Loud knocking at idle

[RBN]

Just picked up a cheap bike as a source of spares, fired it up today to find a loud knocking noise coming from the motor at tickover. It's a deep/low pitched noise so not the sort of noise I'd associate with top end problems, it's so bad that each knock vibrates the frame.

But twist the throttle a little and the knocking stops instantly and the bike sounds smooth as the day it was made.

Am I looking at a shot big end/crank bearing?

[jkv45]

Originally Posted by RBN

Just picked up a cheap bike as a source of spares, fired it up today to find a loud knocking noise coming from the motor at tickover. It's a deep/low pitched noise so not the sort of noise I'd associate with top end problems, it's so bad that each knock vibrates the frame.

But twist the throttle a little and the knocking stops instantly and the bike sounds smooth as the day it was made.

Am I looking at a shot big end/crank bearing?

The last bike I looked at that had a loud knock at idle had a louder knock when revved. The owner said it was a rod bearing.

Ninjas make odd sounds when cold at idle. Does it do the same when warm and the idle is at the correct 1300 RPMs?

Oil level and condition look OK?

[baxtc1]

Does it still do it at idle in gear?
Does the oil light go out in a reasonable time after starting?

[Ninjinsky]

Sounds like clutch clonking from an uneven idle. It is surprisingly loud and sounds just like big ends but it isn't so don't worry.
Does it diminish when you pull the clutch in? That's a good test.

[RBN]

Thanks for the info/suggestions. will post again when I've had a chance to fire it up again (in the middle of moving the shed contents around to make room for it)