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Old 11 December 2011, 02:44   #1
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Understanding the Mercury 30hp 2 stroke

I have just taken a 3.8m inflatable with a Merc motor out for the first time and have a found a couple of things im not sure about.

I had the trim at hole 3 (manual trim) and when rising above mid revs the power ceased and the motor kept revving higher as though the gear had popped out. When I released the throttle and picked the revs up again the power was back (until reaching mid revs again).

I tried trimming to hole 2 and the problem was resolved. Now I should have then tried hole 3 again but I didn't and am left wondering if this issue was due to cavitation or if the gear was really slipping out.

The other question I am having about the motor, is it supposed to have a tilt mechanism? I have simply blocked it out of the water with a piece of wood.
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Old 11 December 2011, 03:45   #2
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i had my 25hp merc trimmed all the way in on my yam360 to control the amount of ventilation occouring. what you get with a sib is that as it rides over each wave, the whole boat bends and flexes - this in itself is constantly changing the trim of the engine which causes the prop to lose its grip in the water causing 'wheelspin'. with plenty of practice you will learn your own boat and will know exactly when its about to happen and will instinctively work your engine to suit.
another point on trim is that as a sib doesnt have a heavy or deep hull you have no reason to try to get the bow up - the boat will easily get itself up onto the plane no matter where the engine is trimmed, but in keeping it trimmed in you are compensating for the flex of the boat.
as for tilt mechanism, it should at least have a locking catch to keep it in the tilted up position. this should engage automatically when the engine is lifted.
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Old 11 December 2011, 15:40   #3
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I found out that the engine is ex surf life saving and the tilt mech is removed for them. I contacted the life saving guys and they just use a broom handle.

I suppose I could buy a new tilt mech.
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Old 11 December 2011, 17:04   #4
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....or use a broom handle!
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Old 12 December 2011, 06:11   #5
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For what it's worth there were a ,lot of 1960s / 70s Johnsorudes just used friction to keep them up.

Needless to say I have applied broom handle!
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