Motorcycle running rich please help??

I bought a 94 honda magna about a month ago and for the most part it has been running perfectly.this is my first bike and while im good at mechanics in cars, im still learning about motorcycle because slightly different set up.anyway i recently replaced spark plugs and put new ones in. One thing i noticed about old spark plugs is it seamed to be running rich.now my bike as plenty of power usually, runs perfect starts right up.never once needed to use the chike yet to start.but there is one problem with bike,it only happened twice so far sand i put well over 1000 mi. On bike, but twice i started to bike and took off,everything seemed perfect and all of a sudden i lose power, it drops rpm and has almost no throttle ,and this is after driving for like ten min. Or so bike already warmed up.it will backfire really bad and just wont go.acts like out of gas or something but put it in reserve and still no luck.its getting plenty of gas and air,first thought could be bad gas but drained tank and gas was good.happened again on highway.then next day ill start the bike and runs perfect. Happened one before spark plug and air filter change and one after.please help.i took apart fuel pump and everything was clean inside it and no holes in it.you think it could be a petcock problem maybe? This bike doesnt have fuel filter instead has screen in petcock,but it was clean.almost spotless. Please some suggestions how to fix this problem