Knock Knock..

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Michael-Searay

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Jun 23, 2012
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Just replaced NGK BPR6EFS spark plugs on a 3.0L Mercruiser and noticed the one that came out of cylinder 4 has two cracks in the white insulator (from the top down) . The plug was very tight so my first thought is too much force on the wrong angle and I must have caused the damage while removing. However while test starting on muffs I am now getting a very distingtive fast knocking sound over 1400 revs. Oil level is OK, hesitant to test in the water to see if it is the same, in case there is some piston or valve problem???

Come to think of it, the boat did seem down on power and sluggish the last time out. Any advice?

Thanks, Michael
 
Don S

Don S

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62,321
Re: Knock Knock..

Do a compression test and see if the time running with a bad plug caused other problems.
Also try pulling the plug wire off the noisy cylinder and see if it stops.
Firing order is right isn't it?
 
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Michael-Searay

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Jun 23, 2012
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Re: Knock Knock..

I don't have a compression tester but will go out and buy one as helpful to have, firing order in manual says 1-3-4-2 which is how it was before I replaced plugs and how I put it back.
 
wrench 3

wrench 3

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Aug 12, 2012
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Re: Knock Knock..

Just a random thought, but do you have the drive down for enough when you are running on the muffs?
 
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Michael-Searay

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Jun 23, 2012
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Re: Knock Knock..

Sounds like a big ah ha moment coming for you guys, but why does the drive need to be down to run on muffs (please be kind here)
 
O

oldsub86

Petty Officer 1st Class
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Sep 9, 2008
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311
Re: Knock Knock..

Because too much of an angle will make the universal joints knock a bit?
 
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Michael-Searay

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Jun 23, 2012
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Re: Knock Knock..

OK thanks heaps for that, I will lower sterndrive unit and see if knocking remains.
 
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Michael-Searay

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Jun 23, 2012
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Re: Knock Knock..

Many thanks Don, wrench 3 & oldsub86. With sterndrive down knocking gone. I am yet to do compression test but will once I get a tester.

Thanks again, you learn something new everyday.
 
elkhunter338

elkhunter338

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Jun 27, 2009
Messages
818
Re: Knock Knock..

Michael-Searay said:
OK thanks heaps for that, I will lower sterndrive unit and see if knocking remains.
Click to expand...

I would check your bellows real good to make sure no tear. i have heard you can tear the bellows if the drive is up too far and you run the engine.
 
Don S

Don S

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Re: Knock Knock..

If the angles are right, you can destroy the transom shield with the drive all the way up.
 
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