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Originally Posted by Motofool

Yes, if valves don't close perfectly or if their guides are worn out, compression will leak out and raw air will leak in (sucking less from the carbs).

Intake valves are typically larger than exhaust valves because the piston is moving away from it, creating a vacuum to pull a mix of air and fuel into the cylinder, which is not as efficient as pushing combustion gases out of the cylinder through the exhaust valves.

When new, valves close perfectly, since the angles of the cones are matching and the surface is perfect.

With time, and especially when working long time and hard out of specs (too little clearance), the surfaces may get damaged from not enough cooling (valve and head not together enough time) and hot exhaust gases leaking out.

Did you use a torque wrench and the recommended torque during your valve service?
Too little may let the nuts get loose and too much may break the rocker arms.

I did not use my torque wrench because I believe the reading was in newton-meters and I didn't know the conversion rate so I tightened sort of like the spark plugs, tight plus 1/8 turn or so