Yam 50hp two stroke problems?

win-or-bin · 05 April 2009, 20:42

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

Poly · 05 April 2009, 20:52

Quote:

Originally Posted by win-or-bin

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

win-or-bin · 05 April 2009, 21:01

Quote:

Originally Posted by Polwart

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

Poly · 05 April 2009, 21:34

what prop do you have on it? (should be marked on the side)

win-or-bin · 06 April 2009, 07:08

The prop doesnt appear to have any markings on it, although looks like a standard yammy prop.
Cheers

doggypaddle · 06 April 2009, 09:06

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"

win-or-bin · 06 April 2009, 10:49

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

PeterM · 06 April 2009, 10:57

Quote:

Originally Posted by doggypaddle

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 ........

9D280 · 06 April 2009, 11:16

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.

win-or-bin · 06 April 2009, 11:41

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?!!

9D280 · 06 April 2009, 12:07

Only 2nd hole? As I say, I've never seen one of those hulls, but on my Humber the old Yam (identical leg, clamp etc as yours) was 4th out, the Suz on my old SR4 was in the furthest out position! (both boats had quite a rake on the transom)

How heavy is the steering? If it's a "foot on the console" job to haul it round, you could definetly move the pin back.

Another thought - what size is your fuel hose? a 50 won't run too well on the smaller 1/4" stuff used for smaller engines....

doggypaddle · 06 April 2009, 12:18

Quote:

Originally Posted by win-or-bin

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

If you give it wide open throttle in neutral it will probably achieve 14,000 rpm briefly before a large chunk of piston flies out the sideDont do it again naughty boy!

You want the RPM to be roughly in the 5000-5500 range, but there is no strict rule , just because an engine can rev safely to 5700rpm does not mean it develops maximum power at thet RPM. Power may peak before that. This allows you to under prop slightly so that when the boat is unloaded your rpm is slightly over the optimum, and as you load the boat if falls back towards max power. This way there is less difference between loaded and unloaded. As suggested try trimming up a hole at a time and see how that affects things. When i look at video of me flat out i am always surprised just how far the engine is trimmed out. you wont be able to trim for max top speed with manual trim as it will probably ventilate in turns and at takeoff , but you can find a good compromise between "bite " and speed. Also worth noting the first time i took my SR out i got 26-27 knots and was disapointed, the sea was not rough only a slight chop, next time out it was like a millpond and i saw 34.9 knots. conditions make a BIG difference to some boats. i am rambling again

win-or-bin · 06 April 2009, 14:59

Thanks very much for the help, I am learning a lot, previously I have just had wooden fishing boats with tiller steer so these rib things are new to me

and when I got it everyone said it would a flying machine, so was a bit disappointed with the speed. I am going to try trimming it as soon as I get the chance to go out again (not easy when you live on the West coast of Scotland i the spring) And will report back, dont think I am going to go with a new prop as a 13" probably isnt going to make much difference and better trying the easy things first!
Thanks again