OK. Here is some interesting poop in the Haynes Fuel systems Techbook.

I think it answers many questions and will put to rest all the bickering over vacuum petcocks.

"There is another cause of fuel starvation: when bikes with vacuum operated fuel taps have a less restrictive airbox or filter fitted. Even though the carburetors were re-jetted, the intake vacuum on WOT can be reduced enough to stop the fuel tap working. A manual type of tap is then needed. Going a stage further, tuned engines require more fuel flow and it isn't unknown for this to exceed the fuel tap's capacity. The tap effectively takes over the job of the main jets!"

I think that says a lot. Re-jet, free-flow air filter and WOT can cause starvation. It isn't really the fault of the fuel tap. It is a result of modifying the bike outside the parameters the fuel tap was designed for. Therefore we have the PRIME setting.

2004+ bikes have larger fuel lines. Maybe to help with modified bikes and fuel starvation.

Makes total sense to me!  I've known that prime has solved issues with starvation at WOT on some models, and now this connects some dots.

- Bboy

Nice find, would never have thought of that.

Good find. This makes sense to me too being mine has a K&N Lunchbox and will cutout when in the ON position. I had bought a new 09 frame petcock and it did the same thing so I always just leave it on PRI but its good to know its not the petcock or carbs