Ive been boating for about 10 years, and after starting with a Yamaha LS2000 twin jet boat, have had a couple I/O boats in between.
Just picked up a tricked out (ie mutilated) 73 Rally Sport Hombre turned hot-rod.
On our 1st time out yesterday, it ran OK at first, then started dying at about 2krpm, and finally would not stay running. We limped back by waiting a minute, after which it would start and run for about 30 seconds. repeated 8-10 times, then pushed with oars and boat hook lol.
Initially i thought it was a midrange jet problem on the Holley double-pumper, but then when it died altogether i suspect fuel starvation.
After I thought about it last night. here is m working theory:
The boat has twin tanks, one on each side. They appear to be plumbed with just a T fitting before the mechanical fuel pump on the olds 455. I filled one tank but not the other before going out. Awesomely cool dual fuel gauges don't seem to work.
So I'm now thinking the other tank was near empty, and when it got below the pickup it started sucking air, resulting in no draw from the other tank. I there EVER a case where a T fitting works with dual tanks? Doesn't make sense to me.
There are many things on this boat that make me think it was set up but never run. The place diverter trim cylinder disconnected from the arm on the short ride. The throttle pedal was set up with about 1" of travel 0-WOT (which I noticed and fixed before our 1st run)
Also, the fuel tank vents (or are they returns) are capped, which seems wrong as well. I understand in a boat these are normally vented to atmosphere.
I'm thinking no matter what, I should replace the T fitting with a selector valve. doing a little web research it seems I should go with a 3/8 valve. Will probably go mechanical. I haven't found a cheap enough electric one (want marine, not automotive), but open to options if someone has a suggestion other than the GM truck one.
I think your theory on the T maybe correct as you only filled one tank. FWIW, I too connect my tanks with a T fitting and no valve and I just always fill both tanks to avoid these issues. They pulled fairly evenly, (Mallory 140 series electric fuel pump) but I just cruise around and WOT a few times out. There are those here on the forum that do suggest a valve, but as I mostly just cruise, I have not seen the need. My tanks are self vented via the caps and your vents being capped does not sound right to me and hopefully some others will chime in.
With only one tank filled....you may have been sucking air.... but that also depends on variables ie T fitting location in relation to the tanks...
One experience I had was a bunch of crud on the bottom of the tanks that would hit the pickups when on a plane and clog the fuel lines or inlets from the tank(s). Best of luck brother, keep after it, you'll get it. MW
Sounds like you are sucking air to me (but I am usually wrong!) I went through two similar seasons chasing down fuel supply demons. My final conclusion was to small of a carb and my cam lobes were worn on the mechanical fuel pump. The vents through the fuel caps were fine but the electronic tank selector valve was sucking air on the 3/8" discharge. I wouldn't think it would make that much diff but it did. Finally not clogging fuel filters! Replacing whole top end this winter though (TR, dual carb and DoveAC heads) so I will figure out the next set of demons. Also check the gasket under your carb.......just don't want to talk about it.