Lotus twin cam. One cylinder knocking. Confused.

[johnfm]

So, getting an odd knock - car sounds a bit like a VW beetle under load. But if I pull #3 plug lead, the sound goes away.


I thought it might be just a slight blow on the exhaust where the header meets the head - so checked and tightened the studs. No changed.

Checked timing and retarded it a bit. No change.

Running 97 RON fuel. No change.

Just pulling the #3 lead works.

Checked valve clearance and ensured #3 exhaust valve actually closes - all good.

Any clues?

[DVandrews]

Give it a compression or leakdown test.

Dave

[johnfm]

Compression is very good, 190, 185, 190, 200

Good leak down too.

[Pumaracing]

Knackered big end.

[PaulKemp]

Yup I'd go with that after gearing a 4age engine do the same

[johnfm]

The reason I think bottom end (or small end) unlikely:

sounds more like an exhause blow

just had (before this sound appeared) new head gasket, replaced sump and so new oil filter, fresh oil etc.

On startup, cranked engine to build oil pressure etc before actually running the engine

Knock issn't very 'knock' sounding and doesn't seem to be coming from the bottom end. Sounds to be coming from around the header outlet.

I might remove/re-install the #3 header to make sure it is sealed properly before panicking about big end or little end bearings.

[johnfm]

Phew!

It is just a blow from where the header meets the head. Just needs a thicker gasket or similar.