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Engine Knock when starting but it goes away...

PostPosted: Wed Oct 09, 2019 8:13 am
Author: BARKER
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?

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

PostPosted: Wed Oct 09, 2019 10:06 am
Author: bstig60
Can you post an audio file? It is difficult to diagnose the problem from your description only. Have you run a compression test?

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

PostPosted: Wed Oct 09, 2019 10:31 am
Author: Arjay
would honing cylinders help? Sounds like its time to commit to complete tear down and check each part.

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

PostPosted: Sun Oct 13, 2019 7:46 pm
Author: BARKER
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.

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

PostPosted: Mon Oct 14, 2019 9:39 am
Author: bstig60
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.

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

PostPosted: Mon Oct 14, 2019 12:04 pm
Author: Arjay
Very true Bill...!

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

Rj

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

PostPosted: Mon Oct 14, 2019 1:57 pm
Author: Jake
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.

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

PostPosted: Tue Oct 15, 2019 6:35 am
Author: BARKER
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!

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

PostPosted: Tue Oct 15, 2019 11:18 am
Author: Jake
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.

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

PostPosted: Tue Oct 15, 2019 4:20 pm
Author: bstig60
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