Engine Knock when warm

[MalD]

Ive recently bought a 1975 CB750F1 as a bit of a project bike. Its a runner so i put it through an MOT so i could run it a bit to see what needs doing. Starts no problem and generally runs fine. However as soon as its warm i get a low knocking sound!!! It does seem to smoothe out if i rev it but as soon as it idles its back. Leave the bike to cool and its fine again. First thoughts were carb balance which has been done, with no effect. Any other advice before the inevitable winter strip down


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[hairygit]

Does it quieten down if you pull the clutch in?

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[MalD]

Does it quieten down if you pull the clutch in?

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Hi, No I get the usual clutch rattle which  quietens down when I pull the clutch in. The knock persists unfortunately, but only when she’s warm!!!


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[MrDavo]

Could be the bottom end, any sign of the oil light when she's running?

[MalD]

Could be the bottom end, any sign of the oil light when she's running?

Funnily enough it did flicker yesterday on idle. But went straight out again. First time it’s done that


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[K2-K6]

They never usually come on at tickover,  needs more investigation as to what's happening with the oil pressure.

Is the oil new?

[MalD]

They never usually come on at tickover,  needs more investigation as to what's happening with the oil pressure.

Is the oil new?

Not sure of the age of the oil, hopefully change the oil this weekend and see how it goes. Anyone using different oil on these?


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[MrDavo]

Examine the oil that comes out carefully for 'glitter' catching the light. That's how I knew my main bearing shells were borked having gone through the oil light, knock and denial stages.

[K2-K6]

Definitely change the oil and filter. If you are uncertain of its origins and age it's probably one of the best things you can do for one of these.

It's not a magic bullet in fixing something like this but you just can't assess it thoroughly without being confident of oil condition, they live on new oil and I'd not run it further without replacement.

They don't need anything special, just 10/40 mineral oil is standard but if it's been started and stopped frequently without much of a run to get miles on it then the oil gets diluted by excess unburnt fuel from using choke. This results in a viscosity reduction that is not good at all for plain bearings.

[MalD]

Thanks for the advice. The bike hasn’t been run a lot so the diluted oil could well be the case!! I shall be scanning that old oil for metallic objects and keeping finger crossed 🤞


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