Misfire

[JamesG]

After a lengthy saga getting new inlet manifolds made (previous ones were distorted and allowing air in) my Jupiter based special is now running, but with a misfire. All plugs are new and sparking correctly. There is no appreciative difference in he heat from all four exhausts (all four are too hot to touch!)
On examining the plugs nos 1,2 and 3 have a darkish look (possibly running a little rich) but no 4 plug is much cleaner with a light coloured tip which suggests to me a leaner mixture. When I do a compression test no 4 is about 20% down on the other three. I thought that maybe that this all indicates a leaking exhaust valve (suction stroke drawing in more air through the valve)?

I would be grateful for any thoughts or suggestions before I remove the head.

[Forumadmin]

Head Gasket?
Valves?
Piston ring?

Leak down test should prove which.