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Old 04-13-2013, 06:16 PM   #1
Boxer Metal OP
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Engine Knock Under Load

So I have a motor that once it is warmed up there is an engine knock only under load and it goes away at higher RPM's No noise when cold or at revving the motor at idle. Sounds like a rod. Ideas?
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Old 04-13-2013, 06:34 PM   #2
Big Bamboo
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Could it be pinging (sounds like rocks rattling in a can) ...or is it a steady knock knock sound? If so, retard the timing a bit and see if that helps.
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Old 04-13-2013, 06:35 PM   #3
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Originally Posted by bmweuro View Post
So I have a motor that once it is warmed up there is an engine knock only under load and it goes away at higher RPM's No noise when cold or at revving the motor at idle. Sounds like a rod. Ideas?
Change the oil. Put in strait 30 or 40wt. and see what happens. If it shuts up or it has to get a whole lot hotter before it starts I would suspect a rod. You can also put a pressure gauge on it (pull the idiot light sender, it's m12x1 thread IIRC) and see if the knock happens as the oil pressure drops under load.

Check with a mechanics stethoscope at the cylinder base at idle and reving in the driveway. It may be knocking then but not loud enough to get through years of hard rock and power tools hearing damage.

Also maybe drain the tank into your truck and run some of the highest test gas you can find in the motor. That is diagnostic for detonation type knocks. Or add a can of octane booster to what is in there.
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Old 04-13-2013, 07:17 PM   #4
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Originally Posted by bmweuro View Post
Sounds like a rod. Ideas?
Sounds like a rod bearing.

I'd pull the oil filter and dissect it for more clues.
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Old 04-13-2013, 08:04 PM   #5
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Quote:
Originally Posted by bmweuro View Post
So I have a motor that once it is warmed up there is an engine knock only under load and it goes away at higher RPM's No noise when cold or at revving the motor at idle. Sounds like a rod. Ideas?
Pull it apart for inspection before any further damage is done ?


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Old 04-13-2013, 08:29 PM   #6
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I'll third the notion of a rod bearing. Quick check would be to read the oil filter. Is the maintenance/history of the bike known?

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Old 04-13-2013, 11:17 PM   #7
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What does it sound like in gear, stationary and starting to let the clutch out while you hold the rear brake on ?
If it knocks under those conditions it will likely be a rod versus detonation.
You know well enough how easy it is to verify and if I remember correctly it's usually the right side that let's go first.
If you catch it early enough you can clean the rod journal up with crocus cloth and just replace the bearing inserts.

Do you have any history on this engine ?
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Old 04-14-2013, 12:04 AM   #8
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Inspect the filter? You might find the problem before you pull the filter out. You know? A $2000 O-ring. BTDT a number of times.

supershaft screwed with this post 04-14-2013 at 12:30 AM
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Old 04-14-2013, 01:38 AM   #9
ME 109
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Originally Posted by supershaft View Post
Inspect the filter? You might find the problem before you pull the filter out. You know? A $2000 O-ring. BTDT a number of times.
Quite possibly so. Or not.
The filter may reveal dark secrets, or not.

We want to look in the filter for colours for big end wear, silver and copper iirc?
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ME 109 screwed with this post 04-14-2013 at 06:26 AM
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Old 04-14-2013, 04:47 AM   #10
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My money's on a rod bearing being marginal. With cold oil there may be enough oil pressure to fill the gap but it gets noisy when the oil heats up.
As for going away at high RPM, again maybe enough oil pressure or maybe the knock "blurs" into a continuous noise that can't be distinguished as a knock.
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Old 04-14-2013, 06:36 AM   #11
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I have never had a rod knock under acceleration. If the knock happens under light load (i.e. constant speed), or it knocks after the engine warms up when stationary and letting the clutch out with the rear brake engaged - I would pull the cylinders and inspect the rod crank journals. The most daunting part of pulling the cylinders is removing the exhaust if it has not been off in a good while. Inspecting the journals is the only way to know for sure. Good luck!
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Old 04-14-2013, 07:06 AM   #12
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Can the big ends be accessed by dropping the sump?
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Old 04-14-2013, 07:13 AM   #13
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Can the big ends be accessed by dropping the sump?
Absolutely.

Have you got a 9" grinder?
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Old 04-14-2013, 08:01 AM   #14
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Absolutely.

Have you got a 9" grinder?
Absolutely.

If Santa brings me a plasma cutter the grinder will become my third favourite tool.
5lb sledge with the 4 foot handle is the number one tool in my shed.
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Old 04-14-2013, 08:23 AM   #15
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Plasma is so...........Star Wars man.

We got some nice abrasive water streams.
Or a 5lb hammer.
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