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Discussion Starter #1 (Edited)
I was riding on the highway, on my way to refueling; the fuel level was low, but the gas station was still within range. I was prepared for having to switch to reserve.

Eventually the engine stopped pulling a couple miles before reaching the gas station. I was in the inner lane and there was upcoming traffic behind me, so I kept pulling the throttle, hoping for some last pulls from the engine, while routinely reached down to the petcock and switched to reserve.

The engine didn't come back. I was desperate to maintain my speed due to the approaching traffic from behind, so -if I remember correctly- I had the throttle wide open, hoping for a quick acceleration. It didn't come. Eventually I gave up and made my way to the side of the road. The engine completely died.

After assessing the situation (I still have some fuel, no prior malfunctions, etc..), I made an attempt to restart the engine, but -for some reason- with full choke. The engine started immediately and vigorously revved up.

Eventually I made it to the gas station and completed the trip without any further issues.

Here's my theory of what happened:
At the time I switched to reserve, I had the throttle wide open, because I was expecting the engine to start pulling, once the carburetors fill up.
However since the engine had just revved down due to the fuel starvation, its suction was probably insufficient for creating vacuum behind the open throttle, so the vacuum-petcock didn't open and didn't let any fuel into the carburetors.

The choke takes fuel from the very bottom of the carburetor, so probably that's the reason why the bike started up with full choke right after dying.

This was a dangerous but useful lesson to me, I'm looking for comments and curious if this has happened to others too.
 

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classic EX tank vent issue. Remove the cap, disassemble and remove the 2 orange flapper valves. End of vent problem.

Search the forum here for pics, how-to. Been documented hundreds, if not, thousands of times.
 

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Discussion Starter #3
Interesting, I think you are right.
  • I opened the fuel cap on the side of the road to check the fuel level, so air could get into the tank.
  • The fuel level wasn't that low - initially I suspected that it could have been a temporary thing, like a downhill section of the highway that moved all the fuel forward, etc..
But the vent issue explains it all
 

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Discussion Starter #4
UPDATE:
I took the fuel cap apart and pulled the two plastic "cups" with the orange rubber valves out. One of the rubber valves is totally flat and looks like a suction cup; the other one has four lobes on the top.
I tried blowing through both of them - the suction-cup looking one needed a surprisingly powerful blow given its size. I could feel it popping open. I can imagine that an almost empty tank doesn't have enough gas in it to create an adequate vacuum, hence this valve won't let any air in.
The other valve (with the lobes) wasn't that stubborn. I was planning to remove it from its plastic cup and install it to the cup of the flat valve but I broke both of them while pulling them through the hole. So I removed both of them, but IMO the 'healthy' valve (with the lobes) going to the bottom is a safe compromise - at least there would still be something there.
 

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a pic or 2 within this old thread, scroll down a bit

In addition, I'd suggest taking a minute and entirely remove the cap from tank so as to not drop tiny parts in to tank. Work on the bench!
 

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just realized I did flap removal a few weeks ago, opted to remove both. See pic. Line shows location removed from, pull the plastic retainer cups which house them, 1 on top of the other. Replace the now empty caps...reassemble. This bike issue free since. The silicone flappers get gooey and stick after being subjected to gas fumes....thus preventing venting fumes (see what I did there?) both inward (as fuel level declines) and out (heat causing expansion and fumes). Each valve is responsible...the reason to do BOTH.



ge7.jpg
 

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My bike stopped whistling in garage after I parked once I did this modifcation years ago. I did a Pingel ON/OFF petcock conversion a while back too which I really enjoy. You just have to cap off one of the carb orifices that the original petcock was hosed to and use a die grinder with a rotary bit to make the gas tank hole a little larger.
 
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