5th gear jumping

[Steris]

Whilst trying to sort my carb issue out I went for a run up the dual carriageway to do a plug chop, all going well  until I was in 5th, after a few miles it started jumping,  it gives the impression its jumping back into 4th momentarily,. But its sporadic,  itll go 2 or 3 miles without jumping then jump a few times then ok again. Is it possible its dropping back into 4th momentarily or could it be a clutch issue?  Only does it in 5th..

[Bryanj]

More likelyto be worn gear dogs, selector or broken shim

[Steris]

Engine out and on the bench, found the middle selector fork really badly worn and has obviously been very hot at some point, 5th gear dogs only meshing by about 2mm. So that's why its been jumping.  Found a complete CB350 twin gearbox on Ebay with as new forks for £60 so happy days.. then it'll be back to sorting the carbs out 🤣🤣

[Bryanj]

Thats about the only bit thatwears on the 500 ans,as you found, not easy to get new.
Carefull assembling the cases, make sure the bearings locate on the pegs and while you are in there check the primary chain as on the 500 it eats into theoil gallery when worn

[Steris]

Hi Brianj  thanks for the advice,  I replaced the primary chain when I rebuilt the engine earlier this year.  I noticed the worn fork then but it didn't look too bad and was selecting 5th gear ok on the  bench.  Only done 150 miles since the rebuilt,  you live and learn 😁😁😁

[murdock]

The other day when I opened my engine I also found the fork a bit bent and a bit worn , thanks to oddjob I bought a new one on ebay for 25 us$ . they sell them in Europe for up to 170 euros , when you open the engine change the rubber plugs.

[Steris]

Hi Murdock
Which rubbers do you mean?

[murdock]

To these

[Steris]

I changed them when I rebuilt the engine earlier this year 👍👍

[Steris]

Well 1500 miles after fitting a new/second hand 5th/4th selector folk the bike is jumping out of 5th again. Surely it cant have worn another one out? So engine will have to come out again this week and see what's happened 

[Johnny4428]

I have just gone through sorting a fifth gear issue on my 750. I had poor insertion of gear 2/3 into 5th which resulted in 5th gear being disengaged when power was applied. Turned out to be a combination of wear and for some reason about 2mm plus lateral movement with 5th gear cog. After replacing worn gears and middle selector fork plus an extra shim I now have 0.4mm of tolerance. In return I now have almost 4mm of gear insertion. It’s gone to take a few runs before I have the confidence that it’s going to stay in gear though. Check selector mech, make sure you are getting full movement when selecting 5th. Most annoying! Good luck with sorting.

[McCabe-Thiele (Ted)]

That must be a nightmare to have the same fault again. I'm not familiar with the 500 gearbox but could it be the selection system that sits at the end of the shaft behind the clutch - on the 400 it looks like a system of levers, springs etc?

[Nurse Julie]

Well 1500 miles after fitting a new/second hand 5th/4th selector folk the bike is jumping out of 5th again. Surely it cant have worn another one out? So engine will have to come out again this week and see what's happened 

What a pain. Let us know what you find.

[Oddjob]

The other day when I opened my engine I also found the fork a bit bent and a bit worn , thanks to oddjob I bought a new one on ebay for 25 us$ . they sell them in Europe for up to 170 euros , when you open the engine change the rubber plugs.

That's a 550 middle fork, the 500 middle fork is not the same.

Rare as hens teeth new.

Did you change any of the gears last time or just the selector fork.?

I'll be selling a NOS L/H selector for and a NOS middle selector fork soon. I bought a complete set of custom made ones from Italy last month so I'll be using them on the rebuild.

[Steris]

The one I fitted last time was from a cb350 twin,  the selector is the same with same part number.  I'll know more once I get the engine on the bench.