Last week I received the Coyote deflators, and today I unpacked them to calibrate and try 'em out. Here they are - recall that Harry also sent me an extra set of softer (silver) springs to try out.
First order of business was to swap the springs. Pull the clip, swap the spring, and reinstall the clip. Easy peasy.
This is the setup I use to calibrate my deflators. It works well as I can watch the pressure change in real time, and also observe when the deflator starts and stops. A simple twist on the deflator cap easily sets the pressure.
The first deflator I did went well, and after a little tweaking was deflating consistently to 10 psi + or - a half pound or so.
But the second deflator gave me trouble right off the bat. With the tank at 25, the deflator would not start. Pulling the pin also did nothing - even when unscrewing the cap nearly all the way (which would be the lowest possible pressure setting). I tried a third deflator. It did start to deflate at 25, but lower than about 20 psi it would not start - again, even when pulling the pin. It also did not stop - it went all the way to zero. I tried this several times varying the cap pressure setting to both extremes with the same result.
So I pulled it apart to see if I could understand what was happening. The Coyote design employs a piston with an O-ring (center, below) to regulate the air flow. The piston slides back and forth in the cylinder (on the right in the picture below), and when the spring exerts enough pressure on the piston it presses a rubber seal (on top of the piston, not visible in this picture) against an orifice at the top of the cylinder, stopping the air flow. When there is enough air pressure against the piston (i.e. - when the pressure in the tire exceeds the pressure setting of the deflator), the piston slides back and allows air to vent through the orifice and out of three holes at the center of the cylinder.
The problem seems to be that the cylinder and o-ring is too tight in the bore to allow the piston to move back and forth freely, meaning it will not start deflating unless there is very high pressure (something over 25), and if it does begin deflating and gets down close to the desired air pressure the spring pressure is not sufficient to move the piston back into the closed position - and it deflates to zero.
Here is the piston in the cylinder: The pin with the spring presses against the divot at the top of the cylinder, pressing it against the orifice as stated above. But notice that the pin does not connect to the piston, so when you pull the pin to start the deflation, all it does it remove pressure from the piston. If the piston does not move, pulling the pin does nothing (hence the reason pulling the pin failed to start the deflation).
And here is the impact of all of this: Notice in the picture below that I have completely removed the cap, pin and spring, so there is nothing creating pressure against, or containing, the piston. And even with 23.6 psi in the tank, it is not enough to move the piston to allow the deflation to start. When I shot a bunch of pressure at it, the piston does shoot out of the cylinder into my hand.
I tried cleaning the cylinder and the piston, and well as lubricating the O-ring with light silicone O-ring lube. No change in the operation. I also reinstalled the original springs with the same result.
Contrast the design of the Staun's in the picture below. The O-ring at the end of the pin creates the seal when pressure is applied by the spring. Since the piston and the pin are one piece, pulling the pin always starts the deflation. Also, the pin/piston are not a tight fit in the cylinder bore (as is the case with the Coyote), and movement is constrained only by opposing spring and air pressure. The Coyotes also have to overcome the friction pressure of the O-ring on the piston - and I believe this is what is causing the problem.
So what am I missing here? I'll give Harry a call on Monday to get his thoughts. Perhaps I received a bad set with an improperly sized O-rings, or bad machining of the piston or the cylinder? No sure. All I know at this point is these do not work predictably at all.
As I sit here typing this, it occurs to me that I may try removing the O-ring from the piston and see what happens. This O-ring does not create a necessary seal - as the air flow is controlled entirely by the seal on top of the piston engaging with the orifice - but it definitely constrains the movement of the piston in the cylinder.
I'm disappointed thus far, but perhaps it is something simple.
