spark knock

mustangfam

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Sep 16, 2010
Messages
7
I just had a rebuilt engine put in my Wellcraft boat. After having it out for only 2hours the boat started losing power. I tried to get the power I could out of it, but after 20minutes we trailered the boat. I took it back to the mechanic who installed it. He called me and said it was spark knock and that I had ran it too long. He said that the plugs were fried on the one side and the one head needed to be redone. I keep asking him what caused it in a brand new motor and he still can't tell me. He is claiming that I ran it too long and it is what messed up the motor, I am claiming there was something wrong with the motor in the first place and the intial problem is his fault. Do have merit? Also, how does spark knock only effect one side of a motor?

oh yeah its a 4.3 mercruiser motor.

Can someone set me straight here?
 

Bt Doctur

Supreme Mariner
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Aug 29, 2004
Messages
19,092
Re: spark knock

And what breakin procedure did you follow with this "new" engine?
 

coheej

Petty Officer 2nd Class
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Sep 15, 2010
Messages
128
Re: spark knock

Could the compression on one head have been higher than the other. Was the fuel on one bank leaner than the other? Could a head gasket have been put on backward blocking some of the cooling passages?

When you heard the spark knock, you should have idled or slowly returned to dock.

There's a good chance that some of the pistons may have been cracked, espescially at the ring lands.
 

Bondo

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70,465
Re: spark knock

I tried to get the power I could out of it,

Ayuh,... More than likely, the original motor died from detonation...
If so,+ the new motor's suffing the same fate,...
I'd think Both motors were Smoked by a Fuel Restriction in the boat's fuel supply system...

When it began to loose power, you should have been Idling it back to the dock,...
Not trying to get more power out of it...
In my mind, your standing with your Mechanic, Stopped at that point, You bought the next repairs...
When there's a Problem,.... Stop, Don't keep pourin' the coal to it....

It could be as simple as $h!t or water in the fuel filter.........
 

Aloysius

Chief Petty Officer
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Aug 21, 2010
Messages
484
Re: spark knock

Listen to bondo on this one.

If you lose a motor, the FIRST thing you do is determine WHY!!!
 

wire2

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Jun 25, 2007
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1,584
Re: spark knock

I hadn't heard the term "spark knock" before but I'll guess it means detonation?
If so, it's not caused by the spark.

Anyways, I also have to question why it seems to have affected only 1 bank. That would tend to point to a fuel restriction in one side of the carb, possibly a blocked jet.
 

Aloysius

Chief Petty Officer
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Aug 21, 2010
Messages
484
Re: spark knock

Spark knock and detonation are pretty much unrelated.

"Spark knock" is the premature ignition of the fuel burn, causing a pressure spike before TDC. Audibly noticable at low rpms.

"Detonation" is the violent spontaneous explosion of the fuel charge, not a controlled burn. Detonation is usually unheard, as it occurs at higher rpms, and is destructive to both pistons from the temperature, and bearings from the shock load.
 

Cpt D

Seaman Apprentice
Joined
Jul 20, 2009
Messages
42
Re: spark knock

Spark knock and detonation are pretty much unrelated.

"Spark knock" is the premature ignition of the fuel burn, causing a pressure spike before TDC. Audibly noticable at low rpms.

"Detonation" is the violent spontaneous explosion of the fuel charge, not a controlled burn. Detonation is usually unheard, as it occurs at higher rpms, and is destructive to both pistons from the temperature, and bearings from the shock load.

Don't quite understand how not related. From your description they sound like the exact same thing, except one is at low RPM's and one is at higher RPM's.
 

mustangfam

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Joined
Sep 16, 2010
Messages
7
Re: spark knock

I asked the mechanic that very thing when i picked it up. This is my first boat and I bought it with the bad engine. I asked to I need to come back to have the carb fine tuned or lifters checked he said no. He said just dont run it full open for about the first 10 hours. Thats it.
 

Aloysius

Chief Petty Officer
Joined
Aug 21, 2010
Messages
484
Re: spark knock

Spark knock, or pinging, or pre-ignition, is still a fuel burn at a finite rate, and doesn't build the intense heat/pressure of detonation, which is instantaneous. You may want to think of detonation as pinging on steroids.

