I agree you are having vacuum actuated petcock issues -- interstate running at 75 mph used to cause me to have "false low fuel incidents" that would put me on reserve and have me headed for a gas station -- same story it stumbled like it was out of gas.
When you are running fast your vacuum level drops a lot, sometimes down to less than what will run your petcock once it gets old and flabby and the rubber diaphragm and valve seals have degraded a bit.
Solution for me was to switch to a $24 mechanical raptor petcock. Now I have no issues at all on the super slab (except my legs get tired and start to cramp after enough of it)
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quote from tech section post
2003 yamaha raptor 660 petcock fits
Franch posting today at 02:47:14
fuelcock part # 5 Lp-24500-01-00
$20.89 from bike bandit plus shipping. it's a direct bolt on.
comes with o-ring.
not as pretty as the original part.
no vacuum line needed.
has on, off and reserve.
must manually turn off.
the nice thing is this petcock doesn't go under the tank towards center of the bike nearly as much as OEM so taking the tank off and on won't be such a tight fit.
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At Ron Ayers Motorsports, it cost less money.
http://www.ronayers.com/browseparts.cfm?SearchString=5LP-24500-01-00&adv=5&kw...
$15.61 isn't that bad a price.
Ron will charge you $9.00 flat rate for shipping.
$24.61 to your door
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Now, Franch gets full credit for his marvelous find. But please, somebody who does this who owns a camera please fill in all the blanks by posting appropriate additional information below.