Topic: Lower end making grinding noise when under heavy load- what's up? cb550k '76

[kippstakes]

If I put too much pressure on the engine (ie starting into a hill, juicing it a bit and opening the clutch quickly, etc.-- it doesn't take much, anything but the most gentle acceleration will produce the effect) the engine will bog down and try to die like something is binding and preventing free movement inside the lower end. This is accompanied by a hair-raising grinding/wrenching sound that seems to emanate from the crankcase or transmission- it can be felt right up through the handlebars; it is tough to miss.
This phenomenon has persisted over two clutches, so I think we can rule that out. Despite clean carbs and 135 compression on all four cyls the bike still runs like it's towing a dead elephant, which makes me wonder if something is binding somewhere.

Any ideas?
Thank you

[Redline it]

I don't know too much, but if you keep riding it, the problem will eventually fall out of the case, then you'll know what it was. Already 2 clutches? It's way beyond any worries. Probably something minor.

[scottly]

Crusty, rusty, worn-out, too-tight chain and sprockets? Thrashed rear wheel bearings?

[kippstakes]

Wheel bearings are solid and both sprockets are in good shape, chain is up to tension. The noise is definitely coming from the crank case somewhere.
I just did a top end rebuild about nine months ago, and would be crushed to have to tear the whole thing down again to rebuild the bottom- it would be great to avoid that.

[Redline it]

was it 2 clutches in 9 months also? 9 months is good, I usually gotta take everything back apart after an hour.

[kippstakes]

Haha no I replaced the clutch that came on the bike- because of this very noise.. It's been making this noise for years, but has gotten much worse over the last month or so.

[Duanob]

Crusty, rusty, worn-out, too-tight chain and sprockets? Thrashed rear wheel bearings?

Chain rubbing against the cases is my first thought. Take the cover off and see.

[jonda500]

Sounds similar to the problem I have with my 500 - the only way to avoid the gnarly grating noise and vibration is to completely avoid putting any load on the engine unless the revs are at at least 5000rpm.
Personally, I am convinced that my primary chain is slipping but I wont be able to verify that until I pull it apart. John

[kippstakes]

I had considered that, except that my bike seems to bind up and lose power when the noise occurs, like something is trying to lock up against something else in my crankcase. My guess would be that if your primary chain slipped, the bike would rev UP, due to the temp loss of load. Does your bike rev up when the noise happens?

[jonda500]

The engine doesn't rev up much when it happens - stays the same or slows down if anything, maybe due to the uneven transmission of power from the engine to the gearbox and the chain riding up on the teeth?
Once the revs are over 6000rpm, I can give it full throttle without it happening.
At idle speed my primary chain rattle sounds really awful when cold and even worse when hot.
John

[Redline it]

It's surprising how much speed can be produced by gear ratios, and how much power can be robbed from a worn bushing.