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There is a utube of a guy repairing a similar pump and it does not have a push spring inside like the omc square pumps just 2 rubber check valves leading me to think it works with both pressure and vacum to suck fuel in to the pump and then a pressure pulse to push fuel OUT to the carb ….
(44) Ferris mower Mikuni fuel pump repair – YouTube
in other words it needs both.. push and pull to flex the large inside plastic diaphragm .
Your pump may not like the absence of a strong vacum pulse from the side air nipple …. it giving out just a pressure pulse since it’s vacum is cancelled out by the reeds openning up on the upstrokes to draw fuel in the crankcase .
anyhow you are on the road to happiness 🙂
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