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Old 05 April 2009, 16:42   #1
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Yam 50hp two stroke problems?

Hi all, new to this forum and I must say it looks a great resource! I wonder if anyone can help me , I have just bought my first rib, a 4.3m Gommautica Rib with an early 90's 50hp yam 2 stroke on the back. Am delighted with the boat as it is immaculate for its age as is the engine-BUT!- was out in it for the first time today and although the engine ran smooth, the boat got on the plane very quickly it would only do 26 knots?(GPS) when I rev the engine in neutral it revs up to 7k ish but out in the boat under load it just sat at 5k revs and felt there was loads more left in it? Had a look at the throttle cables and I am getting full movement at the carbs as well- any ideas?
The boat had been unused for a couple of years although I replaced plugs, all fuel lines etc...
Thanks for your time
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Old 05 April 2009, 16:52   #2
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Originally Posted by win-or-bin View Post
when I rev the engine in neutral it revs up to 7k ish .... any ideas?
Yes don't do that or you'll be needing a new engine soon

Quote:
4.3m Gommautica Rib with an early 90's 50hp yam 2 stroke on the back...
...engine ran smooth...
...the boat got on the plane very quickly...
...under load it just sat at 5k revs...
... it would only do 26 knots?(GPS)...
... in neutral it revs up to 7k...
sounds like the wrong (not ideal) prop to me
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Old 05 April 2009, 17:01   #3
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Yes don't do that or you'll be needing a new engine soon

sounds like the wrong (not ideal) prop to me
Only did it for a second in the barrel honest! any idea what prop to go for?
Cheers
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Old 05 April 2009, 17:34   #4
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what prop do you have on it? (should be marked on the side)
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Old 06 April 2009, 03:08   #5
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The prop doesnt appear to have any markings on it, although looks like a standard yammy prop.
Cheers
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Old 06 April 2009, 05:06   #6
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my k series prop is marked inside the hub, so you may need to remove the prop to get it off.
5K rpm isnt too far out from ideal. you could try one pitch up and see if it changes things.
I guess you have a 13" or 15" pitch so could go to a 15" 0r a 17"
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Old 06 April 2009, 06:49   #7
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Cheers, will check the prop this afternoon, just didint think changing the pitch a bit would give me my missing 10knots! What should the max revs for a 50hp 2 stroke be? i thought if it reved to 7k it should go to 7k under load as well?
Thanks for the advice and help
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Old 06 April 2009, 06:57   #8
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Country: UK - England
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Boat name: Streaker/Orange
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Engine: 50Yam/25 Mariner
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Originally Posted by doggypaddle View Post
my k series prop is marked inside the hub, so you may need to remove the prop to get it off.
5K rpm isnt too far out from ideal. you could try one pitch up and see if it changes things.
I guess you have a 13" or 15" pitch so could go to a 15" 0r a 17"
Isn't it down in pitch gives more revs ? I just have this in my mind from somewhere ........
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Old 06 April 2009, 07:16   #9
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Hi, Welcome to Ribnet.

All the data you give there sounds about right - except for the speed! Not familiar with that flavour of rib hull, but I would expect more form a 4-ish M boat with a 50, even if loaded with dive gear!

Have you played with the trim? If you have Power trim, notch it back slowly & see what happens. The fact you are planing instantly might hint you are either fully trimmed in - or do you have "fins"? - thy will help get you up PDQ as well.

If you have "manual trim" then it's going to involve cold wet fingers. Have a look at the bottom of your clamp, where is the pin? I bet it's as close to the boat as it will go. If so, Pull it out, and move it back a hole or two. Keep going until you either run out of holes or the prop breaks out at the slightest provocation (turns, waves etc) then drop it back in one hole. You may find the revs go up as you do that. If they get to about 5700, you can then go up an inch & get your top speed without throwing a piston.

Where do you normally launch - I have a leftover 14" K series form my old Yam if you want to try it, but I'd play with the trim first.
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Old 06 April 2009, 07:41   #10
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Hi just an update, checked my prop and its a 14", the trim is set at 2nd closest to the hull, didnt have time the other day to mess around with it but will try, everyone I speak to says it should pull 30+ knots easy but dont have an answer as to why its not!! It planes mega quick and pulled 26knots into the sea and wind and 26knots with the sea and wind behind me, no faster at all?
Help please?!!
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