Setting Valve Lash or Valve Clearance is important and should be checked after the first 10 hours of running. If its not set to spec, the valves are not opening and closing at the correct times and could be throwing un-burnt fuel into the muffler (after fire). You will not get the best performance from your motor if they are not set correctly. The 23, 35, and 36 HP valve lash setting range is 0.004 to 0.006, yours may be different so check your owners manual. Modified motors may also have different settings. Some of the common settings for BPS motors:
You set the valves on a cold engine, remove your valve covers (10mm), spark plugs, and engage the clutch (if you are turning the prop to turn the crank). Turn the crank shaft/prop until you see the valves opening and closing. After the exhaust valve closes put your finger over the plug hole and when you feel air coming out (compression stroke) then go slow. Look in your spark plug hole with a flashlight and turn the shaft till you see the cylinder rise to its max point. Turn it just a little bit more till it drops ¼” and then set the valve clearance (this is ¼” past Top Dead Center- TDC). You could also put a wooden dowel in the spark plug hole that will rise till you get the piston to past TDC.
Use a feeler gauge to check/set the clearance one that has 0.004, 0.005, and 0.006 etc will cost less than $8 at Autozone. Go with 0.005 setting (or applicable to your rockers). Loosen the 13mm nut on the rocker adjuster on the rocker arm and use a Torx T40 driver or metric allen wrench (depending on your adjuster type) to adjust the clearance. The feeler gauge should move between the valve and the rocker arm with resistance (like slicing a stick of cool butter with a knife). Once its the correct clearance then hold the adjuster and tighten the 13mm nut. Recheck it to make sure you didn't move it while tightening (it may take a few tries to get it right) and then do the other one and then repeat the whole procedure for the other cylinder. Torque specs: Valve Cover- 70 in. lbs (8 Nm), Rocker Arm Adjuster Lock Nut- 70in. lbs (8 Nm). Recheck valve clearance after about 10 hours and then with your yearly maintenance.
Different valve settings: .006- Better Low end torque .004- Higher Speed .005- Happy medium
Note- you may get away with re-using the valve cover gaskets, if you reinstall the covers and see oil leaking after running, order and install new gaskets.