67 johnson 9.5 thumps

[relics hunter]

OK so this motor is running great with the exception of it thumping while at idle in neutral. I replaced the water impeller, total rebuild of the carb. and the plugs are gapped correct. truthfully the thump kind of reminds of a motor slipping in and out of gear but I have the prop off and I can hold the shaft and feel no movement in it.
it runs smooth but this thumping is killin me here!
any opinions on this problem ? cause I'm stumped

 

[racerone]

Sounds like a lean sneeze --open the low speed mixture needle.-----Or throttle plate is opening too soon and needs to be adjusted.

 

[relics hunter]

just an update I went through all the motions and discovered a crack in the fuel line at the fuel pump which was sucking air in. It runs great now thanks for the help