Air-cooled Yamaha 5hp - alarming engine sounds

I

irishpaddy

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Aug 22, 2002
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I run an old but venerable air-cooled single cylinder 2 stroke Yamaha 5 - not sure of year of manaufacture, presumably 1970's - model 5BCS (european model - 100:1 gas/oil mix)- maybe someone can guess -<br />My main problem is that it has started making an alarmingly loud metallic banging noise from the engine once warmed up - especially noticeable when idle or at trolling speed. There is nothing loose or anything like that - it sounds almost like a 'backfire' or pre-ignition sound - it is being run on 95 octane unleaded gas @ 100:1 - used to be run on leaded 97 octane (European standard) some years ago - never had any problems - could it be one of the main engine bearings - is it going to die ??
 
sloopy

sloopy

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Jul 12, 2002
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Re: Air-cooled Yamaha 5hp - alarming engine sounds

How are you maity!<br />whats the temperature in Irerland!<br /><br />sounds like you have some problems!, never heard of a air cooled yami, but from the sounds you tell me, something is wrong! do you have a digital camera that you could take pictures with so I could see the <br /><br />1. MOTOR<br />2. LOWER UNIT WITH PROP REMOVED<br />3. POWERHEAD<br />4. Spark plugs<br />5. pictures of the motor area :) <br /><br />thanks!
 
I

irishpaddy

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Aug 22, 2002
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Re: Air-cooled Yamaha 5hp - alarming engine sounds

Hi Sloopy,<br />Thanks for the reply .....briefly I can tell you that the weather here in Ireland is likely to be at least 20 degrees colder than whatever it is in North Carolina right now. 60 - 70 most days and raining. Strangely enough its not raining today.....that's a plus.<br />Well my engines problems got much worse over the weekend...I took a look at the motor on the bench (as it's aircooled, rather like a Briggs & Stratton, there's loads of cast alloy cooling shrouds around the engine to be removed first before you can see much) and everything seemed OK ....gave it a few good tugs of the rope...it seized dead ! Worse still, gave the prop a gentle push to see if I could un-seize and the crankshaft sheared off in two just below the flywheel !! There seemed to be my problem originally - the shaft had obviously fractured somehow and the vibration reulsted in the flywheel (or something else) knocking off something. Great - all I need is a new crankshaft now. At least my uncle is a two-stroke mechanic....I think he's gonna get my engine for a winter project ! My unbroken record of never having problems with Yamaha has ended.<br /><br />(ps I don't have access to a digital caqmera right now - my brother's gone on vacation with it. All i can say is that the aircooled Yam 5 looks almost identical to the modern 4 / 5 hp single cylinder yamahas with full blue cowling etc - were very common in Europe until the early 1980's - still used commonly as a standby engine for lots of powerboats )
 
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