Try dialling in your ignition tune. Hold the rpm @ 2000 and gently swing the distributor either way between advance and retard until you notice the engine rpm's spike by themselves, back it off just a touch then lock the distributor in place (it wouldn't hurt to undersize the plug gaps either by about 0.05 - 0.1mm) Should give you more midrange to top end power, an improvement in top speed and pull through gear changes faster. As for the secondary jets, going up might cause a flat spot in the transition into midrange. The exhaust crackle could be an indication that the timing is retarded. Also the exhaust system design has a hand in it too. Less bends and restrictions cause the exhaust pulse to scavenge the combustion chambers a little more aggressively.