Occasional knock from engine...

[RupertW]

Hello all, 

On my chummy project I have noticed an intermittent knock coming from the engine. It's a bit weird because it seems very intermittent and will only do it once every so often, perhaps slightly more when you lift off the throttle. It almost sounds like something is physically striking something else - there's no starter motor fitted so it isn't from the bolts hitting the flywheel. Pushing the clutch down also doesn't seem to make any difference.

Appreciate it's near impossible to diagnose a noise based on what's written down but is there anything obvious I should be looking at? 

All thoughts welcome!

["Slack Alice" Simon]

Loose engine mounts?

Propshaft bolts?

Noises do not necessarily come from where you think they do.

[RupertW]

The sound isn't audible when driving, although I suspect that's simply because I can't hear it over the rest of the engine noise! It sounds exactly like how I'd imagine the flywheel hitting a starter motor bolt would sound, although there's no bolts there for it to hit! To my untrained ear it also doesn't sound like anything to do with the big ends.
Only thought I've had is that I think the clutch thrust mechanism is a bit of a mishmash of 3 and 4 speed parts so it's possible that that's not happy.

[Alan Osborne]

Rupert,
On our Chummy when we bought it there was a knock each time you either accelerated or lifted off. It was ok all the while you you accelerated or decelerated but it was the reversal that caused the knock.
Turned out it was the flywheel that was loose. Since tightening the nut up all has been ok. (Touch wood!)
Hope this helps.
Alan

[Austin in the Shed]

I have a friend with a mag engine chummy which does the same intermediate knock.Our last thoughts was from the cam coming off load.
When I had a loose flywheel the knock went when the clutch was pressed.

[Alan]

If it’s a mag engine is there a difference between dynamo off and on?

[RupertW]

It's a coil engine, approx 1930 off top of head.
It has a newly fitted reconditioned dynamo which isn't currently wired up to anything. I'd hope it's not related to that, but thinking about it I didn't notice the knock before fitting it so it might be worth investigating

[Ian Williams]

You could have a broken main bearing flange allowing the crank to float back and forth, I have heard similar to what you describe when this happens. But if it has started since the dynamo swap I would immediately be suspicious of that as others have suggested.

[Alan]

Probably not a good idea to run a dynamo unconnected - Bob C will know, but if it’s running unloaded the backlash in the gears could be the source of noise.