Triumph Gearbox Woes

1986 Bonneville had a broken mainshaft. Dont know why, hadnt been overly thrashed ,no wheelstands, but do like to wind on the throttle. Looked to be broken half way through for some time and then finally let go. Broke at the first shoulder from clutch end. My dilemma is after rebuild i cannot engage 4th and 5th gears when clutch is used, something seems to jam. When taken for test ride gears will engage if I dont use clutch. On the bench I can get all gears without using clutch, as soon as I use clutch cant get top two gears. Everthing appears to be in good nick visually . 20 thous kilometres . Any ideas please.

Primary chain being tight can sometimes cause odd clutch-related shifting problems like this.

Otherwise, don't know Trumpy gearboxes at all, hopefully someone else does.
Good Luck.

Disengaging the clutch pushes the mainshaft over to the drive side.

First easy check is the tightness of the nut on the timing side end of the mainshaft, under the gearbox outer casing. If that's loose (not rare to find), the shaft gets pushed further to the left when you squeeze the clutch lever, which I think can affect at least top gear engagement. I can't remember which direction the dogs for the other gears go.

Also, ask these guys: http://www.britbike.com/forums/ubbthreads.php?ubb=postlist&Board=4

The whole cluster needs to come out and be inspected carefully. Source a good new mainshaft, and replace all the bearings and bushings, then re-assemble carefully. My guess is that you will find yourself missing at least one part when compared to the parts diagram in the parts book.

Primary chain not too tight. Trumpy gearboxes similar operation to commandos with camplate and selector quadrant. Mainshaft nut definately tight. Have inspected carefully and do have a slight wear mark on one of the selector forks. Will dismantle again. Brand new mainshaft and layshaft was tested for trueness. Back to the shed.

Do a camplate / index plunger assembly & check .
Burrs or distorted camplate , etc possable .
The with selectors but no gears .
The action should have NO binding / tightness .

Straight edge , a square & selectors on shaft to check if twisted / bent .

Check that both rollers are on the pins where the selector forks fit into the cam plate .

So after many wasted hours I found I had washer on mainshaft on inside of inner cover instead of against bearing on outerside of cover. All is working good now, thanks for the replys.