B&S burning oil

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Dec 26, 2007
My family owns a 1997 Lawnboy mower with a commercial grade 6hp B&S engine on it. I'll admit that the oil was not changed every season, and sometimes in its younger days it may have gone 3 years with no oil changes (I was younger at the time and didn't know any better). However, we typically mow 15hrs a season so that's not too bad. Anyway, I've been personally changing the oil consistently for the past few years now every season. Last year I did a Frankenstein mix of Mobil Clean/Pennzoil/Castrol 10w40. It would smoke a little bit on start up so I switched to HD 30. I put Castrol HD 30W in it this time, and it still smokes a little on start up for about 5 seconds...it blows black smoke out of the exhaust. Is this something to be concerned about, or could it possibly be the residual multigrade 10w40 in there getting burned out? Thanks.
Do you prime or choke it? Black smoke is more likely rich combustion than oil burning. But anyway, it's not something to be concerned about.
The air filter is cleaned and re-oiled every year with the oil change. Plug was changed a year ago (I do the plug every other year).
A carb cleaning and tune wouldn't hurt, but then neither does the smoke, so I wouldn't sweat it unless you get bored and want to take apart a lawnmower's carb. Also, this could be leaking valves, in which case you really don't want to deal with it and it doesn't matter anyway.
I'd try a double dose of Seafoam in the gas before I did anything else. Like bepperb said, black smoke is a sign of a rich condition. Before I got my ZTR, I had a lawn tractor with a 15hp Kohler that was running bad, belching a little dark smoke. A tank dosed with Seafoam (I used twice the recommended amount) soon returned it to the smooth running motor to which I was accustomed. BTW, I gave that mower to my brother three years ago and it's still running like a champ.
If it's running too rich this could be why it's burning oil as it's washing the cylinder walls... I would take the Carb apart, thoroughly clean it and replace any worn parts/seals etc. and reassemble. Then setup the fuel mixture if it's an older style and you should be good to go! If it still burns oil, then switch to a heavier weight...
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