Ls400 Glowing Red Hot Catalytic Converters

[Boggsy]

Evening Gentlemen and possibly Gentlewomen

Having an odd problem with an LS400 I recently purchased and just chasing some ideas on what it could be.

Problems include:

- After running for 5-10 minutes both of the cats glow red hot
- For a few seconds on startup seems to backfire through the intake once throttle is applied, however disappears once the cats begin to glow
- Rich smell of unburnt fuel and white smoke stemming from the cats after a while

Now I've got theories and just trying to see if I've missed any?

-Timing may be out. (Purchased with chewed timing belt, water pump and idler bearing, Replaced with brand new items and aligned to the timing marks on the case)
-Cats are clogged. (I've tried removing both oxy sensors with the problem still occurring)

Appreciate any help guys :)