Quote Originally Posted by SquamInboards View Post
When you said it happened last after a long ride (5 - 10 min) at relatively high speed, I was thinking it sounds like a fuel line collapsing internally. Pretty hard to diagnose, other than running it that same way on an external fuel tank to see if the problem goes away. But replacing the main fuel line isn't that bad of a job, maybe it's worth doing, unless you have done it very recently. Just a thought. Ethanol blended fuel is really tough on old fuel lines. But your symptoms make sense, in that it only happens when you're drawing a large volume of fuel for a long continuous time. But that scenario also can stir up crap in your fuel filter, or any residual water too.

I second this idea. I have had issues like this happen with vacuum lines and fuel lines over the years, mainly on older vehicles, mostly carbureted. They can collapse from excessive heat and solvent wear making them weak.
It's a cheap and relatively easy fix. I think it's also about time we crack that carb open for a good cleaning and possibly a rebuild.
I think I've told you about the issue we had on our boat where it turned out a small piece of pencil lead had fallen in the gas tank somehow, gotten sucked through the lines into the carb and would intermittently clog the carb up. It would run fine 9/10 times, but every now and then it would find itself in the right position to block the fuel and you'd get stuttering and surging. Let the boat settle for a bit, try again and it was fine until it got in there again. The problem only occurred under load. You could have some debris in the carb or fuel line.
Let me know, I've rebuilt several carbs and will be happy to lend my expertise.