CA P90 Full Auto Only

[daishi]

So last night at a game my p90 started to fire in bursts, before firing only on full auto regardless of the selector switch.

The selector isnt broken as I took apart the gun and the metal sliding bar moves in the correct way, but when I move the small metal switch on the outside of the gear box or the plastic black nub that moves when you pull the trigger....it always fires in full auto.

I have never worked on an AEG as I own gas guns, so I have no idea what to look for or what might be the cause....

[lurkingknight]

the cutoff lever is worn... the only aftermarket one is the prometheus one. It's expensive, and it will also eventually wear out.

[daishi]

How can I diagnose if that is indeed the problem before I buy one?

[lurkingknight]

On the p90 there's the upper trigger contacts and the lower trigger contacts. The upper is what fires in semi, when you have the selector in semi, the bar does not allow the trigger to be pulled further than the 1st half of the pull. When you pull the trigger, that upper shuttle works like a v2/3 box where it pulls the shuttle into the trigger gate. when the gun cycles the sector flips the cutoff lever and the lever flicks the shuttle out of the gate and the spring resets it back to the original position.

In full auto selected, the trigger will allow you to half pull for a semi shot, and if you keep pulling the lower trigger block is connected.

With the gearbox out of the gun and no battery pull the trigger back so the upper trigger shuttle moves forward. With your push the cutoff arm upwards until you see the trigger shuttle reset. Take note of how far the arm has to come up to reset the shuttle.

Now plug in the battery and pull the trigger so the gun fires in semi, (not all the way back that the rear contacts close the circuit) It might be hard to see depending on how worn the lever is, but if it's not working well, the cutoff will just nudge but not rise far enough to hit the shuttle.

To test further, you can very carefully bend the arm upwards so the little piece flicks the trigger shuttle sooner... what's happened is that the cam on the sector gear has ground the hump on the cutoff flat... so it's not moving as much. If you bend it up and it starts working in semi again, then the cutoff is worn, bending it up extends the life a little bit at your stage of wear, but it will stop working again soon enough.

In other gearboxes when you switch to full auto, the cutoff lever gets pulled out of the way so the sector doesn't touch the cutoff, in a v6 the sector is always grinding away at the cutoff lever so it will wear much faster than any other gun.


Also when you check how much the cutoff lever needs to come up to flip the shuttle out, feel how hard it is to push the lever out of the prongs. I have a feeling you can extend the life of a new lever if you bend the prongs open a little bit, so that they're just close enough to make contact with the shuttle, but not tight enough that they hang onto it. This way it requires just a nudge from the arm to break the circuit in turn requiring less force/friction at the sector gear side of the lever... because if you look at the way it's set up... it's s the worst way to use a lever... the activation side is so close to the fulcrum it requires a lot of energy to lift anything on the far end of the lever.

I've been on the same prometheus cutoff lever for about 30k rounds now, while others report their prommy levers are wearing very quickly.

[daishi]

Well I guess it is the lever...since it doesn't move at all. And moving it doesn't do anything to the firing...and pulling the trigger always results in full auto no matter what.

[lurkingknight]

well it's tricky timing the lever has to come up as the shuttle is making contact with the trigger gate. Depending on the speed of your gears/cycle speed you might not even see it move even when it is working so it's not a perfect test. Even if it's not the lever, every p90 owner will facing having to replace it at least once or twice during their ownership depending how much they use it.

But if you can confirm visually that when you pull the trigger just to the semi point of firing that it keeps firing full auto, the cutoff is the first place to start. If you know the lever is good then it's not bumping the shuttle out of the prongs so the trigger prongs are holding the shuttle too tight.