Practically Classics Forum

We have a Imp engined Ginetta G15 which we're putting together from boxes of bits from an unfinished project.

The engine was said to be fully reconditioned so we didn't expect any real issues, but after much other work the engine definitely has a misfire. We knew we had a good spark at all 4 plugs, took plugs out, 1,2 & 4 all looked fine, No. 3 very wet. A quick compression check showed 200psi on 1,2 & 4 but only 120psi on No. 3. A leak test indicates it's not valve related but leaking back past the piston rings. With the engine running you can disconnect & connect No.3 HT lead and not hear any discernible difference to the running of the engine.

My question is that even though 120psi is a long way down on the other cylinders, would this result in no firing/combustion as it appears, I'm thinking that even with compression down it should be doing something given we have a good spark as opposed to nothing and that connecting/disconnecting No. 3 HT lead you'd hear something change.

Look forward to your thoughts.

So you have proved its number 3 cylinder thats at fault.
3 has a good spark but much lower compression.
how did you do the 'leak test'.??

The leak test was done by setting No.3 Piston to TDC, attaching airline (via adaptor) into spark plug hole. No leaks from exhaust or through carbs, but air could be felt through the oil filler neck.

Well that cylinder clearly has a problem and needs sorting, so I'd do that first and see how it goes afterwards. Hopefully only needs re-ringing and perhaps a bore hone, but it'll need a lot more if you leave it, hence why I say fix that first.

I assume you've swapped plugs, leads etc.?

Cheers

Plugs, leads etc. all swapped round and problem persisted on no.3.