It could be you have carbon build-up in your cylinders from the oil consumption. When you shut it off the carbon still has glow and ignites any residual fuel. Back in the day when an engine had problems like you are describing we would run the engine to operating temperature. Take the air cleaner off and at fast idle, pour (trickle) water down into the carburetor (don't kill it). The theory behind it was to steam clean the carbon out of the cylinders.Whether it worked or not, in my opinion it helped. Then we would give them a thorough tune up.
