(02-10-2018, 09:34 AM)Alan Wrote: It looks to me as if that piston has melted rather than broken. So, there are 3 or 4 possibilities. Either the mixture is very weak - so look for a sticky valve or an obstruction to the airflow, or maybe a head or manifold gasket which has an airleak. Or maybe an inappropriate spark plug or the ignition is way over advanced- and as it’s one cylinder I’d be thinking about a bent or worn distributor.
Precisely. It's hard to be 100% sure looking at a photo but a melted crown is caused by pre-ignition.
Mixture too weak, ign. too far advanced, too much compression, wrong spark plug range, sticking valve (I'd look for exhaust not closing properly), obstructed cooling / airflow issue, gasket leak, fuel octane too low (contaminated with diesel, perhaps).
I'm told excessive oil in the combustion chamber can also cause pre-ignition - so failed or badly worn piston rings too. Though I'm guessing they are pretty knackered now either way.
I like the theory of bent / worn distributor but if so can't understand why mine is still running!