NC30 jumping out of 5th gear

[lukemillar]

Hey all,

Hit a new issue over the weekend where under hard acceleration the bike would jump out of 5th gear and into a false neutral. These were clutches upshifts on a straight. The engine is a new build and I inspected the gearbox before installing and it didn't look worn but something must be up. Can anyone point me to the specifics in this picture as to which gears and selector forks I'd need to check:

http://imageshack.com/a/img922/7412/fucoIr.jpg

Thanks
Luke

[jedidude04]

I would maybe be looking at the shift star and selector forks? Should be located just inside the clutch cover, maybe behind the clutch basket.

[Tim400GB]

Sure its 5th?

I had the same issue but with 6th. The issue was with the corners of the gear dogs being rounded off to the point that the gears would want to ramp away from each other.

Burnt and worn the selector too.

[richyrd5]

yep,i had the same with 6th gear,,rounded of dogs on the 5/6th gears,,,but still check the external componants/selector triangle just to be sure,but could be engine out!!

[lukemillar]

Yep, definitely 5th. Listen to this video:

https://vimeo.com/196065759

Happens at 0:49, 3:36 and 9:38

I shift into 5th, it drives and then slips almost straight away. If I tap the lever, I end up in 6th

[Tim400GB]

I think it'll be much the same issue of worn dogs/bent selector combo.

Get the sump off and see if there's anything glaringly wrong.

When my gearbox was fubar the drum was in perfect working order

[lukemillar]

Ok, finally got to the bottom of this one. It is indeed worn dogs on the 5th gear on the input shaft but the cause is actually a missing thrust washer (first item on the shaft). That allows 5th gear to slide back about 3mm and enough to have it slip.

Kind of annoying as I just dropped this box in donated from an engine I had for spares and I didn't take the input shaft apart, just inspected it. Must have lost that washer in a previous life. Easy fix and I have enough spare gearboxes to make a good one but pain in the arse as it is almost a complete strip down... again!

[Tim400GB]

That's a very easy washer to lose too.
Almost did the same myself when the new 2nd hand box I got was supplied without it and the bearing

How did your selector hold up?

[lukemillar]

Selectors look good. I have a spare set so I measure them up to make sure and swap out where needed. They are an NC35 set and I noticed there is an additional dog leg on the centre fork. I'm assuming it is there to stop it rotating when fitting the cases halves together, so I am going to try it!