Would you say the engine is flooding? If so, I would look at the metering diaphragm again and make sure it was installed on the right side of the gasket (cover plate side) and that the metering fork is engaged on the diaphragm center post. Do you have a way to do a pop off test? That would confirm the metering needle is seated properly.

I had a similar problem with one of my old Q 75. It looked to have plenty of spark but simply would not run. Problem turned out to be a bad mag coil. It had spark but not enough to fire under pressure in the cylinder. Are you sure your CH ignition is putting out what it should? Plug lead and cap in good condition? New plug? Maybe try a different plug?