Thanks for the extra info.
The fact that this all happens after about 5 minutes leads me to thinking heat issues. If reeds are bad, the engine will run crappy from the first flip, which is not the case.
It is possible that the rear carb gets real hot, and vapour lock occurs. Also it is possible that the ignition gets too hot. Both these devices need cool air to work well.
It also is quite possible that there is a  general lack of engine cooling. I have seen too many baked engines where the owner said he had good cooling, so can you please take pictures of how your engine is built in the plane? That would realy help me to evaluate your problem.
BTW, Do you use baffles to guide the cooling air to the cylinders? How is the hot air extracted from the cowl?

90:1 oil is way too little. Factory data: use 40:1 oil. This is as important as good cooling.

The manual states carb needle opening "by clock hands" This is the large hand of course , so "one and a half turn" open is same as "one hour thirty minutes"
Have you checked the plug colour?
How is the plug gapped. Gap should be 0.5 - 0.6mm.

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