My old Yamaha Diversion had a vacuum tap. Left for a few days it would need spinning over for ages to fill the floatbowls. Result was a broken starter gear and a crankcases split job at12,000 miles.
I put it back together with silicone sealant because gasket set was £140. Twenty plus years on and now covered 132,000 miles untouched since reassembly. I fitted an electric fuel pump shortly after the rebuild which seems to have cured it.