[OMC-Boats] Bogging down after 45 mins of running - any ideas?

Howdy --
I own a '68 Sportsman 120 w/ a '69 155 engine kept in Southwest Harbor, Maine. It's fairly original, meaning points, OEM coil, rebuilt carb from Flying Fish if I remember correctly, etc. The major mods at this point are only a new outboard engine style fuel tank (old one rusted out), oil filter-style fuel filter, separate blower/bilge pump and such small stuff.

So, I had the old girl out this Fri for a few hours -- 1 of the typically half dozen days a season I get to use it. The rest of the time it's stored inside. It's always fogged and gas drained in the fall. On Fri, after charging battery and checking all the typical issues (tilt motor, shifting, bilge pump) I filled the tank, dunked it in the lake and it fired right up. Idled low and quiet, pumping water just like it's supposed to. I breathed a sigh of relief and cast off.

I then proceeded to run at 3/4 throttle or so for about 45 mins. At that point, I noticed I could give it more gas but it wouldn't go any faster. It then started to bog down more, and eventually the motor died completely, as if you shut off the key. All this time there were no "hot" or "oil" lights on, both manifolds were warm but not burning, and it didn't smell hot. Wondering what was up and if I'd be paddling to shore, I opened the gas tank cover, topped off the tank from a jerry can, and let it rest for maybe 15 mins. At that point, it started right up and ran well for another 45 mins or so, until same issue. After lunch, I found I could run for at least 2 hours at say 1/4 throttle. So it seems related to higher revs/load.

Any ideas, ladies and gentlemen? I did not change the screw-on fuel filter or sparkplugs this year, although I did both last year. The filter would have been cleared of any old gas by the time I had a problem anyway. It didn't look like the plastic gas tank was sucking in and there was no whoosh when I opened it, so I don't think I have an inop tank vent. Also, my gas line to the outboard style tank has a squeeze bulb inline and that seemed to always be full. Could I be overheating the coil?* I noticed that was very hot to the touch. Dying fuel pump? That's pretty cheap and easy to replace.

Help...my eternal gratitude to any and all replies/ideas!

*Related, I'm considering upgrading to CRT performance's HEI ignition and coil anyway, which would eliminate the spark related causes. I'll be driving by their shop this week, in fact. Please let me know if anyone believes that $250 wouldn't be $ well spent.

Thanks much,

-Micah Donahue

Newburyport, MA
Received on Sunday, 7 July 2013