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Engine Knock when starting but it goes away...

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Engine Knock when starting but it goes away...

PostAuthor: BARKER » Wed Oct 09, 2019 8:13 am

Hi guys,

I picked up a Virago on the weekend that has 75,000 kms on the clock. It was a $150 heap, but I'm a bit of an addict when it comes to bringing old engines back to life. I mainly dabble in old Honda's so please bear with me...

This old bike had a seized engine from sitting for a couple of years. I free'd it with some homemade penetrating oil and a little bit of pressure on the crank shaft bolt (yes... I am aware of the possible/LIKELY ring of rust and the damage it can do to cylinder walls). At this point the carbs are off the bike as is the "missing-when-I-bought-it" front exhaust pipe.

Now when I hit the starter there is a wicked knock coming from the engine. However, when I cover the intake side of the front cylinder head with my hand, the knock goes away. No change when I do this on the rear. Theory being that the vacuum in the cylinder cushions the piston/con-rod movement.

My immediate thought is cooked front cylinder big end bearing/crankshaft. Could this be something else though?

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Re: Engine Knock when starting but it goes away...

PostAuthor: bstig60 » Wed Oct 09, 2019 10:06 am

Can you post an audio file? It is difficult to diagnose the problem from your description only. Have you run a compression test?
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

Please take the time to edit your profile to show your location. City, State/Province, and Country.
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Re: Engine Knock when starting but it goes away...

PostAuthor: Arjay » Wed Oct 09, 2019 10:31 am

would honing cylinders help? Sounds like its time to commit to complete tear down and check each part.

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Re: Engine Knock when starting but it goes away...

PostAuthor: BARKER » Sun Oct 13, 2019 7:46 pm

With a touch of oil squirted in through the spark plug holes I get 150 psi on both front and rear. I'll work on getting a video done to share with you guys.

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Re: Engine Knock when starting but it goes away...

PostAuthor: bstig60 » Mon Oct 14, 2019 9:39 am

Barker (OP): What's the compression reading without the oil and with the throttles wide open?

Arjay wrote:would honing cylinders help? Sounds like its time to commit to complete tear down and check each part.


Aray, quite jumping to conclusions. You always try the simple and less costly things first.
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

Please take the time to edit your profile to show your location. City, State/Province, and Country.
It is easier to help you if we know where you are.


SERVICE MANUAL DOWNLOAD LINK
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Re: Engine Knock when starting but it goes away...

PostAuthor: Arjay » Mon Oct 14, 2019 12:04 pm

Very true Bill...!

But, how bad is that knock? Sometimes its best to rebuild before further damage. If he throws a rod...

Rj

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Re: Engine Knock when starting but it goes away...

PostAuthor: Jake » Mon Oct 14, 2019 1:57 pm

BARKER wrote:Hi guys,

My immediate thought is cooked front cylinder big end bearing/crankshaft. Could this be something else though?


Not too likely to be a con rod bearing unless the crankcase was devoid of oil. Those bearings are roller bearings and alway submerged in oil.

Adjust the valves to spec, then check compression again.
_______________________________________________________________________
“It ain’t what you don’t know that gets you in trouble.
It’s what you know for sure that just ain’t so.”

~Mark Twain


There is an old Chinese proverb - "Man with one watch knows the time. Man with two, not sure."
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Re: Engine Knock when starting but it goes away...

PostAuthor: BARKER » Tue Oct 15, 2019 6:35 am

Jake wrote:
BARKER wrote:Hi guys,

My immediate thought is cooked front cylinder big end bearing/crankshaft. Could this be something else though?


Not too likely to be a con rod bearing unless the crankcase was devoid of oil. Those bearings are roller bearings and alway submerged in oil.

Adjust the valves to spec, then check compression again.


It was definitely had oil in it when I bought it... like 8 quarts! It almost overflowed in my drain pan.

This thing lead a mysterious... likely hard... life!

BARKER
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Re: Engine Knock when starting but it goes away...

PostAuthor: Jake » Tue Oct 15, 2019 11:18 am

BARKER wrote:
It was definitely had oil in it when I bought it... like 8 quarts! It almost overflowed in my drain pan.

This thing lead a mysterious... likely hard... life!



The vacuum petcock is trash, that was oil and gas. I’d replace the petcock with a manual one, or a lawn mower shut off valve for the short term, otherwise, I guarantee it will happen again.
_______________________________________________________________________
“It ain’t what you don’t know that gets you in trouble.
It’s what you know for sure that just ain’t so.”

~Mark Twain


There is an old Chinese proverb - "Man with one watch knows the time. Man with two, not sure."
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Re: Engine Knock when starting but it goes away...

PostAuthor: bstig60 » Tue Oct 15, 2019 4:20 pm

If it had gasoline in the oil and had been run for a while, it is possible a rod bearing has been scored. Really need that vid to hear what the knock sounds like.
https://www.partzilla.com/catalog/yamah ... aft-piston
Bill
"It´s a friggen motorcycle, it´s not supposed to be comfortable, quiet or safe. The wind noise is supposed to hurt your ears, the seat should be hard and riding it should make you s**t your pants every now and then. "

Please take the time to edit your profile to show your location. City, State/Province, and Country.
It is easier to help you if we know where you are.


SERVICE MANUAL DOWNLOAD LINK
http://viragotechforum.com/viewtopic.php?f=220&t=31501#p279545

Image
User avatar
bstig60
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Posts: 13474
Images: 38
Joined: Sun Jun 08, 2008 6:16 pm
Location: Sharpes, Florida Flag
Bike year & model: 1999 Yamaha Virago XV1100LC Cruiser,
2003 Honda Goldwing GL1800 Touring
2007 Honda ST1300A
1987 Honda TLR200
Sex: Male



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