Scratching noise at certain RPMS and loss of power

[sam000lee]

My GS750 (8valve) started making a scratching noise at certain RPMs on a recent trip and has noticeably less power/response. I rode out ~130 miles and back and it happened sometime right in the middle and has been like that since.

The noise will go away in certain gear/RPM combinations. For example, if I'm going ~55 in 4th and then shift up into 5th, there's the noise and a lack of power. Everything at idle and low speeds <15 mph feels normal.

Pulling the plugs, the 1 & 2 cylinders were carbon fowled whereas 3 & 4 look fine.

Just for fun, I pulled the valve cover since I couldn't remember how many miles since my last valve adjustment. Everything's in spec. there, but I did notice some unevenness on the exhaust cam in cylinder one. The actual face looks fine, but the edge looks almost cerrated. Looking at the other cam lobes, this is present on a couple of others, but it's not as noticeable (see cam C for reference). The shims and buckets look normal and I don't think this would affect performance in any way but it was my first time noticing it.

I've also gone ahead and pulled the carbs and done a preliminary spray through with compressed air and carb cleaner. Everything also looks fine from that perspective. Yes, I've done a full disassembly and thorough cleanup with new O-rings, intake boots, etc. and have been riding the bike regularly since. I tore one of the adjustment screw O-rings and totally mangled the float bowl gaskets so am waiting on replacements right now.

Ignition is a Dyna with the green coils installed the year before last and also have been working well. I reset the CCT at some point last summer so don't suspect that's at fault, either.

Any help would be appreciated!

 

[sam000lee]

Oh also I did a compression test - 145 130 130 140

 

[sam000lee]

So I did some checks on the bike and have it back together now. Valves are in spec. Carbs are good. Took it out again and the noise happens exactly when I shift into 5th gear. It'll pull strong in 4th, then I shift into 5th going 65mph and there's a scratching/wining noise and it will barely accelerate. Could the 5th gear clutch plate be damaged and what would that behavior look like?

 

[Gorminrider]

You might want to see if you can turn the engine over with a wrench in various gears (spark plugs out) That might give you an indication that narrows it down.



"Scratchy-whiny in 5th" is not one I've had..not on a chain-drive bike, anyways. Unfortunately, I'd look to engine internals -you "found something" on the cams but gearbox and engine come to mind for me -not carbs or the clutch...
If it isn't the gearbox itself, well...the engine is next seeing as changing to 5th is your highest gear and the highest demand on engine.
. I'm kind of curious if thisis still scratchy-whiny" down hill or coasting rather than accelerating on the flat.

..whiny suggests gears. Scratchy suggests something eating through a bearing or case.
"scratchy" or "whiny" just isn't in my experience of clutch noises. Their failure is usually as slippage or a lag as the plates are slow to engage

gearbox, Camchain (perhaps the tensioner blade is failing) and more- all bad and none easy.