I just repaired the above lawnmower. I cleaned all the needle valves and finally got it running. Now it is running for about five minuites then blowing white smoke everywhere and dying. The engine is also blowing a combination of oil and gas out of the rubber tube on the air intake at the time of engine failure, and there is now gas in the oil system. Any ideas what is wrong with this thing?

Well, you need to replace the breather assembly on the side of the engine. Follow the rubber tube and it will lead you to a small rectangular shaped box. Remove the two screws, replace with a new one. What it is doing is allowing oil to blow through it and enter the air intake system, causing the engine to not run properly if at all, and causing the smoke.