Did you blow out all the passages in the carbs? Only takes a small particle of dirt to block a fuel or air passage.
Did you soak the carbs or just use spray carb cleaner? Soaking seems to work best.
Is carb adjusted properly? Are carbs syncronized?
Any air leaks? Gaskets, boots, etc...? You can try spraying a little WD40 around those areas while the engine is running. If you hear a change in the engine, that could be a suspect area.
On the electrical side; like Jeff suggested. Check for loose wires, bad connections, dirty contacts, etc...
New plugs and wires won't hurt either.
Random thoughts.
Steve