I think I broke my boat - part deux

[oGaryo]

Following on from the other thread where I get knocking at high load and a strange sound from the Alternator.. here's my findings:

it's not the alternator as the sound still exists when I run the engine without the belt on, it seems to be coming from the port side of the engine under the exhaust manifold!!

A compression test looks ok with all cylinders in the green so I suspect it's either the exhaust manifold blowing or the head gasket has actually gone but doesn't get picked up with a compression test (does that make sense if the sound is escaping air?).

Timing had drifted out so I think that's the cause of the knocking and could be the cause of a blown gasket if it has gone.

Here's a couple of mobile phone vid clips where you should be able to hear the regular sound I'm referring to, it increases frequency with engine speed and sounds like a steam train.

At best, I think I'm looking at replacing the exhaust manifold gasket or possibly the head gasket.. it may even be a leaky carb gasket considering I've had that off in the past.. the only part turning on the outside of the engine is the crank shaft pully but I've checked for rubbing all round there and the frequncy of the sound is more attuned to the fall and rise of a piston rather than the faster speed of rotation of the pulley.

I presume both gaskets can be changed without pulling the engine.

Can I have your thoughts please

Hope you can help.. a single word springs to mind, starts with B and ends in ugger oo:

Many Thanks Gary




Engine
Engine Sound 2

Can't get URL clicky working so here's the links to cut and paste in to your browser:
http://www.youtube.com/watch?v=RSMI2NozKmU
http://www.youtube.com/watch?v=cjIU8T67oNc

 

[omega2]

Are the spark plugs tight??

 

[oGaryo]

they are mate and on the other swide of the engine to which the noise is coming from

 

[gordmac]

Maybe my hearing is a bit iffy but I can't really hear any knocking noise! A leaky exhaust manifold makes a sort of chuffing sound, fairly distinctive. Have you checked the manifold bolts? If the head gasket was blowing enough to make a noise I think you would see something on compression test.
If you think the inlet leaks try and seal it off with tape.

 

[JKay]

Good shout even electrical tape will work to plug an airleak

 

[oGaryo]

Maybe my hearing is a bit iffy but I can't really hear any knocking noise! A leaky exhaust manifold makes a sort of chuffing sound, fairly distinctive. Have you checked the manifold bolts? If the head gasket was blowing enough to make a noise I think you would see something on compression test.
If you think the inlet leaks try and seal it off with tape.

I haven't checked them yet but will do.. got my finger's crossed that is the case.. The knocking only occured at high revs or periods of high load (coming on to the plane and > 30knots).. The timing was well out so have adjusted it back to 1 degree BTDC

I also need to check this bulletin to see if it applies to my engine

http://www.boatfix.com/merc/Bullet/98/98_14.pdf

Just need to find the source of the 'chuff' sound now.. if you put your ear close to the pc speaker you'll clearly hear it when running the vids

thanks for the advice

 

[Cliveshep]

A nice piercing whistle is often a very small exhaust manifold blow. Sounds very like a massive water pump squeal. Unlikely to be a head gasket.

Got distracted there, think I've answered a question you didn't ask!

 

[Wiggo]

Sounds like the Snurble Grip Thribley's on the way out. Nick may still have a spare: http://www.ybw.com/forums/showflat.php/Cat/0/Number/664388/page/2/fpart/all/