GS750 TSCC engine knock advice

[Guest]

Hey gang, been lurking a while, finally making a post. I've got a 750l with a terrible knock. The guy I bought it from said it just started out of nowhere on a ride one night. When he bought it, the carbs had been boogered up by someone, so he replaced them and "tuned" them himself... He didn't replace (or install for that matter) the o-rings on the intake ports, cut the mufflers off, and opted for pod filters... How much we wanna bet those carbs still have stock jetting? I haven't looked into it yet, but these things make me wonder if it got ran lean. I got it started to hear the noise, and boy is it rough. Very hash metallic, a bit like a ball peen hammer hitting an anvil and bouncing. I pulled the valve cover off just to see if there was anything obvious like a loose and possibly jumped chain, bent/broken valve, spring, whatever, but no luck. The whole engine is out the frame now, but before I pull the head and totally disassemble, you think I should bother with compression/leak down or will a noise this bad be well noticeable? I'll try to post a video of one of the sadest sounds you'll hear this year. Also, if you respond, add your favorite place to buy engine parts for these old girls. Thanks for any thoughts, ideas, and inputs.

[Kart]

[hjfisk]

That engine is noted for pour oiling to the top end , but that sounds more like a bottom end knock to me. I would turn it upside down and split the cases . My guess crank is had.

[storm 64]

Wow! Something is in there and it wants out! My gess is piston to valve or piston to something. Good luck

[hampshirehog]

I would pull the valve cover first - it might be something in the top end flapping around.

[rapidray]

The 80-82, 16 valve 750s are NOTORIOUS for spinning rod bearings. I used to road race them & tried everything to make them live. I build race engines & THAT engine was just a bad design. You need to split the cases & my guess is the #3 rod bearing is spun.
Ray.

[blowerbike]

in my experience with 80-82 750's.....
the sound is the piston hitting the head...look for carbon removed on which ever cylinder/combustion chamber and you'll find the bad rod.
this is the worse one i have heard...most came to me needing a "valve adjustment"

[storm 64]

Hey Kart, did you ever figure out what was making that knock? There sure been some caterstofice engine failures lately. First this then the valve shim/head damage guy. Oh, the clutch melt down...

[Kart]

Finally got a chance to pull it down a bit, looks like the No.2 cyl lost the rod bearing. I haven't gotten as far as to split the crankcase to remove the rod cap, but there is soooo much play. I don't think its possible the crank can be salvaged. You can see though that the piston was making contact with the head, and while I don't think it bent any valves, it peened the top of the piston and bottom of the head pretty bad. You can actually feel a slight depth on the head. Think the head is shot too? That should change compression on that cylinder... Looking for crank, rods and Pistons for sure. Hope not cylinder head.

[Kart]

[Kart]

image.jpg Figured it out I think.

[Kart]

image.jpgPhoto bucket is the worst these days.... Having a hard time posting pictures.

[rapidray]

Yep, bad rod bearing, the piston is hitting the head. Was it #3 or #2?
Ray.

[rapidray]

Never mind, I see it was #2. Now you have to see if it killed the crank journal. You may find it a better end result to just install a 16 valve 1100 or 1150 motor. Those engines are pretty bullet proof & only need a couple of things done to them to make them near indestructible. The bigger 16 valve engines bolt right in too!
Ray.

[Kart]

Never mind, I see it was #2. Now you have to see if it killed the crank journal. You may find it a better end result to just install a 16 valve 1100 or 1150 motor. Those engines are pretty bullet proof & only need a couple of things done to them to make them near indestructible. The bigger 16 valve engines bolt right in too!
Ray.

I wouldnt be be opposed to that idea, but finding one...