# Briggs and Stratton 3.5 HP Running Rough



## mhendriks (Jul 11, 2010)

I acquired a lawnmower with a Briggs and Stratton 3.5 HP Classic engine. It runs, but it runs extremely rough and emits black smoke. I'm at a bit of a loss as to what to troubleshoot next. Steps I've take so far are:

Change Oil
Clean/Gap spark plug
Fresh fuel
Clean carburetor

I'm starting to wonder if I have a carburetor adjustment problem, or if there's something else going on. What should my next steps be?

Thanks


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## mhendriks (Jul 11, 2010)

I should add that I just completed a compression test - it came back at 90 PSI


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## kbowley (Sep 23, 2009)

If it has the plastic carb it may be warped, either way, you need to purchase a carb rebuild kit which will include a new diaphragm and gaskets. depending on age, there may be an adjustment screw for low speed and possibly a second for high speed. If so, bring engine to full throttle and try turning the screw inward to lean mixture. also ensure that the air cleaner is not plugged up...test by removing air cleaner and running engine to see if there is a change in performance.


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## mhendriks (Jul 11, 2010)

Thanks Kevin,

I've got an inductive tach pickup on the way, and I saw a carb adjustment procedure on the B&S website, so I'll give that a try.


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