Best Valve Timing Chart Explained: Not for Low Attention Spans

If you can visualize this?  Start with IVO (Intake Valve Opens) It's barely opening just BEFORE your piston gets to Top Dead Center.  The reason for this, is 'cause the piston is traveling so fast on it's way up, it takes a bit for the air to get moving and the gas to get into the cylinder.  After the piston passes TDC, it starts sucking the gas and air in there to fill up the cylinder with some flammable material, so when the piston starts coming back up, and compresses the gas - we get a good explosion to push down this piston.  The (IS) Ignition Starts early for this same reason, since it takes a bit to get the fire started.
Now follow that line all the way around and visualize when the valves are opening and closing, and where the piston is . . .  You'll notice there's a small time when both valves are open at the same time(35 + 10 = 45 degrees of rotation) and this is called "Overlap."  If you run straight pipes, or big pipes and open exhaust, the more degrees of overlap you have, the more chance of your engine babbling and stumbling due to low backpressure. 
Now you can look at your current cam specs. and maybe understand what's happening in there by looking at your numbers and comparing.  You may need to choose a different cam based on your cubic inches, exhaust type and carburetor size - or change one or a combination for best performance?

My problem is:  I get one of my motorcycles running just perfect.  It starts great.  It runs great.  It's fast for what I've got done to it.  Everything is working perfectly, a perfect combination of valve timing, ignition timing, carb and pipes.  BUT, I don't know this !  I think that I could change one thing and make it just a wee bit better.  Then it runs worse.  I go backwards.  Since it was a lot of work to change this one thing - I just ride it the way it is, until I get totally sick of the bike running like this, and change it back - or change something else.  If it's not broke - Don't fix it.