My 1986 mazda b2000 "popping" sound and has no power, what should i look at?

i adjusted the valves to the right height and it ran perfectly for about 2 days, then started poppin again, ive checked all s/p's and wires and checked out the vacuum pressure and just replaced the dist. vacuum advance with no luck. it pops always but gets bad on acceleration to the point that i cant even make it up a hill without almost bogging out. also when i give it only a little gas it runs ok but as soon as i get past a certain point it pops and lags.