I had that happen to me. Boat with one motor and two tanks. One tank worked other tank started not going past 3500-4000rpm. Blew fuel line with 100psi. several times, replaced fuel line never fixed it. Could have been vacuum in tank i don't know? Sold boat.

The vent works, I stuck my hose in the gas inlet and the air (although slow) came back out. I will test this also next time out. If still have problem, I will remove gas cap and run it to rule out venting issue.

UPDATE: DONE!! Runs like a top! No more falling on FACE over 4k!!
 

Well, I took her out again today.

Repaired both engine vacuum lines going into plenum, blew out pickups and vent for tank. Also filled the filters half with lucas fuel injector cleaner for the hell of it even though there were cleaned and flow tested 85 hours ago.

Ran out in lake at about 4k, punched it and immediately noticed there was no bogging down above 4k.

I am going to make an assumption that it was the vacuum lines, hell, I didnt even know I had FPR or much less how they worked until I started chasing this down.

About 75 degrees, wind out of north around 15mph and 1 ft chop.
Both running 5k at 65-66mph
:bunnydance:

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Damn I got the exact same problem but mines a carb'd 509 with 35 hours on it and just started doing this! When motor was done everything was replaced new elect. Fuel pump all line after pump, all msd box, dist, coil, etc with new motor! Ran great then about 20 hour out bam can't run it from 4500-5500 where it needs to be! And I don't have vacuum lines! I quess I will check pick up now! Any ideas?