Gen 9 M4 - Single fire not working?

I took her apart and now full auto and safety work but single fire; fires as if it’s on full auto. Can’t work out why?

Check the little spring that connects the selector plate and the cut off lever that’s about all I can think of also did u shim the gears when you had it apart sometimes the sector gear can b shimmed too high which doesn’t allow enough friction for the cut off lever to work as it should


So there’s that little leaver that should catch the selector gear after one full rotation right? When I spin all the gears with the case off and have it in semi mode the gears are still able to spin free. Would that mean the little leaver is not putting enough pressure on the selector gear? All the gears are stock so no shimming done yet. Thanks

The cutoff lever won’t stop the gears from spinning itself.

What happens is that the sector gear hits the cutoff lever during its rotation (that cone shaped bit on the bottom of it is what makes contact with the lever) which releases the trigger contacts from touching thus stopping the power input and stopping the gears from rotating (hope that made sense)

Edit: The selector plate pushes the lever in place using the spring (blue) then the sector gear trips the lever (orange) which removes the trigger contact from play (Red)

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Okay that makes perfect sense! So when the little nub on the sector gear hits the cutoff lever it should move. That wasn’t happening so there’s either no contact between the two or the lever is stiff. Interesting…

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The only way to really test it while the gearbox is open is to hold the selector plate halfway in (semi mode) and hold the trigger down while cycling the gears to see if the trigger plates eventually separate on their own… That would be real hard to do with only 2 hands and an open gearbox…

You can try only using the main pieces in question (i.e. gears, trigger/cutoff piece/selector spring) and you could even temporarily remove the cylinder to make it easier to see/access the rest of it.
Then close the gearbox before holding the selector plate and trigger in place to test manually cycling the gears (you should hear/feel the cutoff if it worked)

Maybe a photo of the inside without the sector gear in the way might show us something we can point out for you (hard to imagine what’s there sometimes :stuck_out_tongue:)

You are right about it knocking the trigger trolley off but the friction does stop the gear from further rotation keeping the piston in a forward position.

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Indeed, I just didn’t want Dave to think the cutoff lever itself locks the gears in place :wink:
Edit: Wow, wink face comes across so creepy sometimes…