I have a 6.5 hp Sears self-propelled mower (large rear, small front tires-don't know model number, but trust not essential?), about 3 years old, which has given me very good service, including starting (most of the time) on the first crank. Air filter is cleaned and oil changed regularly, etc.

I wanted to mow yesterday, and it just did not want to start. Finally, I primed it several times and it burst into life. I did my mowing, but when I released the dead-man bar, it died as it is supposed to, but unusually with a considerable burst of smoke (dark colour). I tried to start it today without any success until I again primed it several times (maybe a dozen) and it started with a cloud of smoke and immediately died. Could not repeat the start.

Changed the old plug for a new one (and tried to visually check the spark, but not so easy with the starter cord on the handle and the spark plug on the other side of the engine, but wife reports seeing a spark), removed the carb and checked/cleaned the float chamber. re-assembled with high hopes, but no cigar.

Any ideas gratefully received. Additional issues (please forgive as I am very much a weekend, VERY occasional, and not-very-knowledgeable mechanic):

What exactly does the "Dead-man" bar do? Is it a choke which drowns the engine (accounting for the cloud of smoke) or does it interrupt the spark, or what?

When I removed the carb, there was a rubber tube coming from the direction of the exhaust connecting to the back plate of the air cleaner housing. What is this? Is it some sort of exhaust recycling, emissions control thing or what?

This is a self-propelled mower which is "pulled" by the front wheels. Does anything in the drive train need periodic cleaning and/or lubrication?

What is the correct spark plug gap? (Bought the mower used [just barely] without an owners manual.)

Thanks for any thoughts you might have.