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With that bore size its likely to be the FF99 model, a relatively long stroke engine. The rotary valve should be positioned so that the inlet port is just fully closed at 70degs after top dead centre, and just starts to open at 55degs after bottom dead centre (the side of the valve that goes against the cover plate is usually the more scratched). assuming you have the red coloured Motoplat ignition, the timing should be set to 3.8mm before tdc. I have no experience with the hubchen carb, but would recommend a Tillotson 334A carb for this engine for which 1.25 turns on the low & 1.5 turns on the high screws would be a fair starting point.
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