Beach buggy with gearbox/clutch problems

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Hope someone can help - we have a beach buggy - 1600 twin port motor. This problem has just developed. We have recently replaced the clutch, release bearing and gearbox oil seals (but don't think this is related). Bug has run sweetly for 6 months. Recently (after replacing a snapped clutch cable) it has been jumping out of gear when pulling off, whilst still sliding/releasing the clutch. The clutch initially releases smoothly. Suddenly just before it is properly/completely released, there is a great shudder from the clutch. If released further it is much like a gearbox when missing a gear and then clunking back into gear - except it has just jumped out of gear ! It doesn't feel selector related. On inspection we have found that the lock bolt next to the clutch release lever (on the gearbox) has unscrewed.
Question - what is this bolt holding ? just the shaft from slipping out sideways, or is it locating the shaft and spring mechanism rotationally in a special position ?
Or is it a red herring ? and something else more dire is hiding in the box ?? We are hoping it is a quick fix, because we want to sell the bug.

[Marc]

The bolt passes through a hole in the bushing which supports the operating shaft to locate it laterally. It also holds it in one place rotationally, but that's not important - the shaft has a circlip which limits its leftword motion by contact with the RH end of the bushing. Without the bolt, it's possible for the shaft & bushing to migrate to the left, causing the release bearing to become misaligned with the clutch cover. If it comes out by more than about ½" the RH end may come out of its bushing in the bellhousing, which can result in all manner of mechanical mayhem. If the lever and the metal cup that the return spring encircles are still riding snugly against the end of the bushing bore, things haven't reached a dire point yet - if you can poke around through the bolt hole with a ~¼" diameter rod (Phillips screwdriver works) and make contact with the shaft, the bushing hole is still aligned and the bolt can be reinserted. If it's out of place, tightening the bolt will crush the bushing and cause the shaft to bind. Did you tighten the bolt, and if so was there any resistance? It should run in all the way with no more than thread friction until the head contacts the trans case. If the bushing is damaged you'll need to remove the engine to address its replacement.
A binding shaft may cause clutch "chatter" upon engagement, but it's rarely as violent as what you've described - from this distance it sounds more like you have a loose or broken front transmission mount.

[[email protected]]

Thanks for your reply - all you said is intact. Problem still persists. Interestingly enough my son jacked the motor up from under the shock mount. Whilst doing so he pulled on the exhaust bend to stand up. He noticed the whole motor move up and down ! On further forensic examination, we found that the front gearbox mount (steel bracket) has completely detached from the chassis !! Ha Ha. A small problem for a man and his welder, and a cheap one. Our relief is great - we were imagining stripping gearboxes and stuff. The gears were jumping out (I imagine) because the upward movement (on torque reaction) was extensive enough for the gear linkage to beat against the tunnel, and knock things out of gear ! Thanks Marc for your suggestions. The loose bolt has been re-tightened with no problem - it was only a turn and a half out from picking up again. Ta very much.