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Excessive knocking on my 2008 Suzuki C50. Sounds like valves or chains. While on the throttle in low RPM (gear slightly under load) the bike will dip in power (almost like backfiring). Medium to hard throttle, no issues at all.

...So new fuel, 2 valve adjustments, oil, plugs, you name it - no change. So now I have the engine sitting on my workbench. Sounds like it might be coming from lower half of rear cylinder (still in upper). Pulling the engine was easy enough, but I've never torn into a bike motor before (although I have my fair share of exp with cars). So now Motorcycle Forum will you come with me on my journey? I'll follow your steps and suggestions, as well as post pics along the way. Oh, and bike has 6100 miles. Knocking started after a stupid ride of 100+ for a few minutes.

The right head is from forward cylinder, left is rear. The forward exhaust and intake vales are dirty. The rear intake are very dirty, the rear exhaust are basically clean. Piston heads are solid black, but dont look too bad. Do these clues help?
 

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Well you're sure diving in!

What other info can you tell us about the bike/engine? How many miles on it? Is it burning oil? Have you done compression or leak-down tests? What controls the spark advance and is it turning freely and returning (if applicable.)

Knocking started after a stupid ride of 100+ for a few minutes.
Hmm. Any chance you hooked up an oil pressure gauge when it was hot after this? Knocking after pushing it hard might (MIGHT) imply some bottom end issues, big ends, etc. I would also look closely at each valve and seat while the heads are off. Do any of the valves sit further in their seat areas than the rest. You can also look at the end of each stem and see if any sit suspiciously higher. These would indicate a sunk seat. Those are probably the easiest to check right now.

It could also be something lower down too like a spun big end bearing. This would involve splitting the bottom end apart and physically checking each one. Personally I would be darn sure it wasn't something easier before diving in that far.
 

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Ticking or knocking? If its valves, u sure the clearances are correct? My bike is loud as hell and i mean the valves . Now its loud as hell cause of the exhaust so i dont hear the valves ;) and yes they are in spec. Could be a valve problem like comeon is describing, could be a seal, etc. . If its the chain , maybe the tension/er? Just a couple of ideas. Is that a wet spot on the forward cylinder at the 9oclock position between the 2 exhaust valves? The picture looks very dark to me, but my desktop monitor is garbage. Looks like typical carbon from what i can tell. I would worry more about the noise. Anyway, keep going. Get your spring compressor and remove the valves and check it out while youre there. Will only take a few minutes. Keep us posted

Next time before you tear it apart:
You tube video! Show us and dont just tell us :) Use slow motions going towards/away from noise at different angles. You might also want to try a mechanics stethescope to pinpoint it. I know they can come in handy. Or drop by the service dept. at the dealer and see if they can take a quick look. Noise does travel to the wierdest locations. ive learned that the hard way over the years by pretty much disassembling almost a whole bike to find out it was my speedomter housing vibrating. I always heard the noise coming from down low at the bottom of the frame. Everybody I talked to said it was a ujoint or tires. Let people ride it that have been around for a while. Took it to the dealer 2 times! Nobody could pinpoint it. Knew it wasnt tires so I had the whole rear end apart (swingarm, ujoint, wheel, fender, etc.). Everything looked great. One day i put my hand on the speedo housing while rolling along and presto! WTF!
 

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Wow, you guys are seriously awesome. When this is over I owe you two a case. I honestly appreciate all the help. Ok, here goes...

In all honesty, I drove the bike for a few miles after I hear the noise. I was in the process of moving cross country because of a military move, so repairs were not my primary concern. I never did a compression check. After two valve adjustments in-frame with no luck I ripped the engine out because I was sure it was the cam chain/tensioners. Those were fine, so slapped everything back together and did a third valve job as I felt it was ever-so-slightly off, before I popped the engine back in. No change sadly. A few months passed as I was off for training and now I pulled the engine again. Now we're in the present (tonight)...

I took out valves. They all look healthy and clean, as well as seals. As you can see from the pictures, my foward piston has some scoring. On both exhaust and intake sides. 1st pic is intake, double scoring pic is exhaust. Scoring on cylinder matchest piston, although it hard to see in pic (cylinder "feels" smooth. On top of the piston was debris, what looked like grayish shavings that I could easily crush in my fingers. Rear cylinder is perfect. I'm guessing this needs to be replaced now and possibly something with the cylinder? Could this be the cause of knocking, or just an effect of trouble below in the lower?
 

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Seeing that, if it were my bike I would probably do a complete tear down at this point and check the lower end stuff. It's tough to say whether that could be the cause of your noise specifically (never having heard it), but it's certainly evidence something went wrong in there - makes me worried about the big ends and such.
 

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Thank you all for the replies and support. Tomorrow I'm taking the cylinder to a show to check its bore. That piston is pretty much toast. From my own digging and advice here I've determined a few possible causes:

#1 Overheating
Not sure if i mentioned this, but i the few miles i drove on it with the knocking, I "smelled" it overheating once (radiator fan was not cutting on). Now this could be from my 1st tear down as I never noticed it until directly after that. Possibly never was an issue until then, which leads me to my second possibility.

#2 Something worn in the lower
Although not entirely plausible, I can see how this could happen as I was riding the bike fairly hard.

#3 Wrist pin, bearing, etc. in upper (minus heads)
I have not dug into the wrist pins, bearing yet, but time will tell. Think this could cause the piston to slap.

#3 Dirty Oil
I did find debris in the damaged jug. However i believe this is simply the scouring piston flaking off.

#4 Low Oil
Very highly unlikely. After I noticed the knocking, I immediately checked the oil level. This was always fine.

So for my remaining diagnosis...

Overheating: either thermostat or fan electrical. Is there a way to check either of these?
Something in lower: would the community advice a lower tear down?
Wrist pin / bearing: how would i check these for serviceability?
 

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Check to make sure you dont have a blown fuse first to make sure the fans are working. For the thermostat, boil a pot of water and put the thermo in there. It should open when its in the hot water (slowly) , and close when you take it out. For the fan, you can use a multimeter to see if youre getting voltage at the connector thats on the switch on the radiator. Also, you can unplug the connector from the switch and stick a small flatblade screwdriver in there and touch the screwdriver shaft to the metal on the radiator with ign. switch on. If fan is running, it works. If not, probably your switch. You need to get a manual to check specs/tolerances for wrist pin. But, if it slides into the connecting rod smoothly and there is no play, youre probably good. Always good to check though while its apart and after the damage. I would check the Bottom end also
 
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