RE: SYSSA 30CC GAS MADE IN USA
Still trying to make sense of what is going on with your engine.
Back in May you had six flights, your engine started and ran good, then in Juneyour motor started running badly; the carb had crap in the filter. It seems odd that your engine would run okay for six flights if the crap in the carb came with the engine.
In Juneyou put on a new carb, your motor ran good,then your motor started running bad again. You said it was running good at high and low speeds. After 15 minutes of flying, time to land, it burps spits sputters, and shuts down while turning base. Could have been overheating; usually you would have noticed the engine sagging. Could also have been cowl pressure. Too late now, but anytime overheating or cowl pressure is suspected, it is best to fly without the cowl.
Did you check the filter screen in carb #2? I'm not clear on your answer to that question yet.
You also mentioned having questions about the cylinder head temps you were getting, from 240 to 350, which seems high, but there are so many variables the measurements are not all that meaningful. You said baffle changes in the cowl did not reduce the temp. When that doesn't work, it is good to make sure the cooling air exit area is at least 3x the inlet area.
Another concern you had was the pressure inside the cowl causing the carb settings to change, that can also be related to not enough cooling air exit area. Or a vent tube must be run from the carb vent back into the fuselage where the pressure is stable. Usually, the engine will not run right during different speeds and attitudes.