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Looks to be a circa 1986 model year motor. Similar to a US 90ETLJ except that this model has the Precision Blend System whereas yours is premix, according to you.
LIT-18616-00-09 is the Service Manual for the USA model motor if that will help you.
The idle mixture setting also has an affect on the amount of gasoline that it provided through the carburetor at WOT.
Fuel level being too high can cause the carburetor to provide too much gasoline.
Someone might have augered out the existing main mixture jet.
Primestart is a Yamaha term for any automatic fuel enrichment system. They are not all the same.
Primestart on an EFI F350 or the latest XF425 direct injection model is completely different than Primestart on two stroke models.