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I have an 18 hp B&S 2 cylinder engine that runs well. Since I put on dual exhausts I can hear that the right cylinder produces an irratic beat with the rain cap flapper, while the left cylinder has a very regular beat. This leads me to believe that the right cylinder is often misfiring. My first thought was to adjust the valve clearance. The B&S "Twin Cylinder "L" Head Repair Manual" shows the following on valve clearance:



There are stated clearance tolerances. I have never been inside an engine. Where do I measure these tolerances? And, if they are out of tolerance and I must replace or shorten the valve stem do I have to remove more parts to get to the valve's stem end to remove the valve?
 

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B&S twins are very prone to have the valve seat come loose engine will run as it gets progrssively worse and beats the block out.I have 2 or 3 of these engines that has happened to.
 

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What you describe is very typical of that series of engine. It is hard to get them to sound the same at idle. IF your engine has an adjustment screw sticking straight out at you just below the air cleaner, when facing the Intake manifold, turn it 1/8 turn counter clockwise and see if that helps. What you have going on does not sound like a valve seat to me but could be.

Check valve clearance by removing the rectangular crankcase breather on engine block behind the Intake Manifold on each side. Right one may just be a cover plate. Now, how you find 1/4" down from the top with piston past TDC is another thing since the spark plugs come in from the side.

Remove both spark plugs, place thumb over one spark plug hole, turn flywheel clockwise until thumb quits being blown off plug hole, you should be able to fee TDC here, turn just a little more, guestimate it. Do same with other cylinder. Top tappet is Intake, closest to flywheel. Since you have the manual, you should be set.

Walt Conner
 
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