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Early Montesa gear selection


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I have read a few posts here about the 123/172/200 Cotas jumping out of gear and other mentions of a flawed design. I haven't seen any information as to why?

Both my 123 and 172 jump out of  1st or 2nd gears, the 123 occasionally and the 172 worse. If it is a design issue, what is it?

I have rebuilt both engines with all new bearings with no apparent wear or damage to the gears etc. They have new springs in the selector and the pawls look fine. I have tried both less tension and more tension on the detent with no change. It does seem like it needs oil in the selector pawls to work, if the bike has been sitting it won't select anything till it starts up.

It seems to happen going from off power to on or vice versa. Even idling along in 1st gear it seems like the on/ off from the individual power pulses from the engine is enough to knock it out of gear. Other times it will jump out as you go from zero throttle to wide open at the bottom of an obstacle. 

Are they critical for oil viscosity perhaps? Is it some sort of torque reaction moving the selector dogs inside the box?

If anyone has some experience with this I would love to hear it.

Cheers 👍 



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