Gearbox Advice sought - 3rd/4th jamming

Hello Malcolm - Have you tried rotating the final drive sprocket a little whilst selecting the next gear ? Sometimes as the forks slide the gears along the shafts the dogs can't engage in the appropriate slots because of their relative positions.  If that doesn't work I should  afix the inner cover to the mainshell with a nut or two.  You can move the camplate by putting a small ring spanner on the retaining nut between the engine plates and applying gentle pressure to select the gears.  If that also doesn't help remove the inner cover and gently slide out the bottom gear pair keeping inward pressure on the two shafts. You may then also gently slide out mainshaft second gear.  Now you may select the gear position on the camplate with the spanner and ensure that the  top and third gears engage.  It is possible that you have a fork in the wrong position or both forks in the same channel but persevere and all will become clear.  The open 'C' of each fork are adjacent central on the selector spindle.  You will need to keep pressure on the layshaft as you rotate the final drive sprocket to keep it hard in the drive-side bearing.  Good luck, Howard 

Howard's comment about keeping pressure the layshaft should be in bold. If it slips out of its bearing it will drop and - if your not careful - the 4th gear wheel will drop off. It is an awful faff trying to get it back on without dismantling the whole thing.

I know......................!!

George

Hi Malcolm

I'm having a similar issue, how did you resolve your selection problem?

Regards

Mark

I would suggest that during the rebuild the selector arms were not inserted correctly into the camplate slots.

You need to have this plate exactly in the middle of gear selection when replacing the selector arms and then insert the the rod these slide along. Many people make the mistake of having first or 4th gear selected when they replace all the chunks and end up finding 4th has disappeared when checking the gear selection process.. You only select 4th when replacing the outer cover and inserting the gear rachet and its springs back in position.

I find that mounting the gearbox vertical, in a vice or clamp, helps to get the various bits back in their correct positions. If you try and do the rebuild with a horizontal box the selector arms keep dropping away from the camplate.

When asked to diagnose why a featherbed twin wouldn't select top gear i eventually discovered that the gearbox case had been welded up after a layshaft bearing failure. The repair was done quite competently except for the fact that the weld metal had intruded into the layshaft bearing location. Not enough to prevent the inner cover fitting and the gearbox rebuilding, but just enough to move the layshaft, hence third gear getting fully engaged which then then stops the selector in its track.

All morning the find the cause. Ten minutes to put right, and another ten minutes to reassemble the gearbox. All done with the box still in the bike.

Another thing to look out for that i came across when sorting out a MK3 box that wouldn't select any gears apart from 1st is that the camplate is not very strong and is easily distorted if mishandled. 

Look up Old Britts Norton, Technical articals it will give you all the information plus pictures.

Covers everything, only an idiot could go wrong, or a blind man.

I used the clubs gear box rebuild CD so many clever tips especially drifting bushes out.