It's a Marine 4160 (list 75009-1), pre 1992 vintage, that has been having intermittent idling "opportunities"..rough idle if it will idle at all and black/blue smoke. I first thought I had a blown Power Valve when I read about the phenomena on pre 92 carbs. So dismantled for rebuild and installed the check valve mod. Re-installed and ran great for a several minutes then back to smoking/rough idle (actually it will generally stall if below ~1000 rpm). FINALLY realized that the J vent on the primary side was flowing gas. So of course I've tried to reset float level (it's not externally adjustable) by dismantling and bending tab several times with no luck. Also changed out the float valve and seat (from rebuild kit 703-28) and also changed the float from secondary side to primary side to no avail. Currently I've ordered a flow regulator (Holley 12-803) and a gauge (Holley 26-504) and hopefully will see that the electric pump is acting up and putting out too high a pressure..intermittently??? The engine manual (5.7L 350 CID Marine Power/GM) says 4-6 psi ..The fuel pump is a part # 152-875 which I believe is a Carter P4389 or P4594. BTW each time I've changed the float setting the float bowl is clean and I have blown air thru passages...I can't believe there is debris getting stuck under seat. SOOOOOO what have my "blinders" been preventing me from seeing????

Thanks in advance for any thoughts.