Well to let you know your not alone. I built a brand new everything 350 sbc roller motor last year and I used world product sportsman 2's and from day one the problems would just get worse over time. I kept getting oil on my plug threads and after about two months between carb issues and the stupid heads I would have to change plugs.
I kept having miss fire problems with engine and kept checking everything and just could not find out what would cause it. Eventually it got so bad my engine would not idle very well anymore.
Anyways long story short world products uses umbrella seals on there dual spring setup heads and that is a no no with dual springs. I took the heads off and had them checked by the machine shop and I only had less then 3000 miles on my heads and the clearances was horrible on the guides and all my intake valves was just full of oil and carbon deposits.
I had all new bronze liners put in to get everything in spec and I also had the heads done up with a port job and valve job and I had them machined for viton positive stop seals. 600 bucks later they are way better then when I got them new. I will never buy a brand new set of world heads again. They are nice in themselves if everything is up to spec and better valve seals. Umbrella are about worthless to me in my opinion.
There tolerances are horrible and they don't put care into there heads like some others do. A lesson I learned when getting new heads take apart if fully assembled and check everything.
I almost lost my engine cause of carbon getting into the piston rings and I have a few scars in my cylinder walls because of it and am having to rebuild whole engine again just so its all clean. All of this cause of world being cheap on the valve seals and too much guide clearance.