Burman BA gearbox 4th gear issue

[Peter.Greene]

Hello Team,

I have an issue with a BA Burman gearbox, Out on a run today everything going well when 4th gear decided not to engage, It ran through the other gears perfectly up and down but when trying to select 4th it seems to engage slightly then disengage leaving me to limp home in third, Any hints would be greatly appreciated.

[nevhunter]

Top is direct and something must have changed to have the dogs not engaging. Have you repositioned the gear lever? It may be obstructed by the footrest, which may have moved also if you have leaned it on a kerb. Next trick . remove the outer cover and see if the gears select by directly rotating the selector drum by the little gear with the dot on it. Have the bike on the rear stand and you might have to rotate the rear wheel back and forwards a bit. Nev

[johnwalsh]

Sleeve bushes on the main shaft coming adrift can cause 4th selection problems.

[john.whiting]

I suggest you dismantle the box and find out what is happening.I often see BA and CP boxes where fourth has been partly engaged,with the ends of the gear teeth bevelled off.The design is such that tricks such as backgrinding cant be used.If the main gear and slider are still ok,you can eliminate sources of endfloat that are allowing the gears to partially engage.We must be getting close to a no more spares situation with the BAs.There is plenty of CP stuff about,thanks to WW2 production.Worn fork grooves,fork pins,endfloat in the selector shaft,and wear in the detent grooves and pawl,as well as a weak plunger spring should be looked at.As well as the sleeeve bushes backing out,or more commonly being very loose on the mainshaft.Without a source of new parts these boxes will become difficult to repair.Regards John.

[Peter.Greene]

Thanks all for your input, sounds like a big day ahead, will get into it and see what unfolds

[Knud.Degnbol]

A loose nut at the rear chain sprocket can cause the same.

[les.brooks]

Just had the same issue, after lots of checking parts I found the spring behind the plunger had fractured. I have no experience of Burman gearboxes and it was not until I compared it with another box that I realised that whilst the plunger was still operating it was not positive enough. Worth checking before you take the box out and apart.

[Peter.Greene]

Thanks Les, I must admit I am not familiar with the plunger spring where is it located

[Peter.Greene]

Well, I found the problem, The layshaft selector fork had fractured welding itself to the sliding gear, or the other way around (It may be salvageable) and breaking away, In the process it damaged gear number 28 breaking off a couple of teeth.

[Peter.Greene]

I suspect looking at the amount of lubrication that came out of the gearbox the grease/oil mix (in fact it was now nearly all oil) had dropped down enough for the layshaft and selector fork to run dry then hot welding the selector fork arm to the sliding gear. Mmmm "there's a lesson"