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Hawk's right also keep in mind torch tip cleaners are a great tool for cleaning carb orfices.
Any chance there's a critter nest/exhaust obstruction, worth a try running w/ pipe off?
If tank interior/anything of the fuel system questionable, bi-pass all w/ a gravity feed/remote fuel supply.
What of engine compression itself, could the decompressor be sticking/cable too tight/brkn. spring?
Ignition, try running in the dark watching for spark jumping also w/ a Spark Tester watching for erratic spark?
Best of luck!
~ Pete
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