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Well Dave, There are just too many things or combinations that can be wrong to come up with an answer long distance. Assuming you did a complete carb overhaul, a mental checklist: 1. power and idle jets are right size. No score marks in screwdriver slot near orifices. 2. power needle has right taper, about 70 degs or same as orignal and not bent. 3. floats set equal distance with gasket in place and are parallel with bowl cover. 4. throttle valve is bevel down with bevel ID nr on top. 5. I don't think the venturi can be installed upside down but may be possible. 6. engine warm, choke open. Mixture screws initial set is about 1 turn off the seat. Idle mixture screw is cw rich, ccw lean, set @ about 500 rpm with throttle valve just slightly open. Power screw is opposite, set at 1000 rpm or higher w/load. Fuel smoke is incomplete combustion as in slow burn rate. Very lean generally causes backfire under load. Less lean or too rich may cause afterfire. Lean very high exh temp, light color haze. Rich cooler exhaust, dark smoke, wet plugs. If engine runs fairly steady w/o positive missing, other engine problems that mimic carb lean & rich are weak coil, very low or uneven compression. That doesn't tell you much does it. I think I would do a compression check and if ok, probably change the coil before going back to the carb. Joe
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