10 seconds of pinging is annoying..10 seconds of detonation is terminal.
 

mustangfam

Cadet
Joined
Sep 16, 2010
Messages
7
Re: spark knock

Ayuh,... More than likely, the original motor died from detonation...
If so,+ the new motor's suffing the same fate,...
I'd think Both motors were Smoked by a Fuel Restriction in the boat's fuel supply system...

When it began to loose power, you should have been Idling it back to the dock,...
Not trying to get more power out of it...
In my mind, your standing with your Mechanic, Stopped at that point, You bought the next repairs...
When there's a Problem,.... Stop, Don't keep pourin' the coal to it....

It could be as simple as $h!t or water in the fuel filter.........
Bondo,
I bought the boat with a bad motor. The guy didn't winterize it correctly and the block cracked. The mechanic I went to was the this guys mechanic too. I honestly didn't hear a knock just felt the lack of power. I probably should have stopped, but I was trying to take off the fuel line in the water, fuel filter and so on and so on trying to fix it and run it. How is that it just happened to one side of the motor?
 

mustangfam

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Joined
Sep 16, 2010
Messages
7
Re: spark knock

Bondo,
I bought the boat with a bad motor. The guy didn't winterize it correctly and the block cracked. The mechanic I went to was the this guys mechanic too. I honestly didn't hear a knock just felt the lack of power. I probably should have stopped, but I was trying to take off the fuel line in the water, fuel filter and so on and so on trying to fix it and run it. How is that it just happened to one side of the motor?
Funny part is that the mechanic can't tell me what caused it. Everytime i ask he just keeps saying you should not have kept running it and I tell him I understand that but what caused it and what is going to keep it from happening again of which he can't tell me.
 

coheej

Petty Officer 2nd Class
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Sep 15, 2010
Messages
128
Re: spark knock

If this destroyed the head and probably the pistons which have not been looked at, then this is much more than preignition or pinging. This is all the signs of detonation. Either the fuel was lacking on that side, or the head temperature was much higher on that side, or the compression was higher on that side.
 

rodbolt

Supreme Mariner
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Sep 1, 2003
Messages
20,066
Re: spark knock

spark knock, detonation, preigntion are all about the same thing depending on the tech your talking too.
they are all uncontroled cumbustion that is harmful to the engine.
timing advacing to far can cause spark knock,detonation or preignition, depending on who your talking to.
lean air/fuel in a cylinder can cause it, as can plugs wires crossfireing and deposits on the head or piston crown.
biggest thing I see do it is the engine over propped for the load that day or poor quality fuel from last year.
 

mustangfam

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Sep 16, 2010
Messages
7
Re: spark knock

If this destroyed the head and probably the pistons which have not been looked at, then this is much more than preignition or pinging. This is all the signs of detonation. Either the fuel was lacking on that side, or the head temperature was much higher on that side, or the compression was higher on that side.
I don't understand how it would be bad gas or over propped when it only affected one side of the motor.

The mechanic had better give me something to go on. B/c if the initial problem is not fixed then what is to prevent it from doing this again?
 

Bondo

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Messages
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Re: spark knock

Ayuh,... It could be 1 side of the carb goin' lean I suppose...
It is a dual plane manifold...
 

Aloysius

Chief Petty Officer
Joined
Aug 21, 2010
Messages
484
Re: spark knock

Yea, bondo, but a dual plane manifold splits differently...2 and 1 on each side on a 4.3.

Only one side..someone milled too much off the head, and compression is too high. Or, used a head with a smaller combustion chamber.
 

mustangfam

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Sep 16, 2010
Messages
7
Re: spark knock

how in the world do i find out if its the head. If the machine shop is fixing the head will they not measure for that? Mechanic says the head should be done early this week. Also, how do I ensure everything is runningwell? Every 30 minutes do i look at the spark plugs when i am on the water?
 

coheej

Petty Officer 2nd Class
Joined
Sep 15, 2010
Messages
128
Re: spark knock

how in the world do i find out if its the head. If the machine shop is fixing the head will they not measure for that? Mechanic says the head should be done early this week. Also, how do I ensure everything is runningwell? Every 30 minutes do i look at the spark plugs when i am on the water?

Ask him to cc the combustion chamber and calculate the compression ratio.
 

mustangfam

Cadet
Joined
Sep 16, 2010
Messages
7
Re: spark knock

ok. New news. Head is cracked and trashed he says due to me running it too long. The motor also has a crack in it and is getting water into it. Mechanic says they are not related, but he is contacting the motor dealer about the warranty on the motor to see what they will do.
 
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