Grunge - I'll take a punt and diagnose carbs that are out of balance. Any competent mechanic can fix this as part of a basic tune-up. You don't have timing belts ( unless you own a Ducati

), its timing chains or maybe gear drive if you are unlucky enough to own a Honda ( double

)
You need to put your location in your profile, so the local KSRC crew can point you in the direction of a good workshop.
the 15 or 50 reference is referring to 'degrees of ignition advance', its a pretty simple concept, it refers to how many degrees of crankshaft rotation occur between ignition spark in the cylinder, and Top Dead Centre ( or TDC - ie the top-most point of the crank rotation.) BTDC is Before Top Dead Centre.
When the bike is started and idling, the degrees of advance are kept to a minimum ( eg 15 deg BTDC), to make the engine work properly at low speed.
As the engine speed rises with revs, the spark must be 'advanced' so that the engine works properly at higher speeds. ( Up to 50 deg BTDC, depends on the motor, some are higher, some lower.)
This is just a very basic description, if you are interested in how a motor works properly, read 'Tuning For Speed' by Phil Irving. This is a great book for understanding the basics of motors.