if you need to keep pumping to keep it going the vacum pulse mod is likely not done properly and the pump does not work
see videos of a similar mod on a smaller HP engine.
(1597) HOW TO CONVERT 50s motors from two line tanks – YouTube
https://www.youtube.com/watch?v=ojOMbwvH55I
same basics
otherwise it is best to use the bypass cover method shown above to get a good pulse for the pump and just plug off the air nipple behind the carb with a stub of rubber hose and a large bolt in it
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