G&P gearbox cycling strangely (video)
 

Hey guys,

(video below)

Helped a friend open his G&P gearbox to inspect for wear & tear, shimming, etc. When we got the gearbox back together, it was firing very strangely, and I'm interested in knowing what we may have messed up or missed while putting this together.

Notes off the top of my head:

1) We tried cycling with several different batteries all freshly charged, same cycling behaviour on all.
2) The gears had to be shimmed a little bit higher to reduce play, which there initially was a lot of (perhaps we need to revert to more play?).
3) Pinion-bevel alignment was checked with the half-gearbox-shell, motor grip attached method. Looked about as perfect as I can imagine it being.
4) Cleaned out some grime on the gears/gearbox shell and off the piston and applied a *very* small amount of Modify gear grease to the cleaned gears.
5) Shimming was checked before putting it back together. Gears rotate freely, with almost no play whatsoever. Shimming was checked individually and together, starting at bevel and adding gears as we went. Sector has clearance over the cutoff lever.
6) Tappet plate was checked before assembly - connected to nozzle, tappet spring connected and lively, etc.
7) The piston is a G&P white, and it was showing some signs of tooth wear right across the entire piston.
8) Basic compression test with finger over cylinder head's nozzle and pushing the piston in seemed pretty good, piston pushes in about half way and stops on a cushion of trapped air. Used ARS O-ring grease after carefully cleaning the o-ring on the piston.
9) Trigger contacts making very good contact, almost *no* residue on the contacts (I'm used to seeing a little bit more when I open my guns up).

Video:

G&P gearbox debugging - YouTube

Any advice would be appreciated. My first instinct would just be to disassemble and very carefully reassemble, and perhaps go easier on shimming.

Your anti reversal latch is not functioning like it's suppose to. Either the spring pinged out of alignment, or the bevel gear is sitting too high for the latch to engage it. Did you also remember to put back the cutoff lever spring? In semi, it seems like the cutoff lever is not resetting the switch assembly which gives you that double trigger pull.

I checked the cutoff lever spring during re-assembly and moved the selector plate back and forth to make sure it was in good shape (is moving the selector plate back and forth when the GB is out ever a bad idea? Should it sit in a given position when re-assembling?).

I think we shimmed the bevel a little bit high off the left side of the shell so with your hints, I'm thinking the ARL seems like a likely suspect to check.

Thank you for your advice.

Also for the double trigger pull, it could be that the switch assembly trolley is worn where it meets the cutoff lever. I've seen it happen where the plastic on the trolley wears away and doesn't allow the cutoff lever to engage it. Check that too.