Idling on 3 cylinders

[dickie14]

My van seems to be idling on 3 cylinders - pulling the HT lead to cylinder 2 whilst idling makes no difference to the engine speed or tone. Pulling any of the others nearly kills it. Through the rev range it is fine (van's driving great), but trying to rev it whilst the lead is still disconnected and it's grumbly and lumpy, as expected.

I've tried swapping leads around, changing the dizzy cap and cleaning the idle jet (fitted with Dells), none of which made a difference. The spark plug is a bit sooty but I figured it's ok as everything works as soon as the engine rpm increases (perhaps an ignorant assumption?).

What else should I be checking?

Cheers,
Dickie

[itchyfeet]

compression
chances qre its low on that cylinder
you can buy a test guage for about 30 quid
I have a Gunson one

[CovKid]

Seconded.

Could just be valve not shutting (tappets) but it still means low compression.