When I start moving after stopping at stop signs etc. my 85 k100 w/ 50,000 mi. sometimes pops into neutral, sometimes repeatedly.

Any theories about cause / how to fix?

I do have a parts bike from same year with 30,000 mi. in case it'd be worth swapping transmissions.

Does it pop out if you try to take off using second gear.
Regards Martin.

The selector fork is bent and the gear dogs are worn. This often happens when the bike falls and the gear lever is pushed when the engine isn't running and the gears are not aligned and the fork bends. Also crashing the gears back to 1st when moving too fast and damaging the dogs then that bends the selector fork.
Either way it's out with the gearbox and repair the gears by having a professional regrind the dogs to match and a new selector fork as they cant be straightened.

RicK G wrote:The selector fork is bent and the gear dogs are worn. This often happens when the bike falls and the gear lever is pushed when the engine isn't running and the gears are not aligned and the fork bends. Also crashing the gears back to 1st when moving too fast and damaging the dogs then that bends the selector fork.
Either way it's out with the gearbox and repair the gears by having a professional regrind the dogs to match and a new selector fork as they cant be straightened.

If that's the case, and I do have a complete transmission from the same year on a parts bike, would it behoove me to just replace the whole transmission, or replace the selector fork and at least one of the gear shafts? 

The parts bike was sitting for a long time but was protected from the weather.

Worth trying

Hey Rick, can you and do you repair gear boxes for customers?
Justin

I'm retired now and busy getting my 4x4 and caravan ready for some travelling. Anybody that is familiar with the workings of a constant mesh gearbox should be able to fix it, they really aren't anything special.

To back up Rick: the K100 is the simplest gearbox I've ever taken apart and I thought Guzzi ones were easy. The one and only thing you have to be really aware of is to follow the instructions on setting the selector spring when putting the end case back on. No set, no case go on, so even that's difficult to miss.

Thanks Rick
Just to make sure I have the right grub screw as the one in the attached pic you sent. I have attached a short clip from youtube tightening the "grubscrew" are they both the one and the same?
Thanks for your guidance.

https://www.youtube.com/watch?v=zVea8TByyT8