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I would overhaul the carby and check for corrosion in the main jet and other circuits
if it is a 2 stroke I would check the crank shaft seals as they may be hard and leaking mixture and running lean ( backfiring)
Look for air leaks on the inlet manifold/carb plate. If any air can leak in here, the motor runs lean and rough. Since you replaced the carby, double check the rubber parts for leaks (may not be obvious off the bike so stretch it a bit) and make sure the gasket to the engine is OK (some bikes use o-rings which shrink over time). Next time you run it, when it starts acting up, reach down and pull/push the carby towards the engine. ANY change in revs or running indicates a leak.
Its not common with YZ but is very common with not starting the bike for long time - you need to take out the pilot jet and replace it with new, You could clean them but why bother they about $5 each?
If I were you whilst you have the carbi off I would change all the jets to new ones especially as been sitting for a while (not essential but worth while as off already?)
running only with choke on and backfire are classic symptoms of blocked pilot jet or incorrect size pilot jet so chuck new one in and you should be good to go
The pilot jet is the jet closest to the engine when carbi was installed (it has perforated holes on the side)
good luck regards Jamie
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