# black smoke when revving



## powerwrench (Aug 29, 2013)

last season i replaced the piston rings on my 1972 8 hp B&S and i noticed a small scratch mark on the wall but every time i revve the engine quickly the exhaust turns black for a second. i think its running to rich or that scratch might be letting in oil to the calendar witch i hope its no t but I just wanted to see what you guys think


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## td5771 (Feb 21, 2011)

if it is only when you rev it up it sounds like it is running rich at idle. when you rev it up it has to burn off the added fuel


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## powerwrench (Aug 29, 2013)

ok good. i didn't want to take the engine apart agin to hone the cylinder especially in the winter lol


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## td5771 (Feb 21, 2011)

Best way to judge the mixture is to run the blower actually blowing snow for a while. When you are done don't let it idle more than a minute or two. Then pull the plug and see what it looks like. Black is too rich. White or very clean is too lean. Any medium brown/light brown is just where it should be.

If oil was getting in the smoke would be blue.


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## CarlB (Jan 2, 2011)

its not that unusual to see a puff of black smoke when quickly transitioning from idle to full throttle. If the engine is a 1972 then you would have both idle air and main jet adjustments. If the engine runs fine under a load I would not touch the high speed adjustment. If you want you can adjust the idle screw (you can listen for engine rpm change as you turn it in or out. when you find the highest rpm back off in the opposite direction about 1/4 turn or so.


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## GustoGuy (Nov 19, 2012)

CarlB said:


> its not that unusual to see a puff of black smoke when quickly transitioning from idle to full throttle. If the engine is a 1972 then you would have both idle air and main jet adjustments. If the engine runs fine under a load I would not touch the high speed adjustment. If you want you can adjust the idle screw (you can listen for engine rpm change as you turn it in or out. when you find the highest rpm back off in the opposite direction about 1/4 turn or so.


Sounds like it is burning off extra fuel. How much did it cost you to re-ring the engine? Did you put in new valve guides too? This is another area were they tend to loose oil. Smoke on start up? Does it puff blue or black smoke? Any lose of oil when running it?


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