You've been quite thorough and detailed in your observations. Have you confirmed fuel flow and levels within the float bowls? The premiers, the rec is to use actual fluid level rather than a measurement of float height. I used clear hose plugged into an old plastic bowl drain plug, held hose against carb to watch where fuel level came to. Should be very close to bowl/body sealing flange (check amal website for exact rec of level) adjust float accordingly.
Have you got good compression?
Are you operating choke correct direction?
Did you test new carbs for blockages in idle circuit? Swarf can be present out the box. Gotta see a good spray out the two tiny holes in carb throat when applying aerosol wd40 etc to fuel pickup port underside of carb, other passages blocked. Also remove, blow through the nw pilot jets to get good spray out the little holes in the jet.
After failed starts, are plugs wet/smelling of fuel or dry? Might identify flooded or starved condition.
Try some starter fluid into intakes to see if helps get pops.
Also, my wassel ei manual states to set timing 28/31 btdc. Assuming that the euro convention, it means 28 to 31 degrees. When I push mine to 31 ish, got a few kick backs and more difficult starts, so now I'm at 29 or 30. Starting never kicks back and 9 out of 10 starts are one kick, even cold, tickled, no chokes installed. Seems to like throttle held open upto 1/8 turn on grip, hot or cold.
