Okay Jaguar friends. I have rebuilt let's just say refreshing up a 1990 Jaguar 5.3 l V12. New timing chain tensioner head works done rings bearings and gaskets. I custom made intakes and have six 40 IDF carburetors on it. With new plugs in it runs completely fine maybe a hiccup on the a side every now and then in a hiccup I mean I miss. I'm not loading up I'm not burning oil but when it runs for 30 minutes all the plugs will file out. I'm taking gas out today and going with the highest octane fuel I can get. I'm curious that my timing marks were on even though I rotated could it be one tooth out on the cam on the a side. Is there a clearance issue problem. Meaning is it a zero clearan
ce for the valves. I have bought five sets of plugs this motor I have only ran for maybe a total of 3 hours. Does the crankcase need to be vacuumed for venting crankcase? I have very good compression on every cylinder. Someone help

 

Since this is a non stock setup how do you tune it without a dyno run? Have you monitored the fuel trims? How did you decide on those carburetors and how did you size them correctly? How did you set up the idle circuits and how did you know how to do that? Does this mod required timing changes? Where is your timing set now? Maybe you need a different model of spark plugs?

Do you see the problem with doing all those aftermarket mods? It's no longer stock so anyone on this forum with your car won't be able to help?
You need to do what all of us do when doing custom stuff and that's get the car tuned properly. To me it sounds WAY over carbureted but that just a guess.

It's NOT an octane problem for sure.
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Is it fuel or oil fouling the plugs? Crankcase venting back into the intake does indeed lower the octane level. It can also oil foul the plugs. Easy to tell the difference between plugs fouled by fuel or by oil.