Small Engines (Lawn Mowers, etc.): B+S 31c707 problem., battery cables, hp engine

Question
Hi. My parents have a Toro mower with a B+S 31c707 17.5 hp engine. For the last year or so it has been very hard to start, even with new battery. Turns about 1 rev and then stops. I spin it past compression by hand then try again until it starts. The last time they used it they reported a fuel leak at the carb/line but could not find it and it all leaked out. Today I got to it, pulled off fuel line and reattached added fuel. No leak now. Hard starting as I said then when it did start it had much popping/backfireing for 5 seconds then real quiet or not fireing but still spinning, then popping, then quiet and so on all the while spewing lots of grey smoke and dripping black fluid from exhaust. Gas + carbon rather then oil I think. They did not have this issue when they last used it. I figure it may be blown but the smoke doesnt smell of oil. Could be bad carb?... but then why the hard start? What do you think? If I know what it is I can fix it, finding that out is my issue. They are not against replacement rather then repair depending on the cost/what it is. Thanks.

Answer
Does the oil smell like gasoline?  How old is the mower?  How old is the battery?  Have you disconnected the battery cables and cleaned the battery posts and cable ends with a wirebrush?  Fixed a lot of hard starting problems just by cleaning the connections.

Remove the spark plugs and see if the starter spins the engine quickly.

Have you had the battery load tested?

Let me know.

Eric