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I have a strange problem with oil or oil vapour getting into distributor and burning on to contacts. Result is increasing misfires until when too many cylinders miss it becomes really bad

Does anyone have any experience of this and is it just down to the £2 'O' ring on the shaft or do I have a bigger problem?

That should be S3

Hi Jagman,
I think the most likely causes are:-

1 That the oil seal on the distributor shaft has hardened, thus not sealing on the bore of the distributor seating.
2 That there is too much crankcase pressure and oil is being blown past worn seals on the distributor shaft. Check and clean the gauze filter behind the engine breather take off on the front of the Cyl head
3 That the phosphor Bronze bush at the lower end of the distributor is worn and oil is coming up the shaft inside the distributor.ds,

Regards