Small Engines (Lawn Mowers, etc.): B&S Missing and Backfiring, drive shaft, valve clearances

Question
I have a B&S 350447 18 HP with about 500 hrs on it.  It has started to miss and backfire badly, very load and constant, especially as it is run longer (after 40 minutes) and when it comes under load even at full throttle.  Changed air cleaner and gas filter.  Have used 4 different tanks of gas.  Plug tips are clean but threads have black soot for 1st ¾ inch.  There is a slight smell of gas from exhaust but no smoke.  Read your answers and checked out valves.  I do not know much about this but removed plugs and rotated drive shaft so the rod appeared to be bottomed, then adjusted.  I figured if anything I wouldn’t make the clearance too tight this way.  Three of 4 valves were at 13 one was 11.  Not knowing what I was doing, to be conservative, I moved them all down to about 7-8 (specs are 4-6).  I know you say 4-6 and 8-10.  Was the 13 possible?  Could that be the problem?  Can I hurt anything by going to 7?  Now of course I think the valves sound “noisy”  just like a car sounds better when you change the oil.  If you think I should go to your specs, how can I tell intake from exhaust?

Answer
Hello Jim:

This Model Engine Valve Specs are .004in for All the Valve Clearances. I Usually Set them at .004 - .006 on this Model Engine. You can View a Breakdown/IPL of the Engine at this Site Addy; http://www.briggsandstratton.com/maint_repair/manual_and_more/doclist.aspx?categ... and Enter the Model Numbers (XXXXXX) and Type Numbers (XXXX) in the Appropriate Boxes and Click GO. Then Select Download PDF. If you Hit a Snag or this Does Not Correct the Problem,  I am Here if You Require More Assistance. Hope this Helps. Let me Know What Happens, Please. May the All Mighty Bless You and Yours. Thanks.

Good Luck

Respectfully

John

PS: I am Allowed to Answer 10 Questions a Day. If you See I am Maxxed Out, then Try Submitting your Question at or Just After 8pm EST (US). My New Day Starts then. Thanks.

Respectfully

John