2002 Yamaha 30hp knocking

[Rosco 11]

Hi all,
I have just bought a 30 DMH Yamaha, and am in the process of ironing out a few bugs in it.
Have put a new water pump kit in, new plugs and t-stat as well.
I am stumped with a knocking that is in the engine. On the muffs, it seems to only knock as the revs are coming down, rather than going up. It seemed to do it more often if I reduced the revs suddenly.
The knocking is very hard to describe in words . . .but it seems like the driveshaft is trying to engage itself back into an already turning spline. It's hard to say if the noise is coming from the powerhead, or the lower unit.
I took it for a test run today, and found that the knock is worse under load. Whenever I tried to go more than say 1500rpm, the knock was there, and the motor even stalled a couple of times as the knock was happening.

Any suggestions as to where I should start ? I have a manual and average mechanical skills ( mostly learnt from iboats ! )
All help gladly appreciated. Thanks . . .

 

[sigma]

Re: 2002 Yamaha 30hp knocking

Since it not only happens in gear but out of gear on the muffs I would say either a loose flywheel or a crank bearing? Possibly a bearing on the lower unit right at the engine like a drive shaft bearing

 

[moonfish]

Re: 2002 Yamaha 30hp knocking

If there isn't enough fuel a motor will cough. You might call the same sound a knock although the motorheads wouldn't. It can be kind of violent.

 

[Rosco 11]

Re: 2002 Yamaha 30hp knocking

Thanks Sigma and Moonfish for your replies.

I'm leaning towards the flywheel at this stage ( makes sense, fits the symptoms and is pretty easy for me to look at ). I will grab the torque wrench and have a discussion with it later today.
I also suspected the pinion bearing/nut . . but I discounted that by removing the lower unit and starting it ( with the water hose straight onto the tube) . . and it still made the knock/clunk sound. Didn't seem to do it as much, but still did it.
So the lower unit doesn't seem to be the guilty party here.

Thanks again for the replies . . I will let you know how the flywheel goes.
In the meantime, any more suggestions ?