Gearbox noise

[daleburt]

Hi
I have a 88 yamaha 70hp. It ticks over perfectly,revs freely out of gear,runs fine in reverse but in forward gear it runs in a jerky manner like there's a tooth(or part of a tooth) missing in the gearbox.It starts as soon as it gets above idle speed.
Likely cause?? & is it hard to fix??

 

[robert graham]

Re: Gearbox noise

could be your gear shift is slightly out of adjustment and you're not fully engaged in forward. also maybe drain lower end oil into a clean container to see condition of oil. If you got water(especially salt water) into your lower gear unit then it could be in bad shape. Good Luck!

 

[daleburt]

Re: Gearbox noise

I drained the oil and found nothing unusual.I disconnected the remotes & worked the gear selector by hand and it's still doing the same thing. I can feel the knock in the gearbox when the engine is in gear.
Would a broken tooth make it behave this way? It clunks ever 5-6 seconds but only in forward gear

 

[robert graham]

Re: Gearbox noise

looks like you're going to have to open up the lower unit and see what's the problem. Good Luck!

 

[daleburt]

Re: Gearbox noise

Update:
It turned out that the shim at the back of the forward drive had worn away which then allowed the bearing to move. Result was trashed shim,trashed bearing and trashed prop sharft splines. Nice

 

[robert graham]

Re: Gearbox noise

Glad you found your problem but sorry it wasn't a cheap/easy fix. Wonder what caused the shim to wear like that? Are you all fixed and running again?

 

[daleburt]

Re: Gearbox noise

She's running well again now.Just in time to be taken out for the winter.
It wasn't such a huge expense.I managed to track down a 70 with powerhead failure(no 2stroke in the petrol) and got the gearbox for ?250.

 

[robert graham]

Re: Gearbox noise

You did great for $250, it would have cost a lot more to repair your old one and you can keep the old one for for parts, in case you ever break your lower unit/skeg.