78 yamaha xs 400 fuel issue

[jscustoms]

Ok, so here's the deal-

I have a 1978 yamaha xs 400. This bike has been modified slightly by yours truly. The frame has been chopped and a solo seat installed and a Sportster peanut tank with Harley petcock (gravity feed, not vacuum) has been fitted. Not much has been changed on the engine other than some aftermarket slip on mufflers and pod air filters and a carb jet kit was installed.


The issue I am having is this:
The bike will sit and idle all day long. I take it out around town and it runs great- has good power (well, for a 400) and runs smooth and strong.
If I take it out on the highway (60-65 MPH), it will run good for 20-30 miles, then it will spit and sputter for a few seconds and then shut off. Look down and the clear fuel tubing is completely dry. Let it sit for 10-15 minutes and the fuel tubing is about half full (rarely gets fuller than that) and the engine will fire right up and run like a champ... until the fuel line goes dry again.

Here's what I've checked so far:
-the tank- fuel cap is venting properly (even tried removing it, no help).

-petcock- is flowing fine, a solid stream that almost fills the 5/6" outlet- ran a whole tank of fuel through it into a gas can and didn't have any issues with the tank vacuum locking. Then I hook the fuel line up to the carbs and barely get a trickle. I would imagine this petcock should flow twice what this engine should ever demand, even at high speeds. This did have an in-line fuel filter at one time, but I removed it thinking that could be the cause.

-carbs- have been disassembled and cleaned like nobody's business. I even had them ultrasonically cleaned. I set the float level at the spec the service manual calls for (32mm from the gasket flange if I remember correctly). I checked the float level with a tube gauge and verified it is correct while installed on the bike. Bowl vents are clean and clear as could be.

After bouncing some ideas off of Dodsfall in another thread (thanks again), I decided to check the float valves to see if they were sticking somehow or just not opening far enough. I made a little trough out of an empty washer fluid bottle and removed the bowl drain plug (one carb at a time). I then opened the petcock and let it flow for a few seconds into the trough. Both carbs seemed to have plenty of fuel flowing into them. I opened the petcock for about 5 seconds on each carb and got about a cup of fuel out of each side. Also, if I accidentally forget to turn the petcock the "OFF" position, it will flood the carbs (it is gravity-fed, afterall).

I've checked everything I can think of (even if I may have forgotten to list it here) and I am just stumped. I've been working on cars and bikes professionally for the last 15 years and have owned my own shop for the last 4+ and have been pretty darned successful, but I'm out of ideas on this one...

Any more ideas out there?