Mariner 9.9hp outboard - ventilating/cavitating issues. Trim?

[willibet]

Hi all, I'm new to this boating lark having just got a new 3.5m inflatable and an old outboard and very new to the forum, although I've stalked it a few times for some sound advice. Anyway, I'm back more formally now, having joined and with a question:-

I have an old (1990s I believe) 2 stroke Mariner 9.9hp outboard and I believe that it's ventilating (or possibly cavitating) at higher rpm. I get a bubbling sound, a little like a kettle boiling and no increase in boat speed associated with the engine accelerating. Looking over the back it seems as if as the boat lifts as speed increases, the prop is partly out of the water. The boat was not light at the time, it was quite heavily loaded. This I think is caused, at least in part, by the trim angle of the engine: a previous owner did some work on it and it doesn't seem to reach the #1, lowest/most vertical setting, staying at position 2 on the little tell-tale decal at the side, although it does seem to be in the lowest of the 5 (I think) notches for the trim settings. Does anyone have any idea how I can adjust the clamp/trim setting relationship so that the engine is trimmed as low as possible?

Any other suggestions as to the cause or prevention of the issue are very welcome!

 

[VicS]

If the prop is partly out of the water it sounds as though the engine is mounted too high. The AV plate should be level with or up to 1" below the bottom of the transom

You should be able to find the answers to your questions in your owners manual. If you have mislaid your copy you should be able to down load one from the Brunswick Marine in EMEA Download Center.
Brunswick Marine in EMEA Download Center - Download Center
You may have to browse a little to find the right one

You will also find year identification lists which will enable you to determine the year model from the serial number

A good site for parts lists and exploded diagrams is Boats.net: Outboard Motors, OEM Marine Parts, Boats for Sale

For questions that cannot be answered on these forums I suggest the Mercury / Mariner forums at iBoats and Marine Engine

Engine Repair and Maintenance Page: 1 - iboats Boating Forums | 15

Outboard Motors Forum

 

[[159032]]

It sounds as though the leg is too tight in against the transom
See this (rather small) graphic.

It could be he was trying to set the trim to overcome sitting too far aft (or being a bit large).

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