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Old 24 March 2005, 17:45   #1
Country: Ireland
Town: Enfield, Co Meath
Boat name: no name
Make: Zodiac Pro
Length: 6m +
Engine: Suzuki 115
Join Date: Sep 2004
Posts: 120
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help with yamaha 50 problem

hello again folks.... its that time of the year again...

I took out the rib last weekend - but have a problem with the motor - I wonder if anybody can help...

it's a 50 HP Yamaha 3 cyl 2 stroke... I tried to buy a manula from Amazon in the US - but it wont ship the book to Ireland - North or South - can anybody point me to where I might be able to buy one ?? (Amazon UK dont have...)

the problem is that the engine will run fine with the throttle pretty wide open, but when I back off the throttle it cuts out. After a minute or two, if I try to restart the engine with no throttle opening - it starts and revs up a lot - with a lot of smoke - and a loud intremittent squeaking noise - and then cuts out again...

I was using fresh petrol. At first I suspected slow running jets being clogged or something. However, I opened the cover and had a look - and I think I may see the problem...

looking at the engine from the front (ie nearest the carbs) there is a linkeage that attaches to all thre carbs, which is actuated by a cam on the side of the lower carbs. The cam is driven by a stub attached to where the throttle cable terminates. This linkeage is connected through, under the carbs to (I think) the autolube pump on the other side. The cam (and hence the linkeage) only operates when the throttle approaches the fully open position. So when I open the throttle wide - the stub moves and makes the cam rotat - hence operating the linkeage... However - when I close the throttle - the cam and linkeage dont return to the 'closed' position. I can close them manually - they're quite stiff - and things appear to return to normal.

So - any opinions ?? what is this linkeage doing ? should it spring closed itself automatically ?? It's quite stiff - should it be free ? should be a spring to close it ??? etc .. any help would be greatly appreciated ?? Of course advice about buying a 200 hp V6 replacment will be appreciated too - but I'm quite happy to hear suggestions on how to continue with the engine I have ...
and any ideas about the squeaking noise ???
Happy Easter and Happ Boating (Ribbing sounds a bit like knitting....)

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Old 25 March 2005, 01:09   #2
Pete7's Avatar
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,837
Sounds like the carbs are gummed up. Petrol left inthe carbs turns to a gel like substance and then sets like varnish if its left for a while. The random sneezing the engine makes is a classic sign and its the reed valves behind the carbs which are not closing properly.

Either try a can of carb cleaner down the throats with the engine running or strip the carbs down and clean.


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Old 25 March 2005, 03:36   #3
Country: UK - England
Town: scotland
Boat name: Leviathan
Make: Phantom
Length: 8m +
Engine: GM Diesels
Join Date: Jan 2003
Posts: 1,437
The 'random sneezing' is either weak mixture, due to blocked jet(s) or other carb fault, or incorrect relationship between ignition timing and throttle position, ie, relationship between rotation of trigger assy under flywheel, and opening of the throttle butterflies.
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Old 07 April 2005, 14:29   #4
Country: UK
Boat name: Seascaper
Make: Hurricane 640
Length: 6.7
Engine: Volvo Penta AD31DP
Join Date: Dec 2002
Posts: 82
I had the same problem with one of the pair of Yamaha 50 that I used to run. The lever that you see on the front of the engine just reachable with a thin finger or screwdiver between the oil tank is the auto choke. This had a tendancy to stick on on one of my engines. If you look at your handbook for the engine there is a diagram showing the on/off status. It is supposed to operate this operation automatically but this was not always the case.


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