hole.