83 GS400 engine knocking

[drumstyx]

So I noticed after coming back from Friday 13th, a knocking in what seems like the bottom end on the left side. It's always seemed a little louder than the right, and the left muffler's even blown out (something may be wrong fuel delivery-wise). I've got a video here, basically I'm worried that it's the bearings and I'll have to look at a major overhaul/engine swap. It's worse when it's colder (mornings), but I was thinking maybe I could get a second opinion...I could still hear the knocking a bit when it was warm, so I took this video:

https://www.youtube.com/watch?v=q7eceqsVpek

The camera's crap and compensates for the noise unfortunately, but you can still hear *taptaptap* occasionally.

EDIT:

Here's a cold start, much more noticeable. Note again that the left exhaust is blown out. It seems to go away when revving, only really present at idle.

http://www.youtube.com/watch?v=hziYb-wIByM

[Gorminrider]

... I think you would have to hold the video a bit longer on the "noise" so the experts here can form an opinion. I'm not one of those-but I listened anyways and the video is too short.

[drumstyx]

Yeah, I'll update soon with a 'cold' start. Should be much clearer.

[drumstyx]

Updated with cold start video.

[Mekanix]

Rod bearing.

Damage is done, replace the bearing if the crank isn't scored up and make sure the oil passages aren't clogged.

Also get some oil pump gears from a GS500, it should help prevent this with higher oil pressure.
You'll notice the noise goes away slightly above idle where the forces and oil pressure are higher.

Another thing that makes this kind of noise but not as bad is if the carb synch is wayyyyy off and one cylinder isn't doing any work at idle.

[drumstyx]

It's very possible the carb synch is way out, since like I said, I've always suspected the left cylinder running lean (exhaust gets hotter faster, corrosion from heat on that side's header etc). Am I grasping at straws here? Is this at all likely to be the issue?

I read on another forum that to test for bearing wear, you can rotate the crank until the piston starts to go down, then push on the piston with something (screwdriver, socket extension?) and if it moves at all, it's a bad bearing. Is this true?

What's involved in changing the bearing out though? The Haynes manual seems to say anything to do with the crank/bearings means the motor comes out and is completely disassembled. Is this so? Might just be worthwhile to swap in my spare motor (assuming it works correctly).

[Gorminrider]

"....I've always suspected the left cylinder running lean (exhaust gets hotter faster,"

x your fingers and hope it's as Mekanix said: "Another thing that makes this kind of noise but not as bad is if the carb synch is wayyyyy off and one cylinder isn't doing any work at idle" and that you can fix it at carbs or ignition....
2nd video does sound like it might be no-firing
so...put your hands on the exhaust pipes when !starting!. you can only really tell then, because even if a cylinder isn't firing, after awhile, the heat will move through the metal and it's harder to tell.

[drumstyx]

Well, the right pipe (without the knock) definitely gets warm faster on start-up, but of course the left one gets hotter after a ride.