Personally I would start at petrol taps to carbs. Then strip carbs has needle dropped or floats wrong height. Just because the carbs are new doesnt mean they are right,. If thats ok then go for valves.
I wouldnt start it with both ticker and choke either one or other. When you tickle it does fuel come out? To be honest it doesnt sound like much of a problem its just been methodical in finding it. when you said it would only do 50mph that sounds like carb trouble,on this one of mine ive actually had a jet come loose. Might sound daft but it has got fuel in has it got ptoper taps,also if the inside of the tanks been treated are the fuel tap filters clogged up, they are very fine mesh.
Happy hunting. When you get it sorted let us know what it was