I once bought a car (too embarrased to admit it was a toyota)with a 2.6lt six that was very tired and burning lots of oil. This had clogged up the whole exhaust unbeknowns to me. When I rebuilt the motor and took it for a good run (it was night time) there was a trail of sparks coming out the back like New Years fireworks at Sydney Harbour. It was burning/blowing carbon out the system and the further I went the better it went as the inside diameter of the exhaust increased progressively!!!!!!
Have you rebuilt your motor lately?
The cop said "Do you know why I pulled you over". I said " Well, that all depends on how long you have been following me!"