Older models, pre-smooth twist, are super sensitive to hammer spring and air pressure. Too much air pressure (200 bar fill) and it won't cycle unless the hammer spring is strong (if the hammer doesn't hit the valve hard enough against the high pressure it doesn't let enough air out to operate the blow back). Once the pressure drops, the volume of air is there but it can't make it through the bushing on the end of the barrel and there is not enough pressure now to push against the heavy hammer spring, and it starts triple firing because the hammer is not getting over the edge of the sear (the blow back pushes the action back enough to cycle a pellet but not enough to set the hammer, so it just flies forward and hits the valve again only to repeat). So, it needs a lighter spring setting at lower pressures.
One must set the hammer spring to a soft enough tension that the action will set the hammer properly at modest pressures, which means the gun won't semi-auto at higher pressures. This means the gun must be filled to only that pressure where the action will cycle. The solution is to install a regulator so that high pressure can be in the tube for shot count, but low enough pressure at the valve so the hammer spring doesn't have to be too strong.
Lane is the only regulator I know that will work on Monsoon. The Altaros regulator for FX is designed to seal against the airtube mount block that attaches to the breech. The Monsoon airtube mounting block has a big screw going through it to hold it to the breech block, which interferes with the oring on the regulator. The Lane regulator needs a breath hole drilled in the airtube.
What this means is that the gun becomes a 25/26 fpe gun with a decent shot count.
Single biggest reason a previously cycling gun stops cycling: lubrication. The part that slides in the shroud and the hammer, hammer follower, and bolt. Rem Oil is the thinnest I've found. Use it liberally.
Fill poppet (J4) -005 Polyurethane 90 Durometer O-Ring from www.captainoring.com
Fill Probe and breach seal: -009 Polyurethane 90 Durometer O-Rings from www.captainoring.com
The valve is expected to seal the high pressure side from only either (or a combination thereof): the valve cap to the valve offset (the block that the valve screws into that is then bolted to the breach block), the bottom edge of the valve (what is exposed around the valve cap) or the threads of the valve itself. I tried sealing the threads, but that doesn't work so well. What did work is I make a small gasket punched from a thin piece of rubber using two different sizes of brass tube, 5/8" and 7/16". This is the thin wall brass tube you can get at Ace or a hobby store. K&S seems to be the standard brand. Wipe both sides of the gasket with Super Lube.
To remove and re-install K1, Reloading Rod, unscrew and re-screw rod and D4, Brass Bushing, with barrel removed. After screwed togehter, install barrel and slip spring over as it goes on.
To change breech seal:
Remove 4 screws that hold barrel in, remove barrel. Insert oring into groove at one spot. Fold seal over and install opposite side into groove. Use an allen wrench to push one side that is sticking out down into the groove, then the other side. Smooth seal into groove all the way around.
To re-install barrel, remove valve and use a 3mm allen wrench through bottom of breech block and through transfer port of barrel.