Originally Posted by schilzy
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The bike inevitably starts to sputter as if it's not gettting any gas and than completely shuts down after being driven for 15 minutes or so with the shut-off valve turned to the "on" position. I simply pull-over to the side of the road, turn the shut off valve to the "reserve" position and the bikestarts up & runs just fine..no problems.
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Assuming that's 15 minutes of high-speed riding, then:
Take a close look at the back of the fuel petcock. There should be a single rubber hose connected to it. If there are two hoses someone's swapped in a vacuum-operated valve that automatically shuts off when the engine vacuum drops.
If so then check for a vacuum leak in the hose from the #1 carburetor to the back of the petcock. The hose may have cracked and as the vacuum drops with rpm/throttle it may be closing the fuel valve.
If you've got a single hose on the petcock that goes down toward the bottom of the carbs, that's the standard manual one.
Take a look at the rear of the tank on the left side. There should be a short rubber hose going into a black plastic device that's a tipover valve and normally vents the tank. The tipover valve must be vertical with the big end down. If not it will seal vent to the gas tank and eventually fuel consumption will create a vacuum in the tank that prevents fuel from flowing into the carbs.
Check that the valve isn't stuck by removing it and blowing through it while holding it vertical. Likewise check that the vent lines from the tank and from the bottom of the tipover valve aren't blocked. There's a chance you'll find a plug in the end of the line from the tipover valve that should have been put in the air filter box drain hose.
BUT, that doesn't explain why switching to 'reserve' makes a difference.
If the problem is happening at low rpms/small throttle operation then there's some sort of blockage in the petcock 'run' plumbing and you'll have to remove the petcock disassemble it and see if you can find the problem. It could be caked up inside with fuel additives or possibly something left over from manufacture.
Since the bike runs ok on reserve then it's probably not a blocked duckbill filter.
Of course, the obvious question (that I've been saving for last) is:
Does this happen when the tank is full?
Jim