Mariner Outboard problem.

[scotty123]

A recently serviced mariner 5 hp outboard, has given some concern recently.
Sometimes, when running, it has suddenly gone into high revs, which are unable to be controlled with the throttle & only stopped using the killcord.

Initially, thought it might be a shear pin failure (suggested by the service guys), but there doesn't seem to be any shearpin. The prop is held on by a castled nut which has a splitpin through it, which when taken apart, just seems to be fitted onto a tapered shaft with splines.

Is there a shear pin anywhere?

What could cause it to go suddenly into high rev mode?

 

[Rigger Mortice]

If it has a splined shaft it is unlikely to have a shear pin and, in any event the engine would still respond to the throttle.

From what you describe I would start looking at the carb.

 

[Seajet]

Yes, sounds like a lean mixture.

 

[macd]

Check that the throttle cable (or linkage) is properly secured on the slide. Sometimes they can 'hook up' in the wrong position, giving the symptoms you describe.

 

[scotty123]

If it has a splined shaft it is unlikely to have a shear pin and, in any event the engine would still respond to the throttle.

From what you describe I would start looking at the carb.

Can understand that fuel if left in the carb overwinter, can be a problem, but considering it's had a recent service, does this suggest the carb wasn't checked properly.

PS
how is the system protected, if no shear pin?

 

[VicS]

Can understand that fuel if left in the carb overwinter, can be a problem, but considering it's had a recent service, does this suggest the carb wasn't checked properly.

PS
how is the system protected, if no shear pin?

You dont say what year or give the serial number but

Older Japanese models ITYWF have a shear pin while later American and European made models have a rubber hub in the prop.

A broken shear pin will manifest itself as a lack of drive although occasionally the broken bits may "catch" and give some intermittent drive.

A failed hub will tend to drive at low throttle openings but slip when more power is applied.

However
If you are saying that the engine simply revs high and cannot be controlled with the thottle then you should look at the linkages ( cable??) between the twist grip and the carb.

Check the throttle valve ( butterfly ??) in the carb and determine why it cannot be closed properly . It wont rev in the way you appear to be suggesting if the throttle valve closes.

 

[Rigger Mortice]

how is the system protected, if no shear pin?

As VicS has hinted at, a rubber hub gives protection against shock loads. AFAIK shear pins are only used on smaller engines. Rubber hubs are used on the props of bigger engines which generally drive the propellor through a splined shaft. If a prop is on splines then a shear pin would do nothing; the prop would still be driven by the splines.

My first thought about the OP's problem was that the rubber hub was failing and the prop was slipping on its hub. However, if the throttle isn't slowing the engine down, that ruled that thought out.

Edit. Sorry, just realised. You are the OP!

 

[Lakesailor]

On the 4 stroke (perhaps the same on the 2stroke) The throttle cables from the tiller have a nipple which locates on the cam (top right in this pic. This has the remotes kit fitted which uses one push/pull cable).
The cam then pushes and pulls that bent wire rod to operate the butterfly. Makes sure that rod is not going over-centre or jamming on anything. Make sure the cables in the tiller arm are moving freely and the outers are properly secured.